17 files changed, 139 insertions, 26 deletions
diff --git a/clang/include/clang/AST/ASTConsumer.h b/clang/include/clang/AST/ASTConsumer.h
index ebcd805..447f259 100644
--- a/clang/include/clang/AST/ASTConsumer.h
+++ b/clang/include/clang/AST/ASTConsumer.h
@@ -23,6 +23,7 @@ namespace clang {
   class ASTDeserializationListener; // layering violation because void* is ugly
   class SemaConsumer; // layering violation required for safe SemaConsumer
   class TagDecl;
+  class DeclaratorDecl;
   class VarDecl;
   class FunctionDecl;
   class ImportDecl;
@@ -105,7 +106,7 @@ public:
   /// CompleteExternalDeclaration - Callback invoked at the end of a translation
   /// unit to notify the consumer that the given external declaration should be
   /// completed.
-  virtual void CompleteExternalDeclaration(VarDecl *D) {}
+  virtual void CompleteExternalDeclaration(DeclaratorDecl *D) {}
 
   /// Callback invoked when an MSInheritanceAttr has been attached to a
   /// CXXRecordDecl.
diff --git a/clang/include/clang/Frontend/MultiplexConsumer.h b/clang/include/clang/Frontend/MultiplexConsumer.h
index 4ed0d86..e49e339 100644
--- a/clang/include/clang/Frontend/MultiplexConsumer.h
+++ b/clang/include/clang/Frontend/MultiplexConsumer.h
@@ -67,7 +67,7 @@ public:
   void HandleTopLevelDeclInObjCContainer(DeclGroupRef D) override;
   void HandleImplicitImportDecl(ImportDecl *D) override;
   void CompleteTentativeDefinition(VarDecl *D) override;
-  void CompleteExternalDeclaration(VarDecl *D) override;
+  void CompleteExternalDeclaration(DeclaratorDecl *D) override;
   void AssignInheritanceModel(CXXRecordDecl *RD) override;
   void HandleVTable(CXXRecordDecl *RD) override;
   ASTMutationListener *GetASTMutationListener() override;
diff --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
index fb3a5d2..75a8054 100644
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -3098,7 +3098,7 @@ public:
   TentativeDefinitionsType TentativeDefinitions;
 
   /// All the external declarations encoutered and used in the TU.
-  SmallVector<VarDecl *, 4> ExternalDeclarations;
+  SmallVector<DeclaratorDecl *, 4> ExternalDeclarations;
 
   /// Generally null except when we temporarily switch decl contexts,
   /// like in \see SemaObjC::ActOnObjCTemporaryExitContainerContext.
diff --git a/clang/lib/CodeGen/BackendConsumer.h b/clang/lib/CodeGen/BackendConsumer.h
index a648bd3..a023d29 100644
--- a/clang/lib/CodeGen/BackendConsumer.h
+++ b/clang/lib/CodeGen/BackendConsumer.h
@@ -107,7 +107,7 @@ public:
   void HandleTagDeclDefinition(TagDecl *D) override;
   void HandleTagDeclRequiredDefinition(const TagDecl *D) override;
   void CompleteTentativeDefinition(VarDecl *D) override;
-  void CompleteExternalDeclaration(VarDecl *D) override;
+  void CompleteExternalDeclaration(DeclaratorDecl *D) override;
   void AssignInheritanceModel(CXXRecordDecl *RD) override;
   void HandleVTable(CXXRecordDecl *RD) override;
 
diff --git a/clang/lib/CodeGen/CGExpr.cpp b/clang/lib/CodeGen/CGExpr.cpp
index 23e5dee..039f60c 100644
--- a/clang/lib/CodeGen/CGExpr.cpp
+++ b/clang/lib/CodeGen/CGExpr.cpp
@@ -3141,21 +3141,8 @@ LValue CodeGenFunction::EmitDeclRefLValue(const DeclRefExpr *E) {
     return LV;
   }
 
-  if (const auto *FD = dyn_cast<FunctionDecl>(ND)) {
-    LValue LV = EmitFunctionDeclLValue(*this, E, FD);
-
-    // Emit debuginfo for the function declaration if the target wants to.
-    if (getContext().getTargetInfo().allowDebugInfoForExternalRef()) {
-      if (CGDebugInfo *DI = CGM.getModuleDebugInfo()) {
-        auto *Fn =
-            cast<llvm::Function>(LV.getPointer(*this)->stripPointerCasts());
-        if (!Fn->getSubprogram())
-          DI->EmitFunctionDecl(FD, FD->getLocation(), T, Fn);
-      }
-    }
-
-    return LV;
-  }
+  if (const auto *FD = dyn_cast<FunctionDecl>(ND))
+    return EmitFunctionDeclLValue(*this, E, FD);
 
