From 0e5fd810958305226ae98e5debb94b2140cc38d5 Mon Sep 17 00:00:00 2001
From: Philip Craig <philipjcraig@gmail.com>
Date: Sat, 14 Mar 2026 20:10:01 +1000
Subject: [PATCH] Only emit value label aliases for lowered instructions

If we emit a value label alias for an instruction that isn't lowered,
then that signals that the value has been optimised out. However, since
it is an alias we know that the value also exists in an earlier vreg, so
we should skip the alias and use that instead.

This situation occurs often for memory indexes on AArch64. We translate
memory stores into instructions such as:

    v8 = iconst.i32 42
    v9 = uextend.i64 v6
    v10 = load.i64 notrap aligned readonly can_move checked v0+56
    v11 = iadd v10, v9
    v12 = iconst.i64 20
    v13 = iadd v11, v12  ; v12 = 20
    store little heap v8, v13  ; v8 = 42

Here, v6 is a memory index (which has a label) and v9 is an
extension of the memory index (which has a label alias, added by
cast_index_to_pointer_ty()). This is lowered to:

     40c:       52800540        mov     w0, #0x2a                       // #42
     410:       f9401c41        ldr     x1, [x2, #56]
     414:       91005021        add     x1, x1, #0x14
     418:       b8384820        str     w0, [x1, w24, uxtw]

The uextend has been folded into the str, so v9 has been optimised
out. But v6 is still present in w24, so the debuginfo should use that
instead.

This fixes the following tests for AArch64:

    native_debug::lldb::dwarf_cold_block
    native_debug::lldb::dwarf_fib_wasm
    native_debug::lldb::dwarf_fib_wasm_dwarf5
    native_debug::lldb::dwarf_fib_wasm_split4
    native_debug::lldb::dwarf_fission
    native_debug::lldb::dwarf_fraction_norm
    native_debug::lldb::dwarf_imported_memory
    native_debug::lldb::dwarf_shared_memory
    native_debug::lldb::dwarf_simple
    native_debug::lldb::dwarf_spilled_frame_base
---
 cranelift/codegen/src/machinst/lower.rs | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/cranelift/codegen/src/machinst/lower.rs b/cranelift/codegen/src/machinst/lower.rs
index c3ce9fa9c96c..dc09f10c4d9c 100644
--- a/cranelift/codegen/src/machinst/lower.rs
+++ b/cranelift/codegen/src/machinst/lower.rs
@@ -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.
@@ -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 }) {
             let labels = label_starts
                 .iter()
                 .map(|&ValueLabelStart { label, .. }| label)
@@ -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;
         }
@@ -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);
         }
     }
 
@@ -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());
     }