[llvm] [GlobalISel] Fix stale debug location in tryCombineTrunc (PR #216411)

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llvmorg-github-actions[bot] wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-aarch64

Author: Adrian Prantl (adrian-prantl)

<details>
<summary>Changes</summary>

LegalizationArtifactCombiner shares one MachineIRBuilder across all combines, and setInstr() only moves the insert point: it leaves the builder's DebugLoc pointing at whatever instruction was combined previously. Instructions built for an artifact that carries no location of its own therefore inherit an unrelated statement's location.

rdar://184762794

Assisted-by: claude

---
Full diff: https://github.com/llvm/llvm-project/pull/216411.diff


2 Files Affected:

- (modified) llvm/include/llvm/CodeGen/GlobalISel/LegalizationArtifactCombiner.h (+1-1) 
- (added) llvm/test/CodeGen/AArch64/GlobalISel/combine-trunc-debugloc.mir (+52) 


``````````diff
diff --git a/llvm/include/llvm/CodeGen/GlobalISel/LegalizationArtifactCombiner.h b/llvm/include/llvm/CodeGen/GlobalISel/LegalizationArtifactCombiner.h
index 08e567f2e640a..4051542833a5e 100644
--- a/llvm/include/llvm/CodeGen/GlobalISel/LegalizationArtifactCombiner.h
+++ b/llvm/include/llvm/CodeGen/GlobalISel/LegalizationArtifactCombiner.h
@@ -265,7 +265,7 @@ class LegalizationArtifactCombiner {
     using namespace llvm::MIPatternMatch;
     assert(MI.getOpcode() == TargetOpcode::G_TRUNC);
 
-    Builder.setInstr(MI);
+    Builder.setInstrAndDebugLoc(MI);
     Register DstReg = MI.getOperand(0).getReg();
     const LLT DstTy = MRI.getType(DstReg);
     Register SrcReg = lookThroughCopyInstrs(MI.getOperand(1).getReg());
diff --git a/llvm/test/CodeGen/AArch64/GlobalISel/combine-trunc-debugloc.mir b/llvm/test/CodeGen/AArch64/GlobalISel/combine-trunc-debugloc.mir
new file mode 100644
index 0000000000000..af373fb5f6489
--- /dev/null
+++ b/llvm/test/CodeGen/AArch64/GlobalISel/combine-trunc-debugloc.mir
@@ -0,0 +1,52 @@
+# RUN: llc -verify-machineinstrs -mtriple arm64-apple-macosx -run-pass=legalizer %s -o - | FileCheck %s
+
+# Folding G_TRUNC results in G_CONSTANT i8 0, which is already in the CSE map,
+# so the combiner emits a COPY.
+#
+# CHECK-LABEL: name: f
+# CHECK: [[ZERO:%[0-9]+]]:_(i8) = G_CONSTANT i8 0
+#
+# Test that the COPY does not inherit the location left in the shared builder by
+# the G_ANYEXT folded just before it.
+#
+# This should have no location:
+# CHECK: [[BYTE:%[0-9]+]]:_(i8) = COPY [[ZERO]](i8){{$}}
+# CHECK: G_STORE [[BYTE]](i8), %{{[0-9]+}}(p0)
+#
+# The instruction that does own a location keeps it.
+# CHECK: G_STORE %{{[0-9]+}}(i32), %{{[0-9]+}}(p0), debug-location
+
+--- |
+  define void @f() !dbg !4 {
+  entry:
+    ret void
+  }
+
+  !llvm.dbg.cu = !{!0}
+  !llvm.module.flags = !{!2}
+
+  !0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, emissionKind: FullDebug)
+  !1 = !DIFile(filename: "t.c", directory: "/")
+  !2 = !{i32 2, !"Debug Info Version", i32 3}
+  !4 = distinct !DISubprogram(name: "f", scope: !1, file: !1, line: 13, type: !6, scopeLine: 13, spFlags: DISPFlagDefinition, unit: !0)
+  !6 = !DISubroutineType(types: !7)
+  !7 = !{}
+  !8 = !DILocation(line: 20, column: 11, scope: !4)
+
+...
+---
+name:            f
+body:             |
+  bb.1.entry:
+    %0:_(p0) = G_IMPLICIT_DEF
+    %1:_(i8) = G_CONSTANT i8 0
+    G_STORE %1(i8), %0(p0) :: (store (i8))
+    %2:_(i64) = G_CONSTANT i64 0
+    %3:_(i8) = G_TRUNC %2(i64)
+    G_STORE %3(i8), %0(p0) :: (store (i8))
+    %4:_(i1) = G_CONSTANT i1 false, debug-location !8
+    %5:_(i32) = G_ANYEXT %4(i1), debug-location !8
+    G_STORE %5(i32), %0(p0), debug-location !8 :: (store (i32))
+    RET_ReallyLR
+
+...

``````````

</details>


https://github.com/llvm/llvm-project/pull/216411