1 //! Instruction predicates/properties, shared by various analyses. 2 use crate::ir::immediates::Offset32; 3 use crate::ir::{self, Block, Function, Inst, InstructionData, Opcode, Type, Value}; 4 5 /// Test whether the given opcode is unsafe to even consider as side-effect-free. 6 #[inline(always)] 7 fn trivially_has_side_effects(opcode: Opcode) -> bool { 8 opcode.is_call() 9 || opcode.is_branch() 10 || opcode.is_terminator() 11 || opcode.is_return() 12 || opcode.can_trap() 13 || opcode.other_side_effects() 14 || opcode.can_store() 15 } 16 17 /// Load instructions without the `notrap` flag are defined to trap when 18 /// operating on inaccessible memory, so we can't treat them as side-effect-free even if the loaded 19 /// value is unused. 20 #[inline(always)] 21 fn is_load_with_defined_trapping(opcode: Opcode, data: &InstructionData) -> bool { 22 if !opcode.can_load() { 23 return false; 24 } 25 match *data { 26 InstructionData::StackLoad { .. } => false, 27 InstructionData::Load { flags, .. } => !flags.notrap(), 28 _ => true, 29 } 30 } 31 32 /// Does the given instruction have any side-effect that would preclude it from being removed when 33 /// its value is unused? 34 #[inline(always)] 35 fn has_side_effect(func: &Function, inst: Inst) -> bool { 36 let data = &func.dfg.insts[inst]; 37 let opcode = data.opcode(); 38 trivially_has_side_effects(opcode) || is_load_with_defined_trapping(opcode, data) 39 } 40 41 /// Does the given instruction behave as a "pure" node with respect to 42 /// aegraph semantics? 43 /// 44 /// - Actual pure nodes (arithmetic, etc) 45 /// - Loads with the `readonly` flag set 46 pub fn is_pure_for_egraph(func: &Function, inst: Inst) -> bool { 47 let is_readonly_load = match func.dfg.insts[inst] { 48 InstructionData::Load { 49 opcode: Opcode::Load, 50 flags, 51 .. 52 } => flags.readonly() && flags.notrap(), 53 _ => false, 54 }; 55 56 // Multi-value results do not play nicely with much of the egraph 57 // infrastructure. They are in practice used only for multi-return 58 // calls and some other odd instructions (e.g. uadd_overflow) which, 59 // for now, we can afford to leave in place as opaque 60 // side-effecting ops. So if more than one result, then the inst 61 // is "not pure". Similarly, ops with zero results can be used 62 // only for their side-effects, so are never pure. (Or if they 63 // are, we can always trivially eliminate them with no effect.) 64 let has_one_result = func.dfg.inst_results(inst).len() == 1; 65 66 let op = func.dfg.insts[inst].opcode(); 67 68 has_one_result && (is_readonly_load || (!op.can_load() && !trivially_has_side_effects(op))) 69 } 70 71 /// Can the given instruction be merged into another copy of itself? 72 /// These instructions may have side-effects, but as long as we retain 73 /// the first instance of the instruction, the second and further 74 /// instances are redundant if they would produce the same trap or 75 /// result. 76 pub fn is_mergeable_for_egraph(func: &Function, inst: Inst) -> bool { 77 let op = func.dfg.insts[inst].opcode(); 78 // We can only merge one-result operators due to the way that GVN 79 // is structured in the egraph implementation. 80 let has_one_result = func.dfg.inst_results(inst).len() == 1; 81 has_one_result 82 // Loads/stores are handled by alias analysis and not 83 // otherwise mergeable. 84 && !op.can_load() 85 && !op.can_store() 86 // Can only have idempotent side-effects. 87 && (!has_side_effect(func, inst) || op.side_effects_idempotent()) 88 } 89 90 /// Does the given instruction have any side-effect as per [has_side_effect], or else is a load, 91 /// but not the get_pinned_reg opcode? 