[KeyInstr][Inline] Don't propagate atoms to inlined nodebug instructions (#133485)

RFC: https://discourse.llvm.org/t/rfc-improving-is-stmt-placement-for-better-interactive-debugging/82668

Author: OCHyams

llvm/include/llvm/IR/DebugInfoMetadata.h

@@ -2284,6 +2284,13 @@ class DILocation : public MDNode {
 #endif
   }
 
+  const DILocation *getWithoutAtom() const {
+    if (!getAtomGroup() && !getAtomRank())
+      return this;
+    return get(getContext(), getLine(), getColumn(), getScope(), getInlinedAt(),
+               isImplicitCode());
+  }
+
   // Disallow replacing operands.
   void replaceOperandWith(unsigned I, Metadata *New) = delete;
 

llvm/lib/Transforms/Utils/InlineFunction.cpp

@@ -1827,10 +1827,16 @@ static DebugLoc inlineDebugLoc(DebugLoc OrigDL, DILocation *InlinedAt,
 /// to encode location where these instructions are inlined.
 static void fixupLineNumbers(Function *Fn, Function::iterator FI,
                              Instruction *TheCall, bool CalleeHasDebugInfo) {
-  const DebugLoc &TheCallDL = TheCall->getDebugLoc();
-  if (!TheCallDL)
+  if (!TheCall->getDebugLoc())
     return;
 
+  // Don't propagate the source location atom from the call to inlined nodebug
+  // instructions, and avoid putting it in the InlinedAt field of inlined
+  // not-nodebug instructions. FIXME: Possibly worth transferring/generating
+  // an atom for the returned value, otherwise we miss stepping on inlined
+  // nodebug functions (which is different to existing behaviour).
+  DebugLoc TheCallDL = TheCall->getDebugLoc().get()->getWithoutAtom();
+
   auto &Ctx = Fn->getContext();
   DILocation *InlinedAtNode = TheCallDL;
 

llvm/test/DebugInfo/KeyInstructions/Generic/inline-nodbg.ll

@@ -0,0 +1,43 @@
+; RUN: opt %s -passes=inline -S -o - | FileCheck %s
+
+;; $ cat test.cpp
+;; int g;
+;; [[clang::always_inline, gnu::nodebug]]  void a() { g = 1; }
+;; void b() { a(); }
+;;
+;; Check the inlined instructions don't inherit the call's atom info.
+;; FIXME: Perhaps we want to do actually do that, to preserve existing
+;; behaviour? Unclear what's best.
+
+; CHECK: _Z1bv()
+; CHECK: store i32 1, ptr @g, align 4, !dbg [[DBG:!.*]]
+; CHECK: [[DBG]] = !DILocation(line: 3, scope: ![[#]])
+
+@g = hidden global i32 0, align 4
+
+define hidden void @_Z1av() {
+entry:
+  store i32 1, ptr @g, align 4
+  ret void
+}
+
+define hidden void @_Z1bv() !dbg !15 {
+entry:
+  call void @_Z1av(), !dbg !18
+  ret void, !dbg !19
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!2, !3}
+!llvm.ident = !{!10}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_17, file: !1, producer: "clang version 19.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: LineTablesOnly, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "test.cpp", directory: "/")
+!2 = !{i32 7, !"Dwarf Version", i32 5}
+!3 = !{i32 2, !"Debug Info Version", i32 3}
+!10 = !{!"clang version 19.0.0"}
+!15 = distinct !DISubprogram(name: "b", scope: !1, file: !1, line: 3, type: !16, scopeLine: 3, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0)
+!16 = !DISubroutineType(types: !17)
+!17 = !{}
+!18 = !DILocation(line: 3, scope: !15, atomGroup: 1, atomRank: 1)
+!19 = !DILocation(line: 3, scope: !15, atomGroup: 2, atomRank: 1)