1 //! Unreachable code elimination.
2 
3 use cranelift_entity::EntitySet;
4 
5 use crate::cursor::{Cursor, FuncCursor};
6 use crate::dominator_tree::DominatorTree;
7 use crate::flowgraph::ControlFlowGraph;
8 use crate::timing;
9 use crate::{ir, trace};
10 
11 /// Eliminate unreachable code.
12 ///
13 /// This pass deletes whole blocks that can't be reached from the entry block. It does not delete
14 /// individual instructions whose results are unused.
15 ///
16 /// The reachability analysis is performed by the dominator tree analysis.
17 pub fn eliminate_unreachable_code(
18     func: &mut ir::Function,
19     cfg: &mut ControlFlowGraph,
20     domtree: &DominatorTree,
21 ) {
22     let _tt = timing::unreachable_code();
23     let mut pos = FuncCursor::new(func);
24     let mut used_tables = EntitySet::with_capacity(pos.func.stencil.dfg.jump_tables.len());
25     let mut used_exception_tables =
26         EntitySet::with_capacity(pos.func.stencil.dfg.exception_tables.len());
27     while let Some(block) = pos.next_block() {
28         if domtree.is_reachable(block) {
29             let inst = pos.func.layout.last_inst(block).unwrap();
30             match pos.func.dfg.insts[inst] {
31                 ir::InstructionData::BranchTable { table, .. } => {
32                     used_tables.insert(table);
33                 }
34                 ir::InstructionData::TryCall { exception, .. }
35                 | ir::InstructionData::TryCallIndirect { exception, .. } => {
36                     used_exception_tables.insert(exception);
37                 }
38                 _ => (),
39             }
40             continue;
41         }
42 
43         trace!("Eliminating unreachable {}", block);
44         // Move the cursor out of the way and make sure the next lop iteration goes to the right
45         // block.
46         pos.prev_block();
47 
48         // Remove all instructions from `block`.
49         while let Some(inst) = pos.func.layout.first_inst(block) {
50             trace!(" - {}", pos.func.dfg.display_inst(inst));
51             pos.func.layout.remove_inst(inst);
52         }
53 
54         // Once the block is completely empty, we can update the CFG which removes it from any
55         // predecessor lists.
56         cfg.recompute_block(pos.func, block);
57 
58         // Finally, remove the block from the layout.
59         pos.func.layout.remove_block(block);
60     }
61 
62     for (table, jt_data) in func.stencil.dfg.jump_tables.iter_mut() {
63         if !used_tables.contains(table) {
64             jt_data.clear();
65         }
66     }
67 
68     for (exception, exception_data) in func.stencil.dfg.exception_tables.iter_mut() {
69         if !used_exception_tables.contains(exception) {
70             exception_data.clear();
71         }
72     }
73 }
74