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//! Unreachable code elimination.
use crate::cursor::{Cursor, FuncCursor};
use crate::dominator_tree::DominatorTree;
use crate::flowgraph::ControlFlowGraph;
use crate::ir;
use crate::timing;
use log::debug;
/// Eliminate unreachable code.
///
/// This pass deletes whole blocks that can't be reached from the entry block. It does not delete
/// individual instructions whose results are unused.
///
/// The reachability analysis is performed by the dominator tree analysis.
pub fn eliminate_unreachable_code(
func: &mut ir::Function,
cfg: &mut ControlFlowGraph,
domtree: &DominatorTree,
) {
let _tt = timing::unreachable_code();
let mut pos = FuncCursor::new(func);
while let Some(block) = pos.next_block() {
if domtree.is_reachable(block) {
continue;
}
debug!("Eliminating unreachable {}", block);
// Move the cursor out of the way and make sure the next lop iteration goes to the right
// block.
pos.prev_block();
// Remove all instructions from `block`.
while let Some(inst) = pos.func.layout.first_inst(block) {
debug!(" - {}", pos.func.dfg.display_inst(inst, None));
pos.func.layout.remove_inst(inst);
}
// Once the block is completely empty, we can update the CFG which removes it from any
// predecessor lists.
cfg.recompute_block(pos.func, block);
// Finally, remove the block from the layout.
pos.func.layout.remove_block(block);
}
}
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