92 pub fn has_lowering_side_effect(func: &Function, inst: Inst) -> bool { 93 let op = func.dfg.insts[inst].opcode(); 94 op != Opcode::GetPinnedReg && (has_side_effect(func, inst) || op.can_load()) 95 } 96 97 /// Is the given instruction a constant value (`iconst`, `fconst`) that can be 98 /// represented in 64 bits? 99 pub fn is_constant_64bit(func: &Function, inst: Inst) -> Option<u64> { 100 match &func.dfg.insts[inst] { 101 &InstructionData::UnaryImm { imm, .. } => Some(imm.bits() as u64), 102 &InstructionData::UnaryIeee16 { imm, .. } => Some(imm.bits() as u64), 103 &InstructionData::UnaryIeee32 { imm, .. } => Some(imm.bits() as u64), 104 &InstructionData::UnaryIeee64 { imm, .. } => Some(imm.bits()), 105 _ => None, 106 } 107 } 108 109 /// Get the address, offset, and access type from the given instruction, if any. 110 pub fn inst_addr_offset_type(func: &Function, inst: Inst) -> Option<(Value, Offset32, Type)> { 111 match &func.dfg.insts[inst] { 112 InstructionData::Load { arg, offset, .. } => { 113 let ty = func.dfg.value_type(func.dfg.inst_results(inst)[0]); 114 Some((*arg, *offset, ty)) 115 } 116 InstructionData::LoadNoOffset { arg, .. } => { 117 let ty = func.dfg.value_type(func.dfg.inst_results(inst)[0]); 118 Some((*arg, 0.into(), ty)) 119 } 120 InstructionData::Store { args, offset, .. } => { 121 let ty = func.dfg.value_type(args[0]); 122 Some((args[1], *offset, ty)) 123 } 124 InstructionData::StoreNoOffset { args, .. } => { 125 let ty = func.dfg.value_type(args[0]); 126 Some((args[1], 0.into(), ty)) 127 } 128 _ => None, 129 } 130 } 131 132 /// Get the store data, if any, from an instruction. 133 pub fn inst_store_data(func: &Function, inst: Inst) -> Option<Value> { 134 match &func.dfg.insts[inst] { 135 InstructionData::Store { args, .. } | InstructionData::StoreNoOffset { args, .. } => { 136 Some(args[0]) 137 } 138 _ => None, 139 } 140 } 141 142 /// Determine whether this opcode behaves as a memory fence, i.e., 143 /// prohibits any moving of memory accesses across it. 144 pub fn has_memory_fence_semantics(op: Opcode) -> bool { 145 match op { 146 Opcode::AtomicRmw 147 | Opcode::AtomicCas 148 | Opcode::AtomicLoad 149 | Opcode::AtomicStore 150 | Opcode::Fence 151 | Opcode::Debugtrap => true, 152 Opcode::Call | Opcode::CallIndirect => true, 153 op if op.can_trap() => true, 154 _ => false, 155 } 156 } 157 158 /// Visit all successors of a block with a given visitor closure. The closure 159 /// arguments are the branch instruction that is used to reach the successor, 160 /// the successor block itself, and a flag indicating whether the block is 161 /// branched to via a table entry. 162 pub(crate) fn visit_block_succs<F: FnMut(Inst, Block, bool)>( 163 f: &Function, 164 block: Block, 165 mut visit: F, 166 ) { 167 if let Some(inst) = f.layout.last_inst(block) { 168 match &f.dfg.insts[inst] { 169 ir::InstructionData::Jump { 170 destination: dest, .. 171 } => { 172 visit(inst, dest.block(&f.dfg.value_lists), false); 173 } 174 175 ir::InstructionData::Brif { 176 blocks: [block_then, block_else], 177 .. 178 } => { 179 visit(inst, block_then.block(&f.dfg.value_lists), false); 180 visit(inst, block_else.block(&f.dfg.value_lists), false); 181 } 182 183 ir::InstructionData::BranchTable { table, .. } => { 184 let pool = &f.dfg.value_lists; 185 let table = &f.stencil.dfg.jump_tables[*table]; 186 187 // The default block is reached via a direct conditional branch, 188 // so it is not part of the table. We visit the default block 189 // first explicitly, to mirror the traversal order of 190 // `JumpTableData::all_branches`, and transitively the order of 191 // `InstructionData::branch_destination`. 192 // 193 // Additionally, this case is why we are unable to replace this 194 // whole function with a loop over `branch_destination`: we need 195 // to report which branch targets come from the table vs the 196 // default. 197 visit(inst, table.default_block().block(pool), false); 198 199 for dest in table.as_slice() { 200 visit(inst, dest.block(pool), true); 201 } 202 } 203 204 inst => debug_assert!(!inst.opcode().is_branch()), 205 } 206 } 207 } 208