[llvm] [DebugInfo][GVN] Propagate DebugLoc from load-of-select to select (PR #114233)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-debuginfo

Author: Stephen Tozer (SLTozer)

<details>
<summary>Changes</summary>

When replacing a load from a selected pointer with a select of the known stored values, we currently assign no DebugLoc to the select; this patch propagates the load's DebugLoc to the new select, since it is a direct replacement.

---
Full diff: https://github.com/llvm/llvm-project/pull/114233.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/Scalar/GVN.cpp (+3) 
- (added) llvm/test/Transforms/GVN/debugloc-load-select.ll (+55) 


``````````diff
diff --git a/llvm/lib/Transforms/Scalar/GVN.cpp b/llvm/lib/Transforms/Scalar/GVN.cpp
index 2ba600497e00d3..291ae882da937c 100644
--- a/llvm/lib/Transforms/Scalar/GVN.cpp
+++ b/llvm/lib/Transforms/Scalar/GVN.cpp
@@ -1122,6 +1122,9 @@ Value *AvailableValue::MaterializeAdjustedValue(LoadInst *Load,
     assert(V1 && V2 && "both value operands of the select must be present");
     Res =
         SelectInst::Create(Sel->getCondition(), V1, V2, "", Sel->getIterator());
+    // We use the DebugLoc from the original load here, as this instruction
+    // materializes the value that would previously have been loaded.
+    cast<SelectInst>(Res)->setDebugLoc(Load->getDebugLoc());
   } else {
     llvm_unreachable("Should not materialize value from dead block");
   }
diff --git a/llvm/test/Transforms/GVN/debugloc-load-select.ll b/llvm/test/Transforms/GVN/debugloc-load-select.ll
new file mode 100644
index 00000000000000..da0def0622ae38
--- /dev/null
+++ b/llvm/test/Transforms/GVN/debugloc-load-select.ll
@@ -0,0 +1,55 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt < %s -p=gvn -S | FileCheck %s
+;; Check that when GVN replaces a load of a select with a select of the values
+;; that would be loaded, we propagate the debug location from the load to the
+;; select, since the select is a replacement that produces the same value.
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @loadtree(ptr %topol, i1 %cmp168) {
+; CHECK-LABEL: define void @loadtree(
+; CHECK-SAME: ptr [[TOPOL:%.*]], i1 [[CMP168:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    [[TMP0:%.*]] = load i32, ptr null, align 4
+; CHECK-NEXT:    [[TMP1:%.*]] = load i32, ptr [[TOPOL]], align 4
+; CHECK-NEXT:    [[CMP1681:%.*]] = icmp sgt i32 [[TMP0]], [[TMP1]]
+; CHECK-NEXT:    [[TMP2:%.*]] = select i1 [[CMP168]], i32 [[TMP1]], i32 [[TMP0]], !dbg [[DBG3:![0-9]+]]
+; CHECK-NEXT:    [[DOT:%.*]] = select i1 [[CMP168]], ptr [[TOPOL]], ptr null, !dbg [[DBG9:![0-9]+]]
+; CHECK-NEXT:    [[CMP174_NOT473:%.*]] = icmp eq i32 [[TMP2]], 0
+; CHECK-NEXT:    ret void
+;
+entry:
+  %0 = load i32, ptr null, align 4
+  %1 = load i32, ptr %topol, align 4
+  %cmp1681 = icmp sgt i32 %0, %1
+  %. = select i1 %cmp168, ptr %topol, ptr null, !dbg !9
+  %2 = load i32, ptr %., align 4, !dbg !3
+  %cmp174.not473 = icmp eq i32 %2, 0
+  ret void
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!2}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C11, file: !1, producer: "clang version 20.0.0git")
+!1 = !DIFile(filename: "/tmp/mltaln9.c", directory: "/tmp")
+!2 = !{i32 2, !"Debug Info Version", i32 3}
+!3 = !DILocation(line: 4, column: 22, scope: !5)
+!4 = !DIFile(filename: "mltaln9.c", directory: "/tmp")
+!5 = distinct !DISubprogram(name: "loadtree", scope: !4, file: !4, line: 869, type: !6, scopeLine: 870, unit: !0, retainedNodes: !8)
+!6 = distinct !DISubroutineType(types: !7)
+!7 = !{null}
+!8 = !{}
+!9 = !DILocation(line: 9, column: 22, scope: !5)
+;.
+; CHECK: [[META0:![0-9]+]] = distinct !DICompileUnit(language: DW_LANG_C11, file: [[META1:![0-9]+]], producer: "{{.*}}clang version {{.*}}", isOptimized: false, runtimeVersion: 0, emissionKind: NoDebug)
+; CHECK: [[META1]] = !DIFile(filename: "/tmp/mltaln9.c", directory: {{.*}})
+; CHECK: [[DBG3]] = !DILocation(line: 4, column: 22, scope: [[META4:![0-9]+]])
+; CHECK: [[META4]] = distinct !DISubprogram(name: "loadtree", scope: [[META5:![0-9]+]], file: [[META5]], line: 869, type: [[META6:![0-9]+]], scopeLine: 870, spFlags: DISPFlagDefinition, unit: [[META0]], retainedNodes: [[META8:![0-9]+]])
+; CHECK: [[META5]] = !DIFile(filename: "mltaln9.c", directory: {{.*}})
+; CHECK: [[META6]] = distinct !DISubroutineType(types: [[META7:![0-9]+]])
+; CHECK: [[META7]] = !{null}
+; CHECK: [[META8]] = !{}
+; CHECK: [[DBG9]] = !DILocation(line: 9, column: 22, scope: [[META4]])
+;.

``````````

</details>


https://github.com/llvm/llvm-project/pull/114233