xref: /wasmtime-44.0.1/cranelift/codegen/src/machinst/vcode.rs (revision 67ce1ef2)

//! This implements the VCode container: a CFG of Insts that have been lowered.
//!
//! VCode is virtual-register code. An instruction in VCode is almost a machine
//! instruction; however, its register slots can refer to virtual registers in
//! addition to real machine registers.
//!
//! VCode is structured with traditional basic blocks, and
//! each block must be terminated by an unconditional branch (one target), a
//! conditional branch (two targets), or a return (no targets). Note that this
//! slightly differs from the machine code of most ISAs: in most ISAs, a
//! conditional branch has one target (and the not-taken case falls through).
//! However, we expect that machine backends will elide branches to the following
//! block (i.e., zero-offset jumps), and will be able to codegen a branch-cond /
//! branch-uncond pair if *both* targets are not fallthrough. This allows us to
//! play with layout prior to final binary emission, as well, if we want.
//!
//! See the main module comment in `mod.rs` for more details on the VCode-based
//! backend pipeline.

use crate::CodegenError;
use crate::FxHashMap;
use crate::ir::pcc::*;
use crate::ir::{self, Constant, ConstantData, ValueLabel, types};
use crate::ranges::Ranges;
use crate::timing;
use crate::trace;
use crate::{LabelValueLoc, ValueLocRange};
use crate::{machinst::*, trace_log_enabled};
use regalloc2::{
    Edit, Function as RegallocFunction, InstOrEdit, InstPosition, InstRange, Operand,
    OperandConstraint, OperandKind, PRegSet, ProgPoint, RegClass,
};

use crate::HashMap;
use crate::hash_map::Entry;
use core::cmp::Ordering;
use core::fmt::{self, Write};
use core::mem::take;
use core::ops::Range;
use cranelift_entity::{Keys, entity_impl};

/// Index referring to an instruction in VCode.
pub type InsnIndex = regalloc2::Inst;

/// Extension trait for `InsnIndex` to allow conversion to a
/// `BackwardsInsnIndex`.
trait ToBackwardsInsnIndex {
    fn to_backwards_insn_index(&self, num_insts: usize) -> BackwardsInsnIndex;
}

impl ToBackwardsInsnIndex for InsnIndex {
    fn to_backwards_insn_index(&self, num_insts: usize) -> BackwardsInsnIndex {
        BackwardsInsnIndex::new(num_insts - self.index() - 1)
    }
}

/// An index referring to an instruction in the VCode when it is backwards,
/// during VCode construction.
#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
#[cfg_attr(
    feature = "enable-serde",
    derive(::serde::Serialize, ::serde::Deserialize)
)]
pub struct BackwardsInsnIndex(InsnIndex);

impl BackwardsInsnIndex {
    pub fn new(i: usize) -> Self {
        BackwardsInsnIndex(InsnIndex::new(i))
    }
}

/// Index referring to a basic block in VCode.
pub type BlockIndex = regalloc2::Block;

/// VCodeInst wraps all requirements for a MachInst to be in VCode: it must be
/// a `MachInst` and it must be able to emit itself at least to a `SizeCodeSink`.
pub trait VCodeInst: MachInst + MachInstEmit {}
impl<I: MachInst + MachInstEmit> VCodeInst for I {}

/// A function in "VCode" (virtualized-register code) form, after
/// lowering.  This is essentially a standard CFG of basic blocks,
/// where each basic block consists of lowered instructions produced
/// by the machine-specific backend.
///
/// Note that the VCode is immutable once produced, and is not
/// modified by register allocation in particular. Rather, register
/// allocation on the `VCode` produces a separate `regalloc2::Output`
/// struct, and this can be passed to `emit`. `emit` in turn does not
/// modify the vcode, but produces an `EmitResult`, which contains the
/// machine code itself, and the associated disassembly and/or
/// metadata as requested.
pub struct VCode<I: VCodeInst> {
    /// VReg IR-level types.
    vreg_types: Vec<Type>,

    /// Lowered machine instructions in order corresponding to the original IR.
    insts: Vec<I>,

    /// A map from backwards instruction index to the user stack map for that
    /// instruction.
    ///
    /// This is a sparse side table that only has entries for instructions that
    /// are safepoints, and only for a subset of those that have an associated
    /// user stack map.
    user_stack_maps: FxHashMap<BackwardsInsnIndex, ir::UserStackMap>,

    /// A map from backwards instruction index to the debug tags for
    /// that instruction. Each entry indexes a range in the
    /// `debug_tag_pool`.
    debug_tags: FxHashMap<BackwardsInsnIndex, Range<u32>>,

    /// Pooled storage for sequences of debug tags; indexed by entries
    /// in `debug_tags`.
    debug_tag_pool: Vec<ir::DebugTag>,

    /// Operands: pre-regalloc references to virtual registers with
    /// constraints, in one flattened array. This allows the regalloc
    /// to efficiently access all operands without requiring expensive
    /// matches or method invocations on insts.
    operands: Vec<Operand>,

    /// Operand index ranges: for each instruction in `insts`, there
    /// is a tuple here providing the range in `operands` for that
    /// instruction's operands.
    operand_ranges: Ranges,

    /// Clobbers: a sparse map from instruction indices to clobber masks.
    clobbers: FxHashMap<InsnIndex, PRegSet>,

    /// Source locations for each instruction. (`SourceLoc` is a `u32`, so it is
    /// reasonable to keep one of these per instruction.)
    srclocs: Vec<RelSourceLoc>,

    /// Entry block.
    entry: BlockIndex,

    /// Block instruction indices.
    block_ranges: Ranges,

    /// Block successors: index range in the `block_succs` list.
    block_succ_range: Ranges,

    /// Block successor lists, concatenated into one vec. The
    /// `block_succ_range` list of tuples above gives (start, end)
    /// ranges within this list that correspond to each basic block's
    /// successors.
    block_succs: Vec<regalloc2::Block>,

    /// Block predecessors: index range in the `block_preds` list.
    block_pred_range: Ranges,

    /// Block predecessor lists, concatenated into one vec. The
    /// `block_pred_range` list of tuples above gives (start, end)
    /// ranges within this list that correspond to each basic block's
    /// predecessors.
    block_preds: Vec<regalloc2::Block>,

    /// Block parameters: index range in `block_params` below.
    block_params_range: Ranges,

    /// Block parameter lists, concatenated into one vec. The
    /// `block_params_range` list of tuples above gives (start, end)
    /// ranges within this list that correspond to each basic block's
    /// blockparam vregs.
    block_params: Vec<regalloc2::VReg>,

    /// Outgoing block arguments on branch instructions, concatenated
    /// into one list.
    ///
    /// Note that this is conceptually a 3D array: we have a VReg list
    /// per block, per successor. We flatten those three dimensions
    /// into this 1D vec, then store index ranges in two levels of
    /// indirection.
    ///
    /// Indexed by the indices in `branch_block_arg_succ_range`.
    branch_block_args: Vec<regalloc2::VReg>,

    /// Array of sequences of (start, end) tuples in
    /// `branch_block_args`, one for each successor; these sequences
    /// for each block are concatenated.
    ///
    /// Indexed by the indices in `branch_block_arg_succ_range`.
    branch_block_arg_range: Ranges,

    /// For a given block, indices in `branch_block_arg_range`
    /// corresponding to all of its successors.
    branch_block_arg_succ_range: Ranges,

    /// Block-order information.
    block_order: BlockLoweringOrder,

    /// ABI object.
    pub(crate) abi: Callee<I::ABIMachineSpec>,

    /// Constant information used during code emission. This should be
    /// immutable across function compilations within the same module.
    emit_info: I::Info,

    /// Constants.
    pub(crate) constants: VCodeConstants,

    /// Value labels for debuginfo attached to vregs.
    debug_value_labels: Vec<(VReg, InsnIndex, InsnIndex, u32)>,

    pub(crate) sigs: SigSet,

    /// Facts on VRegs, for proof-carrying code verification.
    facts: Vec<Option<Fact>>,

    log2_min_function_alignment: u8,
}

/// The result of `VCode::emit`. Contains all information computed
/// during emission: actual machine code, optionally a disassembly,
/// and optionally metadata about the code layout.
pub struct EmitResult {
    /// The MachBuffer containing the machine code.
    pub buffer: MachBufferFinalized<Stencil>,

    /// Offset of each basic block, recorded during emission. Computed
    /// only if `machine_code_cfg_info` is enabled.
    pub bb_offsets: Vec<CodeOffset>,

    /// Final basic-block edges, in terms of code offsets of
    /// bb-starts. Computed only if `machine_code_cfg_info` is enabled.
    pub bb_edges: Vec<(CodeOffset, CodeOffset)>,

    /// The pretty-printed disassembly, if any. This uses the same
    /// pretty-printing for MachInsts as the pre-regalloc VCode Debug
    /// implementation, but additionally includes the prologue and
    /// epilogue(s), and makes use of the regalloc results.
    pub disasm: Option<String>,

    /// Value-labels information (debug metadata).
    pub value_labels_ranges: ValueLabelsRanges,
}