   // FIXME: While we're emitting a binding from an enclosing scope, all other
   // DeclRefExprs we see should be implicitly treated as if they also refer to
diff --git a/clang/lib/CodeGen/CodeGenAction.cpp b/clang/lib/CodeGen/CodeGenAction.cpp
index 0b92c53..e87226e 100644
--- a/clang/lib/CodeGen/CodeGenAction.cpp
+++ b/clang/lib/CodeGen/CodeGenAction.cpp
@@ -376,7 +376,7 @@ void BackendConsumer::CompleteTentativeDefinition(VarDecl *D) {
   Gen->CompleteTentativeDefinition(D);
 }
 
-void BackendConsumer::CompleteExternalDeclaration(VarDecl *D) {
+void BackendConsumer::CompleteExternalDeclaration(DeclaratorDecl *D) {
   Gen->CompleteExternalDeclaration(D);
 }
 
diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index 99e986d..dc9dd03 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -5185,8 +5185,11 @@ void CodeGenModule::EmitTentativeDefinition(const VarDecl *D) {
   EmitGlobalVarDefinition(D);
 }
 
-void CodeGenModule::EmitExternalDeclaration(const VarDecl *D) {
-  EmitExternalVarDeclaration(D);
+void CodeGenModule::EmitExternalDeclaration(const DeclaratorDecl *D) {
+  if (auto const *V = dyn_cast<const VarDecl>(D))
+    EmitExternalVarDeclaration(V);
+  if (auto const *FD = dyn_cast<const FunctionDecl>(D))
+    EmitExternalFunctionDeclaration(FD);
 }
 
 CharUnits CodeGenModule::GetTargetTypeStoreSize(llvm::Type *Ty) const {
@@ -5622,6 +5625,18 @@ void CodeGenModule::EmitExternalVarDeclaration(const VarDecl *D) {
     }
 }
 
+void CodeGenModule::EmitExternalFunctionDeclaration(const FunctionDecl *FD) {
+  if (CGDebugInfo *DI = getModuleDebugInfo())
+    if (getCodeGenOpts().hasReducedDebugInfo()) {
+      auto *Ty = getTypes().ConvertType(FD->getType());
+      StringRef MangledName = getMangledName(FD);
+      auto *Fn = dyn_cast<llvm::Function>(
+          GetOrCreateLLVMFunction(MangledName, Ty, FD, /* ForVTable */ false));
+      if (!Fn->getSubprogram())
+        DI->EmitFunctionDecl(FD, FD->getLocation(), FD->getType(), Fn);
+    }
+}
+
 static bool isVarDeclStrongDefinition(const ASTContext &Context,
                                       CodeGenModule &CGM, const VarDecl *D,
                                       bool NoCommon) {
diff --git a/clang/lib/CodeGen/CodeGenModule.h b/clang/lib/CodeGen/CodeGenModule.h
index 4796d42..0444f9f 100644
--- a/clang/lib/CodeGen/CodeGenModule.h
+++ b/clang/lib/CodeGen/CodeGenModule.h
@@ -1338,7 +1338,7 @@ public:
 
   void EmitTentativeDefinition(const VarDecl *D);
 
-  void EmitExternalDeclaration(const VarDecl *D);
+  void EmitExternalDeclaration(const DeclaratorDecl *D);
 
   void EmitVTable(CXXRecordDecl *Class);
 
@@ -1690,6 +1690,7 @@ private:
 
   void EmitGlobalVarDefinition(const VarDecl *D, bool IsTentative = false);
   void EmitExternalVarDeclaration(const VarDecl *D);
+  void EmitExternalFunctionDeclaration(const FunctionDecl *D);
   void EmitAliasDefinition(GlobalDecl GD);
   void emitIFuncDefinition(GlobalDecl GD);
   void emitCPUDispatchDefinition(GlobalDecl GD);
diff --git a/clang/lib/CodeGen/ModuleBuilder.cpp b/clang/lib/CodeGen/ModuleBuilder.cpp
index df85295..d4e0ab0 100644
--- a/clang/lib/CodeGen/ModuleBuilder.cpp
+++ b/clang/lib/CodeGen/ModuleBuilder.cpp
@@ -310,7 +310,7 @@ namespace {
       Builder->EmitTentativeDefinition(D);
     }
 
