1 //! Unreachable code elimination.
2
3 use crate::cursor::{Cursor, FuncCursor};
4 use crate::dominator_tree::DominatorTree;
5 use crate::flowgraph::ControlFlowGraph;
6 use crate::ir;
7 use crate::timing;
8
9 /// Eliminate unreachable code.
10 ///
11 /// This pass deletes whole blocks that can't be reached from the entry block. It does not delete
12 /// individual instructions whose results are unused.
13 ///
14 /// The reachability analysis is performed by the dominator tree analysis.
eliminate_unreachable_code( func: &mut ir::Function, cfg: &mut ControlFlowGraph, domtree: &DominatorTree, )15 pub fn eliminate_unreachable_code(
16 func: &mut ir::Function,
17 cfg: &mut ControlFlowGraph,
18 domtree: &DominatorTree,
19 ) {
20 let _tt = timing::unreachable_code();
21 let mut pos = FuncCursor::new(func);
22 while let Some(block) = pos.next_block() {
23 if domtree.is_reachable(block) {
24 continue;
25 }
26
27 log::trace!("Eliminating unreachable {}", block);
28 // Move the cursor out of the way and make sure the next lop iteration goes to the right
29 // block.
30 pos.prev_block();
31
32 // Remove all instructions from `block`.
33 while let Some(inst) = pos.func.layout.first_inst(block) {
34 log::trace!(" - {}", pos.func.dfg.display_inst(inst, None));
35 pos.func.layout.remove_inst(inst);
36 }
37
38 // Once the block is completely empty, we can update the CFG which removes it from any
39 // predecessor lists.
40 cfg.recompute_block(pos.func, block);
41
42 // Finally, remove the block from the layout.
43 pos.func.layout.remove_block(block);
44 }
45
46 // Remove all jumptable block-list contents that refer to unreachable
47 // blocks; the jumptable itself must have been unused (or used only in an
48 // unreachable block) if so. Note that we are not necessarily removing *all*
49 // unused jumptables, because that would require computing their
50 // reachability as well; we are just removing enough to clean up references
51 // to deleted blocks.
52 for jt_data in func.jump_tables.values_mut() {
53 let invalid_ref = jt_data.iter().any(|block| !domtree.is_reachable(*block));
54 if invalid_ref {
55 jt_data.clear();
56 }
57 }
58 }
59