/// A builder for a VCode function body.
///
/// This builder has the ability to accept instructions in either
/// forward or reverse order, depending on the pass direction that
/// produces the VCode. The lowering from CLIF to VCode<MachInst>
/// ordinarily occurs in reverse order (in order to allow instructions
/// to be lowered only if used, and not merged) so a reversal will
/// occur at the end of lowering to ensure the VCode is in machine
/// order.
///
/// If built in reverse, block and instruction indices used once the
/// VCode is built are relative to the final (reversed) order, not the
/// order of construction. Note that this means we do not know the
/// final block or instruction indices when building, so we do not
/// hand them out. (The user is assumed to know them when appending
/// terminator instructions with successor blocks.)
pub struct VCodeBuilder<I: VCodeInst> {
    /// In-progress VCode.
    pub(crate) vcode: VCode<I>,

    /// In what direction is the build occurring?
    direction: VCodeBuildDirection,

    /// Debug-value label in-progress map, keyed by label. For each
    /// label, we keep disjoint ranges mapping to vregs. We'll flatten
    /// this into (vreg, range, label) tuples when done.
    debug_info: FxHashMap<ValueLabel, Vec<(InsnIndex, InsnIndex, VReg)>>,
}

/// Direction in which a VCodeBuilder builds VCode.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum VCodeBuildDirection {
    // TODO: add `Forward` once we need it and can test it adequately.
    /// Backward-build pass: we expect the producer to call `emit()`
    /// with instructions in reverse program order within each block.
    Backward,
}

impl<I: VCodeInst> VCodeBuilder<I> {
    /// Create a new VCodeBuilder.
    pub fn new(
        sigs: SigSet,
        abi: Callee<I::ABIMachineSpec>,
        emit_info: I::Info,
        block_order: BlockLoweringOrder,
        constants: VCodeConstants,
        direction: VCodeBuildDirection,
        log2_min_function_alignment: u8,
    ) -> Self {
        let vcode = VCode::new(
            sigs,
            abi,
            emit_info,
            block_order,
            constants,
            log2_min_function_alignment,
        );

        VCodeBuilder {
            vcode,
            direction,
            debug_info: FxHashMap::default(),
        }
    }

    pub fn init_retval_area(&mut self, vregs: &mut VRegAllocator<I>) -> CodegenResult<()> {
        self.vcode.abi.init_retval_area(&self.vcode.sigs, vregs)
    }

    /// Access the ABI object.
    pub fn abi(&self) -> &Callee<I::ABIMachineSpec> {
        &self.vcode.abi
    }

    /// Access the ABI object.
    pub fn abi_mut(&mut self) -> &mut Callee<I::ABIMachineSpec> {
        &mut self.vcode.abi
    }

    pub fn sigs(&self) -> &SigSet {
        &self.vcode.sigs
    }

    pub fn sigs_mut(&mut self) -> &mut SigSet {
        &mut self.vcode.sigs
    }

    /// Access to the BlockLoweringOrder object.
    pub fn block_order(&self) -> &BlockLoweringOrder {
        &self.vcode.block_order
    }

    /// Set the current block as the entry block.
    pub fn set_entry(&mut self, block: BlockIndex) {
        self.vcode.entry = block;
    }

    /// End the current basic block. Must be called after emitting vcode insts
    /// for IR insts and prior to ending the function (building the VCode).
    pub fn end_bb(&mut self) {
        let end_idx = self.vcode.insts.len();
        // Add the instruction index range to the list of blocks.
        self.vcode.block_ranges.push_end(end_idx);
        // End the successors list.
        let succ_end = self.vcode.block_succs.len();
        self.vcode.block_succ_range.push_end(succ_end);
        // End the blockparams list.
        let block_params_end = self.vcode.block_params.len();
        self.vcode.block_params_range.push_end(block_params_end);
        // End the branch blockparam args list.
        let branch_block_arg_succ_end = self.vcode.branch_block_arg_range.len();
        self.vcode
            .branch_block_arg_succ_range
            .push_end(branch_block_arg_succ_end);
    }

    pub fn add_block_param(&mut self, param: VirtualReg) {
        self.vcode.block_params.push(param.into());
    }

    fn add_branch_args_for_succ(&mut self, args: &[Reg]) {
        self.vcode
            .branch_block_args
            .extend(args.iter().map(|&arg| VReg::from(arg)));
        let end = self.vcode.branch_block_args.len();
        self.vcode.branch_block_arg_range.push_end(end);
    }

    /// Push an instruction for the current BB and current IR inst
    /// within the BB.
    pub fn push(&mut self, insn: I, loc: RelSourceLoc) {
        assert!(!insn.is_low_level_branch()); // These are not meant to be in VCode.
        self.vcode.insts.push(insn);
        self.vcode.srclocs.push(loc);
    }

    /// Add a successor block with branch args.
    pub fn add_succ(&mut self, block: BlockIndex, args: &[Reg]) {
        self.vcode.block_succs.push(block);
        self.add_branch_args_for_succ(args);
    }

    /// Add a debug value label to a register.
    pub fn add_value_label(&mut self, reg: Reg, label: ValueLabel) {
        // 1) In the reversed order, we consider the instructions
        //    that define ranges in the "debug_info" array to refer
        //    to the IP **after** them (when reversed):
        //      IP[2]__| Inst 3 |
        //      IP[1]__| Inst 2 |
        //      IP[0]__| Inst 1 |
        //             | Inst 0 |
        //    This is so that we can represent IP[<function start>],
        //    done at the cost of not being to represent IP[<function end>],
        //    which is OK since no values will be live at that point.
        // 2) The live range for "reg" begins at the current IP
        //    and continues until the next, in execution order,
        //    VReg that defines "label". Since the ranges are open
        //    at the end, the subtraction of 1 cancels out:
        //      [last..current IP] <=>
        //      [last..last emitted inst index] <=>
        //      [last..next_inst_index - 1] <=>
        //      [last..next_inst_index)
        //
        let next_inst_index = self.vcode.insts.len();
        if next_inst_index == 0 {
            // This would produce a defective [0..0) range.
            return;
        }
        let next_inst = InsnIndex::new(next_inst_index);
        let labels = self.debug_info.entry(label).or_insert_with(|| vec![]);
        let last = labels
            .last()
            .map(|(_start, end, _vreg)| *end)
            .unwrap_or(InsnIndex::new(0));
        labels.push((last, next_inst, reg.into()));
    }

    /// Access the constants.
    pub fn constants(&mut self) -> &mut VCodeConstants {
        &mut self.vcode.constants
    }

    fn compute_preds_from_succs(&mut self) {
        // Do a linear-time counting sort: first determine how many
        // times each block appears as a successor.
        let mut starts = vec![0u32; self.vcode.num_blocks()];
        for succ in &self.vcode.block_succs {
            starts[succ.index()] += 1;
        }

        // Determine for each block the starting index where that
        // block's predecessors should go. This is equivalent to the
        // ranges we need to store in block_pred_range.
        self.vcode.block_pred_range.reserve(starts.len());
        let mut end = 0;
        for count in starts.iter_mut() {
            let start = end;
            end += *count;
            *count = start;
            self.vcode.block_pred_range.push_end(end as usize);
        }
        let end = end as usize;
        debug_assert_eq!(end, self.vcode.block_succs.len());

        // Walk over the successors again, this time grouped by
        // predecessor, and push the predecessor at the current
        // starting position of each of its successors. We build
        // each group of predecessors in whatever order Ranges::iter
        // returns them; regalloc2 doesn't care.
        self.vcode.block_preds.resize(end, BlockIndex::invalid());
        for (pred, range) in self.vcode.block_succ_range.iter() {
            let pred = BlockIndex::new(pred);
            for succ in &self.vcode.block_succs[range] {
                let pos = &mut starts[succ.index()];
                self.vcode.block_preds[*pos as usize] = pred;
                *pos += 1;
            }
        }
        debug_assert!(self.vcode.block_preds.iter().all(|pred| pred.is_valid()));
    }

    /// Called once, when a build in Backward order is complete, to
    /// perform the overall reversal (into final forward order) and
    /// finalize metadata accordingly.
    fn reverse_and_finalize(&mut self, vregs: &VRegAllocator<I>) {
        let n_insts = self.vcode.insts.len();
        if n_insts == 0 {
            return;
        }

        // Reverse the per-block and per-inst sequences.
        self.vcode.block_ranges.reverse_index();
        self.vcode.block_ranges.reverse_target(n_insts);
        // block_params_range is indexed by block (and blocks were
        // traversed in reverse) so we reverse it; but block-param
        // sequences in the concatenated vec can remain in reverse
        // order (it is effectively an arena of arbitrarily-placed
        // referenced sequences).
        self.vcode.block_params_range.reverse_index();
        // Likewise, we reverse block_succ_range, but the block_succ
        // concatenated array can remain as-is.
        self.vcode.block_succ_range.reverse_index();
        self.vcode.insts.reverse();
        self.vcode.srclocs.reverse();
        // Likewise, branch_block_arg_succ_range is indexed by block
        // so must be reversed.
        self.vcode.branch_block_arg_succ_range.reverse_index();

        // To translate an instruction index *endpoint* in reversed
        // order to forward order, compute `n_insts - i`.
        //
        // Why not `n_insts - 1 - i`? That would be correct to
        // translate an individual instruction index (for ten insts 0
        // to 9 inclusive, inst 0 becomes 9, and inst 9 becomes
        // 0). But for the usual inclusive-start, exclusive-end range
        // idiom, inclusive starts become exclusive ends and
        // vice-versa, so e.g. an (inclusive) start of 0 becomes an
        // (exclusive) end of 10.
        let translate = |inst: InsnIndex| InsnIndex::new(n_insts - inst.index());

