8d0224341c
* Don't use `log::trace` directly but a feature-enabled `trace` macro * Don't emit disassembly based on the log level
59 lines
2.1 KiB
Rust
59 lines
2.1 KiB
Rust
//! Unreachable code elimination.
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use crate::cursor::{Cursor, FuncCursor};
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use crate::dominator_tree::DominatorTree;
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use crate::flowgraph::ControlFlowGraph;
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use crate::timing;
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use crate::{ir, trace};
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/// Eliminate unreachable code.
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///
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/// This pass deletes whole blocks that can't be reached from the entry block. It does not delete
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/// individual instructions whose results are unused.
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///
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/// The reachability analysis is performed by the dominator tree analysis.
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pub fn eliminate_unreachable_code(
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func: &mut ir::Function,
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cfg: &mut ControlFlowGraph,
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domtree: &DominatorTree,
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) {
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let _tt = timing::unreachable_code();
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let mut pos = FuncCursor::new(func);
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while let Some(block) = pos.next_block() {
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if domtree.is_reachable(block) {
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continue;
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}
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trace!("Eliminating unreachable {}", block);
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// Move the cursor out of the way and make sure the next lop iteration goes to the right
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// block.
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pos.prev_block();
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// Remove all instructions from `block`.
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while let Some(inst) = pos.func.layout.first_inst(block) {
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trace!(" - {}", pos.func.dfg.display_inst(inst));
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pos.func.layout.remove_inst(inst);
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}
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// Once the block is completely empty, we can update the CFG which removes it from any
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// predecessor lists.
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cfg.recompute_block(pos.func, block);
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// Finally, remove the block from the layout.
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pos.func.layout.remove_block(block);
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}
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// Remove all jumptable block-list contents that refer to unreachable
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// blocks; the jumptable itself must have been unused (or used only in an
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// unreachable block) if so. Note that we are not necessarily removing *all*
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// unused jumptables, because that would require computing their
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// reachability as well; we are just removing enough to clean up references
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// to deleted blocks.
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for jt_data in func.jump_tables.values_mut() {
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let invalid_ref = jt_data.iter().any(|block| !domtree.is_reachable(*block));
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if invalid_ref {
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jt_data.clear();
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}
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}
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}
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