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Only emit value label aliases for lowered instructions #12779

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Merged
alexcrichton merged 1 commit into from
Mar 16, 2026
+8 −6
Merged

Only emit value label aliases for lowered instructions #12779

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14 changes: 8 additions & 6 deletions cranelift/codegen/src/machinst/lower.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -802,7 +802,9 @@ impl<'func, I: VCodeInst> Lower<'func, I> {

// Value defined by "inst" becomes live after it in normal
// order, and therefore **before** in reversed order.
self.emit_value_label_live_range_start_for_inst(inst);
// Only emit value label aliases if the instruction will be lowered
// (otherwise we want to keep using the earlier label instead).
self.emit_value_label_live_range_start_for_inst(inst, has_side_effect || value_needed);

// Normal instruction: codegen if the instruction is side-effecting
// or any of its outputs is used.
Expand Down Expand Up @@ -949,14 +951,14 @@ impl<'func, I: VCodeInst> Lower<'func, I> {
}
}

fn emit_value_label_marks_for_value(&mut self, val: Value) {
fn emit_value_label_marks_for_value(&mut self, val: Value, allow_alias: bool) {
let regs = self.value_regs[val];
if regs.len() > 1 {
return;
}
let reg = regs.only_reg().unwrap();

if let Some(label_starts) = self.get_value_labels(val, 0) {
if let Some(label_starts) = self.get_value_labels(val, if allow_alias { 0 } else { !0 }) {
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@philipc philipc Mar 14, 2026

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This is a bit of a hack that avoids changing get_value_labels. Can do it differently if desired.

let labels = label_starts
.iter()
.map(|&ValueLabelStart { label, .. }| label)
Expand All @@ -971,7 +973,7 @@ impl<'func, I: VCodeInst> Lower<'func, I> {
}
}

fn emit_value_label_live_range_start_for_inst(&mut self, inst: Inst) {
fn emit_value_label_live_range_start_for_inst(&mut self, inst: Inst, allow_alias: bool) {
if self.f.dfg.values_labels.is_none() {
return;
}
Expand All @@ -982,7 +984,7 @@ impl<'func, I: VCodeInst> Lower<'func, I> {
inst
);
for &val in self.f.dfg.inst_results(inst) {
self.emit_value_label_marks_for_value(val);
self.emit_value_label_marks_for_value(val, allow_alias);
}
}

Expand All @@ -993,7 +995,7 @@ impl<'func, I: VCodeInst> Lower<'func, I> {

trace!("value labeling: block {}", block);
for &arg in self.f.dfg.block_params(block) {
self.emit_value_label_marks_for_value(arg);
self.emit_value_label_marks_for_value(arg, true);
}
self.finish_ir_inst(Default::default());
}
Expand Down
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