        // Generate debug-value labels based on per-label maps.
        for (label, tuples) in &self.debug_info {
            for &(start, end, vreg) in tuples {
                let vreg = vregs.resolve_vreg_alias(vreg);
                let fwd_start = translate(end);
                let fwd_end = translate(start);
                self.vcode
                    .debug_value_labels
                    .push((vreg, fwd_start, fwd_end, label.as_u32()));
            }
        }

        // Now sort debug value labels by VReg, as required
        // by regalloc2.
        self.vcode
            .debug_value_labels
            .sort_unstable_by_key(|(vreg, _, _, _)| *vreg);
    }

    fn collect_operands(&mut self, vregs: &VRegAllocator<I>) {
        let allocatable = PRegSet::from(self.vcode.abi.machine_env());
        for (i, insn) in self.vcode.insts.iter_mut().enumerate() {
            // Push operands from the instruction onto the operand list.
            //
            // We rename through the vreg alias table as we collect
            // the operands. This is better than a separate post-pass
            // over operands, because it has more cache locality:
            // operands only need to pass through L1 once. This is
            // also better than renaming instructions'
            // operands/registers while lowering, because here we only
            // need to do the `match` over the instruction to visit
            // its register fields (which is slow, branchy code) once.

            let mut op_collector =
                OperandCollector::new(&mut self.vcode.operands, allocatable, |vreg| {
                    vregs.resolve_vreg_alias(vreg)
                });
            insn.get_operands(&mut op_collector);
            let (ops, clobbers) = op_collector.finish();
            self.vcode.operand_ranges.push_end(ops);

            if clobbers != PRegSet::default() {
                self.vcode.clobbers.insert(InsnIndex::new(i), clobbers);
            }

            if let Some((dst, src)) = insn.is_move() {
                // We should never see non-virtual registers present in move
                // instructions.
                assert!(
                    src.is_virtual(),
                    "the real register {src:?} was used as the source of a move instruction"
                );
                assert!(
                    dst.to_reg().is_virtual(),
                    "the real register {:?} was used as the destination of a move instruction",
                    dst.to_reg()
                );
            }
        }

        // Translate blockparam args via the vreg aliases table as well.
        for arg in &mut self.vcode.branch_block_args {
            let new_arg = vregs.resolve_vreg_alias(*arg);
            trace!("operandcollector: block arg {:?} -> {:?}", arg, new_arg);
            *arg = new_arg;
        }
    }

    /// Build the final VCode.
    pub fn build(mut self, mut vregs: VRegAllocator<I>) -> VCode<I> {
        self.vcode.vreg_types = take(&mut vregs.vreg_types);
        self.vcode.facts = take(&mut vregs.facts);

        if self.direction == VCodeBuildDirection::Backward {
            self.reverse_and_finalize(&vregs);
        }
        self.collect_operands(&vregs);

        self.compute_preds_from_succs();
        self.vcode.debug_value_labels.sort_unstable();

        // At this point, nothing in the vcode should mention any
        // VReg which has been aliased. All the appropriate rewriting
        // should have happened above. Just to be sure, let's
        // double-check each field which has vregs.
        // Note: can't easily check vcode.insts, resolved in collect_operands.
        // Operands are resolved in collect_operands.
        vregs.debug_assert_no_vreg_aliases(self.vcode.operands.iter().map(|op| op.vreg()));
        // Currently block params are never aliased to another vreg.
        vregs.debug_assert_no_vreg_aliases(self.vcode.block_params.iter().copied());
        // Branch block args are resolved in collect_operands.
        vregs.debug_assert_no_vreg_aliases(self.vcode.branch_block_args.iter().copied());
        // Debug value labels are resolved in reverse_and_finalize.
        vregs.debug_assert_no_vreg_aliases(
            self.vcode.debug_value_labels.iter().map(|&(vreg, ..)| vreg),
        );
        // Facts are resolved eagerly during set_vreg_alias.
        vregs.debug_assert_no_vreg_aliases(
            self.vcode
                .facts
                .iter()
                .zip(&vregs.vreg_types)
                .enumerate()
                .filter(|(_, (fact, _))| fact.is_some())
                .map(|(vreg, (_, &ty))| {
                    let (regclasses, _) = I::rc_for_type(ty).unwrap();
                    VReg::new(vreg, regclasses[0])
                }),
        );

        self.vcode
    }

    /// Add a user stack map for the associated instruction.
    pub fn add_user_stack_map(
        &mut self,
        inst: BackwardsInsnIndex,
        entries: &[ir::UserStackMapEntry],
    ) {
        let stack_map = ir::UserStackMap::new(entries, self.vcode.abi.sized_stackslot_offsets());
        let old_entry = self.vcode.user_stack_maps.insert(inst, stack_map);
        debug_assert!(old_entry.is_none());
    }

    /// Add debug tags for the associated instruction.
    pub fn add_debug_tags(&mut self, inst: BackwardsInsnIndex, entries: &[ir::DebugTag]) {
        let start = u32::try_from(self.vcode.debug_tag_pool.len()).unwrap();
        self.vcode.debug_tag_pool.extend(entries.iter().cloned());
        let end = u32::try_from(self.vcode.debug_tag_pool.len()).unwrap();
        self.vcode.debug_tags.insert(inst, start..end);
    }
}

const NO_INST_OFFSET: CodeOffset = u32::MAX;

impl<I: VCodeInst> VCode<I> {
    /// New empty VCode.
    fn new(
        sigs: SigSet,
        abi: Callee<I::ABIMachineSpec>,
        emit_info: I::Info,
        block_order: BlockLoweringOrder,
        constants: VCodeConstants,
        log2_min_function_alignment: u8,
    ) -> Self {
        let n_blocks = block_order.lowered_order().len();
        VCode {
            sigs,
            vreg_types: vec![],
            insts: Vec::with_capacity(10 * n_blocks),
            user_stack_maps: FxHashMap::default(),
            debug_tags: FxHashMap::default(),
            debug_tag_pool: vec![],
            operands: Vec::with_capacity(30 * n_blocks),
            operand_ranges: Ranges::with_capacity(10 * n_blocks),
            clobbers: FxHashMap::default(),
            srclocs: Vec::with_capacity(10 * n_blocks),
            entry: BlockIndex::new(0),
            block_ranges: Ranges::with_capacity(n_blocks),
            block_succ_range: Ranges::with_capacity(n_blocks),
            block_succs: Vec::with_capacity(n_blocks),
            block_pred_range: Ranges::default(),
            block_preds: Vec::new(),
            block_params_range: Ranges::with_capacity(n_blocks),
            block_params: Vec::with_capacity(5 * n_blocks),
            branch_block_args: Vec::with_capacity(10 * n_blocks),
            branch_block_arg_range: Ranges::with_capacity(2 * n_blocks),
            branch_block_arg_succ_range: Ranges::with_capacity(n_blocks),
            block_order,
            abi,
            emit_info,
            constants,
            debug_value_labels: vec![],
            facts: vec![],
            log2_min_function_alignment,
        }
    }

    /// Get the number of blocks. Block indices will be in the range `0 ..
    /// (self.num_blocks() - 1)`.
    pub fn num_blocks(&self) -> usize {
        self.block_ranges.len()
    }

    /// The number of lowered instructions.
    pub fn num_insts(&self) -> usize {
        self.insts.len()
    }

    fn compute_clobbers_and_function_calls(
        &self,
        regalloc: &regalloc2::Output,
    ) -> (Vec<Writable<RealReg>>, FunctionCalls) {
        let mut clobbered = PRegSet::default();
        let mut function_calls = FunctionCalls::None;

        // All moves are included in clobbers.
        for (_, Edit::Move { to, .. }) in &regalloc.edits {
            if let Some(preg) = to.as_reg() {
                clobbered.add(preg);
            }
        }

        for (i, range) in self.operand_ranges.iter() {
            let operands = &self.operands[range.clone()];
            let allocs = &regalloc.allocs[range];
            for (operand, alloc) in operands.iter().zip(allocs.iter()) {
                if operand.kind() == OperandKind::Def {
                    if let Some(preg) = alloc.as_reg() {
                        clobbered.add(preg);
                    }
                }
            }

            function_calls.update(self.insts[i].call_type());

            // Also add explicitly-clobbered registers.
            //
            // Skip merging this instruction's clobber list if not
            // "included in clobbers" as per the MachInst. (Some
            // backends use this to implement ABI specifics; e.g.,
            // excluding calls of the same ABI as the current function
            // from clobbers, because by definition everything
            // clobbered by the call can be clobbered by this function
            // without saving as well.
            //
            // This is important for a particular optimization: when
            // some registers are "half-clobbered", e.g. vector/float
            // registers on aarch64, we want them to be seen as
            // clobbered by regalloc so it avoids carrying values
            // across calls in these registers but not seen as
            // clobbered by prologue generation here (because the
            // actual half-clobber implied by the clobber list fits
            // within the clobbers that we allow without
            // clobber-saves).
            if self.insts[i].is_included_in_clobbers() {
                if let Some(&inst_clobbered) = self.clobbers.get(&InsnIndex::new(i)) {
                    clobbered.union_from(inst_clobbered);
                }
            }
        }

        let clobbered_regs = clobbered
            .into_iter()
            .map(|preg| Writable::from_reg(RealReg::from(preg)))
            .collect();

