[clang] [Debug Info] Fix a Verifier failure with redeclared nodebug functions (PR #218070)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang-codegen

Author: Adrian Prantl (adrian-prantl)

<details>
<summary>Changes</summary>

When a forward-declard function is used before its definition, and the definition has a nodebug attribute on it, the existing heuristic in EmitFuncDeclForCallSite would fail, because it only saw the non-nodebug forward declaration, thus emitting a conflicting DISubprogram for the call site.

rdar://184780682

Assisted-by: claude

---
Full diff: https://github.com/llvm/llvm-project/pull/218070.diff


2 Files Affected:

- (modified) clang/lib/CodeGen/CGDebugInfo.cpp (+7-2) 
- (modified) clang/test/DebugInfo/Generic/attr-nodebug2.c (+19) 


``````````diff
diff --git a/clang/lib/CodeGen/CGDebugInfo.cpp b/clang/lib/CodeGen/CGDebugInfo.cpp
index 37ee3765fd7c4..091a403043b61 100644
--- a/clang/lib/CodeGen/CGDebugInfo.cpp
+++ b/clang/lib/CodeGen/CGDebugInfo.cpp
@@ -5166,13 +5166,18 @@ void CGDebugInfo::EmitFuncDeclForCallSite(llvm::CallBase *CallOrInvoke,
     return;
   if (Func->getSubprogram())
     return;
+  // If the function has a definition, it either already has a
+  // subprogram or it is a nodebug function.
+  if (!Func->isDeclaration())
+    return;
 
   const FunctionDecl *CalleeDecl =
       cast<FunctionDecl>(CalleeGlobalDecl.getDecl());
 
   // Do not emit a declaration subprogram for a function with nodebug
-  // attribute, or if call site info isn't required.
-  if (CalleeDecl->hasAttr<NoDebugAttr>() ||
+  // attribute, or if call site info isn't required.  The attribute
+  // could be on a later redeclaration than the one the call resolves to.
+  if (CalleeDecl->getMostRecentDecl()->hasAttr<NoDebugAttr>() ||
       getCallSiteRelatedAttrs() == llvm::DINode::FlagZero)
     return;
 
diff --git a/clang/test/DebugInfo/Generic/attr-nodebug2.c b/clang/test/DebugInfo/Generic/attr-nodebug2.c
index a17e1e6cbff7d..4d191f0fc0cf1 100644
--- a/clang/test/DebugInfo/Generic/attr-nodebug2.c
+++ b/clang/test/DebugInfo/Generic/attr-nodebug2.c
@@ -14,6 +14,19 @@ __attribute__((nodebug)) void t1(void) {
   a++;
 }
 
+// A deferred caller is emitted after the nodebug definition, so its call site
+// declaration would attach to a function that already has a body.
+void t2(void);
+
+__attribute__((nodebug)) void t2(void) {
+  int a = 10;
+  a++;
+}
+
+static inline void deferred_caller(void) { t2(); }
+
+void use2(void) { deferred_caller(); }
+
 #ifdef __cplusplus
 }
 #endif
@@ -32,3 +45,9 @@ __attribute__((nodebug)) void t1(void) {
 // CHECK-SAME:  {
 // CHECK-NOT:   !dbg
 // CHECK:       }
+
+// CHECK-LABEL: define{{.*}} void @t2()
+// CHECK-NOT:   !dbg
+// CHECK-SAME:  {
+// CHECK-NOT:   !dbg
+// CHECK:       }

``````````

</details>


https://github.com/llvm/llvm-project/pull/218070