//! Unreachable code elimination.
use cursor::{Cursor, FuncCursor};
use dominator_tree::DominatorTree;
use flowgraph::ControlFlowGraph;
use ir;
use timing;
/// Eliminate unreachable code.
///
/// This pass deletes whole EBBs 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(ebb) = pos.next_ebb() {
if domtree.is_reachable(ebb) {
continue;
}
dbg!("Eliminating unreachable {}", ebb);
// Move the cursor out of the way and make sure the next lop iteration goes to the right
// EBB.
pos.prev_ebb();
// Remove all instructions from `ebb`.
while let Some(inst) = pos.func.layout.first_inst(ebb) {
dbg!(" - {}", pos.func.dfg.display_inst(inst, None));
pos.func.layout.remove_inst(inst);
}
// Once the EBB is completely empty, we can update the CFG which removes it from any
// predecessor lists.
cfg.recompute_ebb(pos.func, ebb);
// Finally, remove the EBB from the layout.
pos.func.layout.remove_ebb(ebb);
}
}