        (clobbered_regs, function_calls)
    }

    /// Emit the instructions to a `MachBuffer`, containing fixed-up
    /// code and external reloc/trap/etc. records ready for use. Takes
    /// the regalloc results as well.
    ///
    /// Returns the machine code itself, and optionally metadata
    /// and/or a disassembly, as an `EmitResult`. The `VCode` itself
    /// is consumed by the emission process.
    pub fn emit(
        mut self,
        regalloc: &regalloc2::Output,
        want_disasm: bool,
        flags: &settings::Flags,
        ctrl_plane: &mut ControlPlane,
    ) -> EmitResult
    where
        I: VCodeInst,
    {
        let _tt = timing::vcode_emit();
        let mut buffer = MachBuffer::new();
        buffer.set_log2_min_function_alignment(self.log2_min_function_alignment);
        let mut bb_starts: Vec<Option<CodeOffset>> = vec![];

        // The first M MachLabels are reserved for block indices.
        buffer.reserve_labels_for_blocks(self.num_blocks());

        // Register all allocated constants with the `MachBuffer` to ensure that
        // any references to the constants during instructions can be handled
        // correctly.
        buffer.register_constants(&self.constants);

        // Construct the final order we emit code in: cold blocks at the end.
        let mut final_order: SmallVec<[BlockIndex; 16]> = smallvec![];
        let mut cold_blocks: SmallVec<[BlockIndex; 16]> = smallvec![];
        for block in 0..self.num_blocks() {
            let block = BlockIndex::new(block);
            if self.block_order.is_cold(block) {
                cold_blocks.push(block);
            } else {
                final_order.push(block);
            }
        }
        final_order.extend(cold_blocks.clone());

        // Compute/save info we need for the prologue: clobbers and
        // number of spillslots.
        //
        // We clone `abi` here because we will mutate it as we
        // generate the prologue and set other info, but we can't
        // mutate `VCode`. The info it usually carries prior to
        // setting clobbers is fairly minimal so this should be
        // relatively cheap.
        let (clobbers, function_calls) = self.compute_clobbers_and_function_calls(regalloc);
        self.abi.compute_frame_layout(
            &self.sigs,
            regalloc.num_spillslots,
            clobbers,
            function_calls,
        );

        // Emit blocks.
        let mut cur_srcloc = None;
        let mut last_offset = None;
        let mut inst_offsets = vec![];
        let mut state = I::State::new(&self.abi, core::mem::take(ctrl_plane));

        let mut disasm = String::new();

        if !self.debug_value_labels.is_empty() {
            inst_offsets.resize(self.insts.len(), NO_INST_OFFSET);
        }

        // Count edits per block ahead of time; this is needed for
        // lookahead island emission. (We could derive it per-block
        // with binary search in the edit list, but it's more
        // efficient to do it in one pass here.)
        let mut ra_edits_per_block: SmallVec<[u32; 64]> = smallvec![];
        let mut edit_idx = 0;
        for block in 0..self.num_blocks() {
            let end_inst = InsnIndex::new(self.block_ranges.get(block).end);
            let start_edit_idx = edit_idx;
            while edit_idx < regalloc.edits.len() && regalloc.edits[edit_idx].0.inst() < end_inst {
                edit_idx += 1;
            }
            let end_edit_idx = edit_idx;
            ra_edits_per_block.push((end_edit_idx - start_edit_idx) as u32);
        }

        let is_forward_edge_cfi_enabled = self.abi.is_forward_edge_cfi_enabled();
        let mut bb_padding = match flags.bb_padding_log2_minus_one() {
            0 => Vec::new(),
            n => vec![0; 1 << (n - 1)],
        };
        let mut total_bb_padding = 0;

        for (block_order_idx, &block) in final_order.iter().enumerate() {
            trace!("emitting block {:?}", block);

            // Call the new block hook for state
            state.on_new_block();

            // Emit NOPs to align the block.
            let new_offset = I::align_basic_block(buffer.cur_offset());
            while new_offset > buffer.cur_offset() {
                // Pad with NOPs up to the aligned block offset.
                let nop = I::gen_nop((new_offset - buffer.cur_offset()) as usize);
                nop.emit(&mut buffer, &self.emit_info, &mut Default::default());
            }
            assert_eq!(buffer.cur_offset(), new_offset);

            let do_emit = |inst: &I,
                           disasm: &mut String,
                           buffer: &mut MachBuffer<I>,
                           state: &mut I::State| {
                if want_disasm && !inst.is_args() {
                    let mut s = state.clone();
                    writeln!(disasm, "  {}", inst.pretty_print_inst(&mut s)).unwrap();
                }
                inst.emit(buffer, &self.emit_info, state);
            };

            // Is this the first block? Emit the prologue directly if so.
            if block == self.entry {
                trace!(" -> entry block");
                buffer.start_srcloc(Default::default());
                for inst in &self.abi.gen_prologue() {
                    do_emit(&inst, &mut disasm, &mut buffer, &mut state);
                }
                buffer.end_srcloc();
            }

            // Now emit the regular block body.

            buffer.bind_label(MachLabel::from_block(block), state.ctrl_plane_mut());

            if want_disasm {
                writeln!(&mut disasm, "block{}:", block.index()).unwrap();
            }

            if flags.machine_code_cfg_info() {
                // Track BB starts. If we have backed up due to MachBuffer
                // branch opts, note that the removed blocks were removed.
                let cur_offset = buffer.cur_offset();
                if last_offset.is_some() && cur_offset <= last_offset.unwrap() {
                    for i in (0..bb_starts.len()).rev() {
                        if bb_starts[i].is_some() && cur_offset > bb_starts[i].unwrap() {
                            break;
                        }
                        bb_starts[i] = None;
                    }
                }
                bb_starts.push(Some(cur_offset));
                last_offset = Some(cur_offset);
            }

            if let Some(block_start) = I::gen_block_start(
                self.block_order.is_indirect_branch_target(block),
                is_forward_edge_cfi_enabled,
            ) {
                do_emit(&block_start, &mut disasm, &mut buffer, &mut state);
            }

            for inst_or_edit in regalloc.block_insts_and_edits(&self, block) {
                match inst_or_edit {
                    InstOrEdit::Inst(iix) => {
                        if !self.debug_value_labels.is_empty() {
                            // If we need to produce debug info,
                            // record the offset of each instruction
                            // so that we can translate value-label
                            // ranges to machine-code offsets.

                            // Cold blocks violate monotonicity
                            // assumptions elsewhere (that
                            // instructions in inst-index order are in
                            // order in machine code), so we omit
                            // their offsets here. Value-label range
                            // generation below will skip empty ranges
                            // and ranges with to-offsets of zero.
                            if !self.block_order.is_cold(block) {
                                inst_offsets[iix.index()] = buffer.cur_offset();
                            }
                        }

                        // Update the srcloc at this point in the buffer.
                        let srcloc = self.srclocs[iix.index()];
                        if cur_srcloc != Some(srcloc) {
                            if cur_srcloc.is_some() {
                                buffer.end_srcloc();
                            }
                            buffer.start_srcloc(srcloc);
                            cur_srcloc = Some(srcloc);
                        }

                        // If this is a safepoint, compute a stack map
                        // and pass it to the emit state.
                        let stack_map_disasm = if self.insts[iix.index()].is_safepoint() {
                            let (user_stack_map, user_stack_map_disasm) = {
                                // The `user_stack_maps` is keyed by reverse
                                // instruction index, so we must flip the
                                // index. We can't put this into a helper method
                                // due to borrowck issues because parts of
                                // `self` are borrowed mutably elsewhere in this
                                // function.
                                let index = iix.to_backwards_insn_index(self.num_insts());
                                let user_stack_map = self.user_stack_maps.remove(&index);
                                let user_stack_map_disasm = if want_disasm {
                                    user_stack_map.as_ref().map(|m| format!("  ; {m:?}"))
                                } else {
                                    None
                                };
                                (user_stack_map, user_stack_map_disasm)
                            };

                            state.pre_safepoint(user_stack_map);

                            user_stack_map_disasm
                        } else {
                            None
                        };

                        // Place debug tags in the emission buffer
                        // either at the offset prior to the
                        // instruction or after the instruction,
                        // depending on whether this is a call. See
                        // the documentation on [`MachDebugTagPos`]
                        // for details on why.
                        let mut debug_tag_disasm = None;
                        let mut place_debug_tags =
                            |this: &VCode<I>, pos: MachDebugTagPos, buffer: &mut MachBuffer<I>| {
                                // As above, translate the forward instruction
                                // index to a backward index for the lookup.
                                let debug_tag_range = {
                                    let index = iix.to_backwards_insn_index(this.num_insts());
                                    this.debug_tags.get(&index)
                                };
                                if let Some(range) = debug_tag_range {
                                    let start = usize::try_from(range.start).unwrap();
                                    let end = usize::try_from(range.end).unwrap();
                                    let tags = &this.debug_tag_pool[start..end];

                                    if want_disasm {
                                        debug_tag_disasm =
                                            Some(format!("  ; ^-- debug @ {pos:?}: {tags:?}"));
                                    }
                                    buffer.push_debug_tags(pos, tags);
                                }
                            };
                        let debug_tag_pos =
                            if self.insts[iix.index()].call_type() == CallType::Regular {
                                MachDebugTagPos::Post
                            } else {
                                MachDebugTagPos::Pre
                            };

                        if debug_tag_pos == MachDebugTagPos::Pre {
                            place_debug_tags(&self, debug_tag_pos, &mut buffer);
                        }

                        // If the instruction we are about to emit is
                        // a return, place an epilogue at this point
                        // (and don't emit the return; the actual
                        // epilogue will contain it).
                        if self.insts[iix.index()].is_term() == MachTerminator::Ret {
                            log::trace!("emitting epilogue");
                            for inst in self.abi.gen_epilogue() {
                                do_emit(&inst, &mut disasm, &mut buffer, &mut state);
                            }
                        } else {
                            // Update the operands for this inst using the
                            // allocations from the regalloc result.
                            let mut allocs = regalloc.inst_allocs(iix).iter();
                            self.insts[iix.index()].get_operands(
                                &mut |reg: &mut Reg, constraint, _kind, _pos| {
                                    let alloc =
                                        allocs.next().expect("enough allocations for all operands");

                                    if let Some(alloc) = alloc.as_reg() {
                                        let alloc: Reg = alloc.into();
                                        if let OperandConstraint::FixedReg(rreg) = constraint {
                                            debug_assert_eq!(Reg::from(rreg), alloc);
                                        }
                                        *reg = alloc;
                                    } else if let Some(alloc) = alloc.as_stack() {
                                        let alloc: Reg = alloc.into();
                                        *reg = alloc;
                                    }
                                },
                            );
                            debug_assert!(allocs.next().is_none());

                            log::trace!("emitting: {:?}", self.insts[iix.index()]);

                            // Emit the instruction!
                            do_emit(
                                &self.insts[iix.index()],
                                &mut disasm,
                                &mut buffer,
                                &mut state,
                            );

                            if debug_tag_pos == MachDebugTagPos::Post {
                                place_debug_tags(&self, debug_tag_pos, &mut buffer);
                            }

                            if let Some(stack_map_disasm) = stack_map_disasm {
                                disasm.push_str(&stack_map_disasm);
                                disasm.push('\n');
                            }
                            if let Some(debug_tag_disasm) = debug_tag_disasm {
                                disasm.push_str(&debug_tag_disasm);
                                disasm.push('\n');
                            }
                        }
                    }

                    InstOrEdit::Edit(Edit::Move { from, to }) => {
                        // Create a move/spill/reload instruction and
                        // immediately emit it.
                        match (from.as_reg(), to.as_reg()) {
                            (Some(from), Some(to)) => {
                                // Reg-to-reg move.
                                let from_rreg = Reg::from(from);
                                let to_rreg = Writable::from_reg(Reg::from(to));
                                debug_assert_eq!(from.class(), to.class());
                                let ty = I::canonical_type_for_rc(from.class());
                                let mv = I::gen_move(to_rreg, from_rreg, ty);
                                do_emit(&mv, &mut disasm, &mut buffer, &mut state);
                            }
                            (Some(from), None) => {
                                // Spill from register to spillslot.
                                let to = to.as_stack().unwrap();
                                let from_rreg = RealReg::from(from);
                                let spill = self.abi.gen_spill(to, from_rreg);
                                do_emit(&spill, &mut disasm, &mut buffer, &mut state);
                            }
                            (None, Some(to)) => {
                                // Load from spillslot to register.
                                let from = from.as_stack().unwrap();
                                let to_rreg = Writable::from_reg(RealReg::from(to));
                                let reload = self.abi.gen_reload(to_rreg, from);
                                do_emit(&reload, &mut disasm, &mut buffer, &mut state);
                            }
                            (None, None) => {
                                panic!("regalloc2 should have eliminated stack-to-stack moves!");
                            }
                        }
                    }
                }
            }

            if cur_srcloc.is_some() {
                buffer.end_srcloc();
                cur_srcloc = None;
            }

            // Do we need an island? Get the worst-case size of the next BB, add
            // it to the optional padding behind the block, and pass this to the
            // `MachBuffer` to determine if an island is necessary.
            let worst_case_next_bb = if block_order_idx < final_order.len() - 1 {
                let next_block = final_order[block_order_idx + 1];
                let next_block_range = self.block_ranges.get(next_block.index());
                let next_block_size = next_block_range.len() as u32;
                let next_block_ra_insertions = ra_edits_per_block[next_block.index()];
                I::worst_case_size() * (next_block_size + next_block_ra_insertions)
            } else {
                0
            };
            let padding = if bb_padding.is_empty() {
                0
            } else {
                bb_padding.len() as u32 + I::LabelUse::ALIGN - 1
            };
            if buffer.island_needed(padding + worst_case_next_bb) {
                buffer.emit_island(padding + worst_case_next_bb, ctrl_plane);
            }

            // Insert padding, if configured, to stress the `MachBuffer`'s
            // relocation and island calculations.
            //
            // Padding can get quite large during fuzzing though so place a
            // total cap on it where when a per-function threshold is exceeded
            // the padding is turned back down to zero. This avoids a small-ish
            // test case generating a GB+ memory footprint in Cranelift for
            // example.
            if !bb_padding.is_empty() {
                buffer.put_data(&bb_padding);
                buffer.align_to(I::LabelUse::ALIGN);
                total_bb_padding += bb_padding.len();
                if total_bb_padding > (150 << 20) {
                    bb_padding = Vec::new();
                }
            }
        }

        debug_assert!(
            self.user_stack_maps.is_empty(),
            "any stack maps should have been consumed by instruction emission, still have: {:#?}",
            self.user_stack_maps,
        );

        // Do any optimizations on branches at tail of buffer, as if we had
        // bound one last label.
        buffer.optimize_branches(ctrl_plane);

        // emission state is not needed anymore, move control plane back out
        *ctrl_plane = state.take_ctrl_plane();

        let func_body_len = buffer.cur_offset();

        // Create `bb_edges` and final (filtered) `bb_starts`.
        let mut bb_edges = vec![];
        let mut bb_offsets = vec![];
        if flags.machine_code_cfg_info() {
            for block in 0..self.num_blocks() {
                if bb_starts[block].is_none() {
                    // Block was deleted by MachBuffer; skip.
                    continue;
                }
                let from = bb_starts[block].unwrap();

                bb_offsets.push(from);
                // Resolve each `succ` label and add edges.
                let succs = self.block_succs(BlockIndex::new(block));
                for &succ in succs.iter() {
                    let to = buffer.resolve_label_offset(MachLabel::from_block(succ));
                    bb_edges.push((from, to));
                }
            }
        }

        self.monotonize_inst_offsets(&mut inst_offsets[..], func_body_len);
        let value_labels_ranges =
            self.compute_value_labels_ranges(regalloc, &inst_offsets[..], func_body_len);

        // Store metadata about frame layout in the MachBuffer.
        buffer.set_frame_layout(self.abi.frame_slot_metadata());

        EmitResult {
            buffer: buffer.finish(&self.constants, ctrl_plane),
            bb_offsets,
            bb_edges,
            disasm: if want_disasm { Some(disasm) } else { None },
            value_labels_ranges,
        }
    }

    fn monotonize_inst_offsets(&self, inst_offsets: &mut [CodeOffset], func_body_len: u32) {
        if self.debug_value_labels.is_empty() {
            return;
        }

        // During emission, branch removal can make offsets of instructions incorrect.
        // Consider the following sequence: [insi][jmp0][jmp1][jmp2][insj]
        // It will be recorded as (say):    [30]  [34]  [38]  [42]  [<would be 46>]
        // When the jumps get removed we are left with (in "inst_offsets"):
        // [insi][jmp0][jmp1][jmp2][insj][...]
        // [30]  [34]  [38]  [42]  [34]
        // Which violates the monotonicity invariant. This method sets offsets of these
        // removed instructions such as to make them appear zero-sized:
        // [insi][jmp0][jmp1][jmp2][insj][...]
        // [30]  [34]  [34]  [34]  [34]
        //
        let mut next_offset = func_body_len;
        for inst_index in (0..(inst_offsets.len() - 1)).rev() {
            let inst_offset = inst_offsets[inst_index];

            // Not all instructions get their offsets recorded.
            if inst_offset == NO_INST_OFFSET {
                continue;
            }

            if inst_offset > next_offset {
                trace!(
                    "Fixing code offset of the removed Inst {}: {} -> {}",
                    inst_index, inst_offset, next_offset
                );
                inst_offsets[inst_index] = next_offset;
                continue;
            }

            next_offset = inst_offset;
        }
    }

    fn compute_value_labels_ranges(
        &self,
        regalloc: &regalloc2::Output,
        inst_offsets: &[CodeOffset],
        func_body_len: u32,
    ) -> ValueLabelsRanges {
        if self.debug_value_labels.is_empty() {
            return ValueLabelsRanges::default();
        }

        if trace_log_enabled!() {
            self.log_value_labels_ranges(regalloc, inst_offsets);
        }

        let mut value_labels_ranges: ValueLabelsRanges = HashMap::new();
        for &(label, from, to, alloc) in &regalloc.debug_locations {
            let label = ValueLabel::from_u32(label);
            let ranges = value_labels_ranges.entry(label).or_insert_with(|| vec![]);
            let prog_point_to_inst = |prog_point: ProgPoint| {
                let mut inst = prog_point.inst();
                if prog_point.pos() == InstPosition::After {
                    inst = inst.next();
                }
                inst.index()
            };
            let from_inst_index = prog_point_to_inst(from);
            let to_inst_index = prog_point_to_inst(to);
            let from_offset = inst_offsets[from_inst_index];
            let to_offset = if to_inst_index == inst_offsets.len() {
                func_body_len
            } else {
                inst_offsets[to_inst_index]
            };

            // Empty ranges or unavailable offsets can happen
            // due to cold blocks and branch removal (see above).
            if from_offset == NO_INST_OFFSET
                || to_offset == NO_INST_OFFSET
                || from_offset == to_offset
            {
                continue;
            }

            let loc = if let Some(preg) = alloc.as_reg() {
                LabelValueLoc::Reg(Reg::from(preg))
            } else {
                #[cfg(not(feature = "unwind"))]
                continue;

                #[cfg(feature = "unwind")]
                {
                    let slot = alloc.as_stack().unwrap();
                    let slot_offset = self.abi.get_spillslot_offset(slot);
                    let slot_base_to_caller_sp_offset = self.abi.slot_base_to_caller_sp_offset();
                    let caller_sp_to_cfa_offset =
                        crate::isa::unwind::systemv::caller_sp_to_cfa_offset();
                    // NOTE: this is a negative offset because it's relative to the caller's SP
                    let cfa_to_sp_offset =
                        -((slot_base_to_caller_sp_offset + caller_sp_to_cfa_offset) as i64);
                    LabelValueLoc::CFAOffset(cfa_to_sp_offset + slot_offset)
                }
            };

            // Coalesce adjacent ranges that for the same location
            // to minimize output size here and for the consumers.
            if let Some(last_loc_range) = ranges.last_mut() {
                if last_loc_range.loc == loc && last_loc_range.end == from_offset {
                    trace!(
                        "Extending debug range for {:?} in {:?} to Inst {} ({})",
                        label, loc, to_inst_index, to_offset
                    );
                    last_loc_range.end = to_offset;
                    continue;
                }
            }

            trace!(
                "Recording debug range for {:?} in {:?}: [Inst {}..Inst {}) [{}..{})",
                label, loc, from_inst_index, to_inst_index, from_offset, to_offset
            );

            ranges.push(ValueLocRange {
                loc,
                start: from_offset,
                end: to_offset,
            });
        }

        value_labels_ranges
    }

    fn log_value_labels_ranges(&self, regalloc: &regalloc2::Output, inst_offsets: &[CodeOffset]) {
        debug_assert!(trace_log_enabled!());

        // What debug labels do we have? Note we'll skip those that have not been
        // allocated any location at all. They will show up as numeric gaps in the table.
        let mut labels = vec![];
        for &(label, _, _, _) in &regalloc.debug_locations {
            if Some(&label) == labels.last() {
                continue;
            }
            labels.push(label);
        }

        // Reformat the data on what VRegs were the VLs assigned to by lowering, since
        // the array we have is sorted by VReg, and we want it sorted by VL for easy
        // access in the loop below.
        let mut vregs = vec![];
        for &(vreg, start, end, label) in &self.debug_value_labels {
            if matches!(labels.binary_search(&label), Ok(_)) {
                vregs.push((label, start, end, vreg));
            }
        }
        vregs.sort_unstable_by(
            |(l_label, l_start, _, _), (r_label, r_start, _, _)| match l_label.cmp(r_label) {
                Ordering::Equal => l_start.cmp(r_start),
                cmp => cmp,
            },
        );

        #[derive(PartialEq)]
        enum Mode {
            Measure,
            Emit,
        }
        #[derive(PartialEq)]
        enum Row {
            Head,
            Line,
            Inst(usize, usize),
        }

        let mut widths = vec![0; 3 + 2 * labels.len()];
        let mut row = String::new();
        let mut output_row = |row_kind: Row, mode: Mode| {
            let mut column_index = 0;
            row.clear();

            macro_rules! output_cell_impl {
                ($fill:literal, $span:literal, $($cell_fmt:tt)*) => {
                    let column_start = row.len();
                    {
                        row.push('|');
                        write!(row, $($cell_fmt)*).unwrap();
                    }

                    let next_column_index = column_index + $span;
                    let expected_width: usize = widths[column_index..next_column_index].iter().sum();
                    if mode == Mode::Measure {
                        let actual_width = row.len() - column_start;
                        if actual_width > expected_width {
                            widths[next_column_index - 1] += actual_width - expected_width;
                        }
                    } else {
                        let column_end = column_start + expected_width;
                        while row.len() != column_end {
                            row.push($fill);
                        }
                    }
                    column_index = next_column_index;
                };
            }
            macro_rules! output_cell {
                ($($cell_fmt:tt)*) => {
                    output_cell_impl!(' ', 1, $($cell_fmt)*);
                };
            }

            match row_kind {
                Row::Head => {
                    output_cell!("BB");
                    output_cell!("Inst");
                    output_cell!("IP");
                    for label in &labels {
                        output_cell_impl!(' ', 2, "{:?}", ValueLabel::from_u32(*label));
                    }
                }
                Row::Line => {
                    debug_assert!(mode == Mode::Emit);
                    for _ in 0..3 {
                        output_cell_impl!('-', 1, "");
                    }
                    for _ in &labels {
                        output_cell_impl!('-', 2, "");
                    }
                }
                Row::Inst(block_index, inst_index) => {
                    debug_assert!(inst_index < self.num_insts());
                    if self.block_ranges.get(block_index).start == inst_index {
                        output_cell!("B{}", block_index);
                    } else {
                        output_cell!("");
                    }
                    output_cell!("Inst {inst_index} ");
                    output_cell!("{} ", inst_offsets[inst_index]);

                    for label in &labels {
                        // First, the VReg.
                        use regalloc2::Inst;
                        let vreg_cmp = |inst: usize,
                                        vreg_label: &u32,
                                        range_start: &Inst,
                                        range_end: &Inst| {
                            match vreg_label.cmp(&label) {
                                Ordering::Equal => {
                                    if range_end.index() <= inst {
                                        Ordering::Less
                                    } else if range_start.index() > inst {
                                        Ordering::Greater
                                    } else {
                                        Ordering::Equal
                                    }
                                }
                                cmp => cmp,
                            }
                        };
                        let vreg_index =
                            vregs.binary_search_by(|(l, s, e, _)| vreg_cmp(inst_index, l, s, e));
                        if let Ok(vreg_index) = vreg_index {
                            let mut prev_vreg = None;
                            if inst_index > 0 {
                                let prev_vreg_index = vregs.binary_search_by(|(l, s, e, _)| {
                                    vreg_cmp(inst_index - 1, l, s, e)
                                });
                                if let Ok(prev_vreg_index) = prev_vreg_index {
                                    prev_vreg = Some(vregs[prev_vreg_index].3);
                                }
                            }

                            let vreg = vregs[vreg_index].3;
                            if Some(vreg) == prev_vreg {
                                output_cell!("*");
                            } else {
                                output_cell!("{}", vreg);
                            }
                        } else {
                            output_cell!("");
                        }

                        // Second, the allocated location.
                        let inst_prog_point = ProgPoint::before(Inst::new(inst_index));
                        let range_index = regalloc.debug_locations.binary_search_by(
                            |(range_label, range_start, range_end, _)| match range_label.cmp(label)
                            {
                                Ordering::Equal => {
                                    if *range_end <= inst_prog_point {
                                        Ordering::Less
                                    } else if *range_start > inst_prog_point {
                                        Ordering::Greater
                                    } else {
                                        Ordering::Equal
                                    }
                                }
                                cmp => cmp,
                            },
                        );
                        if let Ok(range_index) = range_index {
                            // Live at this instruction, print the location.
                            if let Some(reg) = regalloc.debug_locations[range_index].3.as_reg() {
                                output_cell!("{:?}", Reg::from(reg));
                            } else {
                                output_cell!("Stk");
                            }
                        } else {
                            // Not live at this instruction.
                            output_cell!("");
                        }
                    }
                }
            }
            row.push('|');

            if mode == Mode::Emit {
                trace!("{}", row.as_str());
            }
        };

        for block_index in 0..self.num_blocks() {
            for inst_index in self.block_ranges.get(block_index) {
                output_row(Row::Inst(block_index, inst_index), Mode::Measure);
            }
        }
        output_row(Row::Head, Mode::Measure);

        output_row(Row::Head, Mode::Emit);
        output_row(Row::Line, Mode::Emit);
        for block_index in 0..self.num_blocks() {
            for inst_index in self.block_ranges.get(block_index) {
                output_row(Row::Inst(block_index, inst_index), Mode::Emit);
            }
        }
    }

    /// Get the IR block for a BlockIndex, if one exists.
    pub fn bindex_to_bb(&self, block: BlockIndex) -> Option<ir::Block> {
        self.block_order.lowered_order()[block.index()].orig_block()
    }

    /// Get the type of a VReg.
    pub fn vreg_type(&self, vreg: VReg) -> Type {
        self.vreg_types[vreg.vreg()]
    }

    /// Get the fact, if any, for a given VReg.
    pub fn vreg_fact(&self, vreg: VReg) -> Option<&Fact> {
        self.facts[vreg.vreg()].as_ref()
    }

    /// Set the fact for a given VReg.
    pub fn set_vreg_fact(&mut self, vreg: VReg, fact: Fact) {
        trace!("set fact on {}: {:?}", vreg, fact);
        self.facts[vreg.vreg()] = Some(fact);
    }

    /// Does a given instruction define any facts?
    pub fn inst_defines_facts(&self, inst: InsnIndex) -> bool {
        self.inst_operands(inst)
            .iter()
            .filter(|o| o.kind() == OperandKind::Def)
            .map(|o| o.vreg())
            .any(|vreg| self.facts[vreg.vreg()].is_some())
    }

    /// Get the user stack map associated with the given forward instruction index.
    pub fn get_user_stack_map(&self, inst: InsnIndex) -> Option<&ir::UserStackMap> {
        let index = inst.to_backwards_insn_index(self.num_insts());
        self.user_stack_maps.get(&index)
    }
}

impl<I: VCodeInst> core::ops::Index<InsnIndex> for VCode<I> {
    type Output = I;
    fn index(&self, idx: InsnIndex) -> &Self::Output {
        &self.insts[idx.index()]
    }
}

impl<I: VCodeInst> RegallocFunction for VCode<I> {
    fn num_insts(&self) -> usize {
        self.insts.len()
    }

    fn num_blocks(&self) -> usize {
        self.block_ranges.len()
    }

    fn entry_block(&self) -> BlockIndex {
        self.entry
    }

    fn block_insns(&self, block: BlockIndex) -> InstRange {
        let range = self.block_ranges.get(block.index());
        InstRange::new(InsnIndex::new(range.start), InsnIndex::new(range.end))
    }

    fn block_succs(&self, block: BlockIndex) -> &[BlockIndex] {
        let range = self.block_succ_range.get(block.index());
        &self.block_succs[range]
    }

    fn block_preds(&self, block: BlockIndex) -> &[BlockIndex] {
        let range = self.block_pred_range.get(block.index());
        &self.block_preds[range]
    }

    fn block_params(&self, block: BlockIndex) -> &[VReg] {
        // As a special case we don't return block params for the entry block, as all the arguments
        // will be defined by the `Inst::Args` instruction.
        if block == self.entry {
            return &[];
        }

        let range = self.block_params_range.get(block.index());
        &self.block_params[range]
    }

    fn branch_blockparams(&self, block: BlockIndex, _insn: InsnIndex, succ_idx: usize) -> &[VReg] {
        let succ_range = self.branch_block_arg_succ_range.get(block.index());
        debug_assert!(succ_idx < succ_range.len());
        let branch_block_args = self.branch_block_arg_range.get(succ_range.start + succ_idx);
        &self.branch_block_args[branch_block_args]
    }

    fn is_ret(&self, insn: InsnIndex) -> bool {
        match self.insts[insn.index()].is_term() {
            // We treat blocks terminated by an unconditional trap like a return for regalloc.
            MachTerminator::None => self.insts[insn.index()].is_trap(),
            MachTerminator::Ret | MachTerminator::RetCall => true,
            MachTerminator::Branch => false,
        }
    }

    fn is_branch(&self, insn: InsnIndex) -> bool {
        match self.insts[insn.index()].is_term() {
            MachTerminator::Branch => true,
            _ => false,
        }
    }

    fn inst_operands(&self, insn: InsnIndex) -> &[Operand] {
        let range = self.operand_ranges.get(insn.index());
        &self.operands[range]
    }

    fn inst_clobbers(&self, insn: InsnIndex) -> PRegSet {
        self.clobbers.get(&insn).cloned().unwrap_or_default()
    }

    fn num_vregs(&self) -> usize {
        self.vreg_types.len()
    }

    fn debug_value_labels(&self) -> &[(VReg, InsnIndex, InsnIndex, u32)] {
        &self.debug_value_labels
    }

    fn spillslot_size(&self, regclass: RegClass) -> usize {
        self.abi.get_spillslot_size(regclass) as usize
    }

    fn allow_multiple_vreg_defs(&self) -> bool {
        // At least the s390x backend requires this, because the
        // `Loop` pseudo-instruction aggregates all Operands so pinned
        // vregs (RealRegs) may occur more than once.
        true
    }
}

impl<I: VCodeInst> Debug for VRegAllocator<I> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        writeln!(f, "VRegAllocator {{")?;

        let mut alias_keys = self.vreg_aliases.keys().cloned().collect::<Vec<_>>();
        alias_keys.sort_unstable();
        for key in alias_keys {
            let dest = self.vreg_aliases.get(&key).unwrap();
            writeln!(f, "  {:?} := {:?}", Reg::from(key), Reg::from(*dest))?;
        }

        for (vreg, fact) in self.facts.iter().enumerate() {
            if let Some(fact) = fact {
                writeln!(f, "  v{vreg} ! {fact}")?;
            }
        }

        writeln!(f, "}}")
    }
}

impl<I: VCodeInst> fmt::Debug for VCode<I> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        writeln!(f, "VCode {{")?;
        writeln!(f, "  Entry block: {}", self.entry.index())?;

        let mut state = Default::default();

        for block in 0..self.num_blocks() {
            let block = BlockIndex::new(block);
            writeln!(
                f,
                "Block {}({:?}):",
                block.index(),
                self.block_params(block)
            )?;
            if let Some(bb) = self.bindex_to_bb(block) {
                writeln!(f, "    (original IR block: {bb})")?;
            }
            for (succ_idx, succ) in self.block_succs(block).iter().enumerate() {
                writeln!(
                    f,
                    "    (successor: Block {}({:?}))",
                    succ.index(),
                    self.branch_blockparams(block, InsnIndex::new(0) /* dummy */, succ_idx)
                )?;
            }
            for inst in self.block_ranges.get(block.index()) {
                writeln!(
                    f,
                    "  Inst {}: {}",
                    inst,
                    self.insts[inst].pretty_print_inst(&mut state)
                )?;
                if !self.operands.is_empty() {
                    for operand in self.inst_operands(InsnIndex::new(inst)) {
                        if operand.kind() == OperandKind::Def {
                            if let Some(fact) = &self.facts[operand.vreg().vreg()] {
                                writeln!(f, "    v{} ! {}", operand.vreg().vreg(), fact)?;
                            }
                        }
                    }
                }
                if let Some(user_stack_map) = self.get_user_stack_map(InsnIndex::new(inst)) {
                    writeln!(f, "    {user_stack_map:?}")?;
                }
            }
        }

        writeln!(f, "}}")?;
        Ok(())
    }
}

/// This structure manages VReg allocation during the lifetime of the VCodeBuilder.
pub struct VRegAllocator<I> {
    /// VReg IR-level types.
    vreg_types: Vec<Type>,

    /// VReg aliases. When the final VCode is built we rewrite all
    /// uses of the keys in this table to their replacement values.
    ///
    /// We use these aliases to rename an instruction's expected
    /// result vregs to the returned vregs from lowering, which are
    /// usually freshly-allocated temps.
    vreg_aliases: FxHashMap<regalloc2::VReg, regalloc2::VReg>,

    /// A deferred error, to be bubbled up to the top level of the
    /// lowering algorithm. We take this approach because we cannot
    /// currently propagate a `Result` upward through ISLE code (the
    /// lowering rules) or some ABI code.
    deferred_error: Option<CodegenError>,

    /// Facts on VRegs, for proof-carrying code.
    facts: Vec<Option<Fact>>,

    /// The type of instruction that this allocator makes registers for.
    _inst: core::marker::PhantomData<I>,
}

impl<I: VCodeInst> VRegAllocator<I> {
    /// Make a new VRegAllocator.
    pub fn with_capacity(capacity: usize) -> Self {
        let capacity = first_user_vreg_index() + capacity;
        let mut vreg_types = Vec::with_capacity(capacity);
        vreg_types.resize(first_user_vreg_index(), types::INVALID);
        Self {
            vreg_types,
            vreg_aliases: FxHashMap::with_capacity_and_hasher(capacity, Default::default()),
            deferred_error: None,
            facts: Vec::with_capacity(capacity),
            _inst: core::marker::PhantomData::default(),
        }
    }

    /// Allocate a fresh ValueRegs.
    pub fn alloc(&mut self, ty: Type) -> CodegenResult<ValueRegs<Reg>> {
        if self.deferred_error.is_some() {
            return Err(CodegenError::CodeTooLarge);
        }
        let v = self.vreg_types.len();
        let (regclasses, tys) = I::rc_for_type(ty)?;
        if v + regclasses.len() >= VReg::MAX {
            return Err(CodegenError::CodeTooLarge);
        }

        let regs: ValueRegs<Reg> = match regclasses {
            &[rc0] => ValueRegs::one(VReg::new(v, rc0).into()),
            &[rc0, rc1] => ValueRegs::two(VReg::new(v, rc0).into(), VReg::new(v + 1, rc1).into()),
            // We can extend this if/when we support 32-bit targets; e.g.,
            // an i128 on a 32-bit machine will need up to four machine regs
            // for a `Value`.
            _ => panic!("Value must reside in 1 or 2 registers"),
        };
        for (&reg_ty, &reg) in tys.iter().zip(regs.regs().iter()) {
            let vreg = reg.to_virtual_reg().unwrap();
            debug_assert_eq!(self.vreg_types.len(), vreg.index());
            self.vreg_types.push(reg_ty);
        }

        // Create empty facts for each allocated vreg.
        self.facts.resize(self.vreg_types.len(), None);

        Ok(regs)
    }

    /// Allocate a fresh ValueRegs, deferring any out-of-vregs
    /// errors. This is useful in places where we cannot bubble a
    /// `CodegenResult` upward easily, and which are known to be
    /// invoked from within the lowering loop that checks the deferred
    /// error status below.
    pub fn alloc_with_deferred_error(&mut self, ty: Type) -> ValueRegs<Reg> {
        match self.alloc(ty) {
            Ok(x) => x,
            Err(e) => {
                self.deferred_error = Some(e);
                self.bogus_for_deferred_error(ty)
            }
        }
    }

    /// Take any deferred error that was accumulated by `alloc_with_deferred_error`.
    pub fn take_deferred_error(&mut self) -> Option<CodegenError> {
        self.deferred_error.take()
    }

    /// Produce an bogus VReg placeholder with the proper number of
    /// registers for the given type. This is meant to be used with
    /// deferred allocation errors (see `Lower::alloc_tmp()`).
    fn bogus_for_deferred_error(&self, ty: Type) -> ValueRegs<Reg> {
        let (regclasses, _tys) = I::rc_for_type(ty).expect("must have valid type");
        match regclasses {
            &[rc0] => ValueRegs::one(VReg::new(0, rc0).into()),
            &[rc0, rc1] => ValueRegs::two(VReg::new(0, rc0).into(), VReg::new(1, rc1).into()),
            _ => panic!("Value must reside in 1 or 2 registers"),
        }
    }

    /// Rewrite any mention of `from` into `to`.
    pub fn set_vreg_alias(&mut self, from: Reg, to: Reg) {
        let from = from.into();
        let resolved_to = self.resolve_vreg_alias(to.into());
        // Disallow cycles (see below).
        assert_ne!(resolved_to, from);

        // Maintain the invariant that PCC facts only exist on vregs
        // which aren't aliases. We want to preserve whatever was
        // stated about the vreg before its producer was lowered.
        if let Some(fact) = self.facts[from.vreg()].take() {
            self.set_fact(resolved_to, fact);
        }

        let old_alias = self.vreg_aliases.insert(from, resolved_to);
        debug_assert_eq!(old_alias, None);
    }

    fn resolve_vreg_alias(&self, mut vreg: regalloc2::VReg) -> regalloc2::VReg {
        // We prevent cycles from existing by resolving targets of
        // aliases eagerly before setting them. If the target resolves
        // to the origin of the alias, then a cycle would be created
        // and the alias is disallowed. Because of the structure of
        // SSA code (one instruction can refer to another's defs but
        // not vice-versa, except indirectly through
        // phis/blockparams), cycles should not occur as we use
        // aliases to redirect vregs to the temps that actually define
        // them.
        while let Some(to) = self.vreg_aliases.get(&vreg) {
            vreg = *to;
        }
        vreg
    }

    #[inline]
    fn debug_assert_no_vreg_aliases(&self, mut list: impl Iterator<Item = VReg>) {
        debug_assert!(list.all(|vreg| !self.vreg_aliases.contains_key(&vreg)));
    }

    /// Set the proof-carrying code fact on a given virtual register.
    ///
    /// Returns the old fact, if any (only one fact can be stored).
    fn set_fact(&mut self, vreg: regalloc2::VReg, fact: Fact) -> Option<Fact> {
        trace!("vreg {:?} has fact: {:?}", vreg, fact);
        debug_assert!(!self.vreg_aliases.contains_key(&vreg));
        self.facts[vreg.vreg()].replace(fact)
    }

    /// Set a fact only if one doesn't already exist.
    pub fn set_fact_if_missing(&mut self, vreg: VirtualReg, fact: Fact) {
        let vreg = self.resolve_vreg_alias(vreg.into());
        if self.facts[vreg.vreg()].is_none() {
            self.set_fact(vreg, fact);
        }
    }

    /// Allocate a fresh ValueRegs, with a given fact to apply if
    /// the value fits in one VReg.
    pub fn alloc_with_maybe_fact(
        &mut self,
        ty: Type,
        fact: Option<Fact>,
    ) -> CodegenResult<ValueRegs<Reg>> {
        let result = self.alloc(ty)?;

        // Ensure that we don't lose a fact on a value that splits
        // into multiple VRegs.
        assert!(result.len() == 1 || fact.is_none());
        if let Some(fact) = fact {
            self.set_fact(result.regs()[0].into(), fact);
        }

        Ok(result)
    }
}

/// This structure tracks the large constants used in VCode that will be emitted separately by the
/// [MachBuffer].
///
/// First, during the lowering phase, constants are inserted using
/// [VCodeConstants.insert]; an intermediate handle, `VCodeConstant`, tracks what constants are
/// used in this phase. Some deduplication is performed, when possible, as constant
/// values are inserted.
///
/// Secondly, during the emission phase, the [MachBuffer] assigns [MachLabel]s for each of the
/// constants so that instructions can refer to the value's memory location. The [MachBuffer]
/// then writes the constant values to the buffer.
#[derive(Default)]
pub struct VCodeConstants {
    constants: PrimaryMap<VCodeConstant, VCodeConstantData>,
    pool_uses: HashMap<Constant, VCodeConstant>,
    well_known_uses: HashMap<*const [u8], VCodeConstant>,
    u64s: HashMap<[u8; 8], VCodeConstant>,
}
impl VCodeConstants {
    /// Initialize the structure with the expected number of constants.
    pub fn with_capacity(expected_num_constants: usize) -> Self {
        Self {
            constants: PrimaryMap::with_capacity(expected_num_constants),
            pool_uses: HashMap::with_capacity(expected_num_constants),
            well_known_uses: HashMap::new(),
            u64s: HashMap::new(),
        }
    }

    /// Insert a constant; using this method indicates that a constant value will be used and thus
    /// will be emitted to the `MachBuffer`. The current implementation can deduplicate constants
    /// that are [VCodeConstantData::Pool] or [VCodeConstantData::WellKnown] but not
    /// [VCodeConstantData::Generated].
    pub fn insert(&mut self, data: VCodeConstantData) -> VCodeConstant {
        match data {
            VCodeConstantData::Generated(_) => self.constants.push(data),
            VCodeConstantData::Pool(constant, _) => match self.pool_uses.get(&constant) {
                None => {
                    let vcode_constant = self.constants.push(data);
                    self.pool_uses.insert(constant, vcode_constant);
                    vcode_constant
                }
                Some(&vcode_constant) => vcode_constant,
            },
            VCodeConstantData::WellKnown(data_ref) => {
                match self.well_known_uses.entry(data_ref as *const [u8]) {
                    Entry::Vacant(v) => {
                        let vcode_constant = self.constants.push(data);
                        v.insert(vcode_constant);
                        vcode_constant
                    }
                    Entry::Occupied(o) => *o.get(),
                }
            }
            VCodeConstantData::U64(value) => match self.u64s.entry(value) {
                Entry::Vacant(v) => {
                    let vcode_constant = self.constants.push(data);
                    v.insert(vcode_constant);
                    vcode_constant
                }
                Entry::Occupied(o) => *o.get(),
            },
        }
    }

    /// Return the number of constants inserted.
    pub fn len(&self) -> usize {
        self.constants.len()
    }

    /// Iterate over the `VCodeConstant` keys inserted in this structure.
    pub fn keys(&self) -> Keys<VCodeConstant> {
        self.constants.keys()
    }

    /// Iterate over the `VCodeConstant` keys and the data (as a byte slice) inserted in this
    /// structure.
    pub fn iter(&self) -> impl Iterator<Item = (VCodeConstant, &VCodeConstantData)> {
        self.constants.iter()
    }

    /// Returns the data associated with the specified constant.
    pub fn get(&self, c: VCodeConstant) -> &VCodeConstantData {
        &self.constants[c]
    }

    /// Checks if the given [VCodeConstantData] is registered as
    /// used by the pool.
    pub fn pool_uses(&self, constant: &VCodeConstantData) -> bool {
        match constant {
            VCodeConstantData::Pool(c, _) => self.pool_uses.contains_key(c),
            _ => false,
        }
    }
}

/// A use of a constant by one or more VCode instructions; see [VCodeConstants].
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct VCodeConstant(u32);
entity_impl!(VCodeConstant);

/// Identify the different types of constant that can be inserted into [VCodeConstants]. Tracking
/// these separately instead of as raw byte buffers allows us to avoid some duplication.
pub enum VCodeConstantData {
    /// A constant already present in the Cranelift IR
    /// [ConstantPool](crate::ir::constant::ConstantPool).
    Pool(Constant, ConstantData),
    /// A reference to a well-known constant value that is statically encoded within the compiler.
    WellKnown(&'static [u8]),
    /// A constant value generated during lowering; the value may depend on the instruction context
    /// which makes it difficult to de-duplicate--if possible, use other variants.
    Generated(ConstantData),
    /// A constant of at most 64 bits. These are deduplicated as
    /// well. Stored as a fixed-size array of `u8` so that we do not
    /// encounter endianness problems when cross-compiling.
    U64([u8; 8]),
}
impl VCodeConstantData {
    /// Retrieve the constant data as a byte slice.
    pub fn as_slice(&self) -> &[u8] {
        match self {
            VCodeConstantData::Pool(_, d) | VCodeConstantData::Generated(d) => d.as_slice(),
            VCodeConstantData::WellKnown(d) => d,
            VCodeConstantData::U64(value) => &value[..],
        }
    }

    /// Calculate the alignment of the constant data.
    pub fn alignment(&self) -> u32 {
        if self.as_slice().len() <= 8 { 8 } else { 16 }
    }
}

#[cfg(test)]
mod test {
    use super::*;
    use core::mem::size_of;

    #[test]
    fn size_of_constant_structs() {
        assert_eq!(size_of::<Constant>(), 4);
        assert_eq!(size_of::<VCodeConstant>(), 4);
        assert_eq!(size_of::<ConstantData>(), 3 * size_of::<usize>());
        assert_eq!(size_of::<VCodeConstantData>(), 4 * size_of::<usize>());
        assert_eq!(
            size_of::<PrimaryMap<VCodeConstant, VCodeConstantData>>(),
            3 * size_of::<usize>()
        );
        // TODO The VCodeConstants structure's memory size could be further optimized.
        // With certain versions of Rust, each `HashMap` in `VCodeConstants` occupied at
        // least 48 bytes, making an empty `VCodeConstants` cost 120 bytes.
    }
}