-    void CompleteExternalDeclaration(VarDecl *D) override {
+    void CompleteExternalDeclaration(DeclaratorDecl *D) override {
       Builder->EmitExternalDeclaration(D);
     }
 
diff --git a/clang/lib/Frontend/MultiplexConsumer.cpp b/clang/lib/Frontend/MultiplexConsumer.cpp
index 8fdc7f5..651c55a 100644
--- a/clang/lib/Frontend/MultiplexConsumer.cpp
+++ b/clang/lib/Frontend/MultiplexConsumer.cpp
@@ -357,7 +357,7 @@ void MultiplexConsumer::CompleteTentativeDefinition(VarDecl *D) {
     Consumer->CompleteTentativeDefinition(D);
 }
 
-void MultiplexConsumer::CompleteExternalDeclaration(VarDecl *D) {
+void MultiplexConsumer::CompleteExternalDeclaration(DeclaratorDecl *D) {
   for (auto &Consumer : Consumers)
     Consumer->CompleteExternalDeclaration(D);
 }
diff --git a/clang/lib/Interpreter/IncrementalParser.cpp b/clang/lib/Interpreter/IncrementalParser.cpp
index a8d0294..b7c809c 100644
--- a/clang/lib/Interpreter/IncrementalParser.cpp
+++ b/clang/lib/Interpreter/IncrementalParser.cpp
@@ -79,7 +79,7 @@ public:
   void CompleteTentativeDefinition(VarDecl *D) override final {
     Consumer->CompleteTentativeDefinition(D);
   }
-  void CompleteExternalDeclaration(VarDecl *D) override final {
+  void CompleteExternalDeclaration(DeclaratorDecl *D) override final {
     Consumer->CompleteExternalDeclaration(D);
   }
   void AssignInheritanceModel(CXXRecordDecl *RD) override final {
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index b3bfdac..aa44608 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -10818,6 +10818,14 @@ Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
       break;
     }
 
+  // Similar to no_builtin logic above, at this point of the code
+  // FunctionDecl::isThisDeclarationADefinition() always returns `false`
+  // because Sema::ActOnStartOfFunctionDef has not been called yet.
+  if (Context.getTargetInfo().allowDebugInfoForExternalRef() &&
+      !NewFD->isInvalidDecl() &&
+      D.getFunctionDefinitionKind() == FunctionDefinitionKind::Declaration)
+    ExternalDeclarations.push_back(NewFD);
+
   return NewFD;
 }
 
diff --git a/clang/test/CodeGen/bpf-debug-info-extern-func.c b/clang/test/CodeGen/bpf-debug-info-extern-func.c
new file mode 100644
index 0000000..e87c8be
--- /dev/null
+++ b/clang/test/CodeGen/bpf-debug-info-extern-func.c
@@ -0,0 +1,9 @@
+// RUN: %clang -g -target bpf -S -emit-llvm %s -o - | FileCheck %s
+//
+// When linking BPF object files via bpftool, BTF info is required for
+// every symbol. BTF is generated from debug info. Ensure that debug info
+// is emitted for extern functions referenced via variable initializers.
+//
+// CHECK: !DISubprogram(name: "fn"
+extern void fn(void);
+void (*pfn) (void) = &fn;
diff --git a/clang/test/CodeGen/bpf-debug-info-unref.c b/clang/test/CodeGen/bpf-debug-info-unref.c
new file mode 100644
index 0000000..91f761e
--- /dev/null
+++ b/clang/test/CodeGen/bpf-debug-info-unref.c
@@ -0,0 +1,11 @@
+// RUN: %clang -g -target bpf -S -emit-llvm %s -o - | FileCheck %s
+//
+// No debug info is produced for unreferenced functions.
+// CHECK-NOT: !DISubprogram
+void unref(void);
+void unref2(typeof(unref));
+
+// No debug info for unused extern variables as well.
+// CHECK-NOT: !DiGlobalVariable
+extern int unused;
+extern int unused2[sizeof(unused)];
diff --git a/llvm/lib/Target/BPF/BTFDebug.cpp b/llvm/lib/Target/BPF/BTFDebug.cpp
index 34581e3..4d847ab 100644
--- a/llvm/lib/Target/BPF/BTFDebug.cpp
+++ b/llvm/lib/Target/BPF/BTFDebug.cpp
@@ -1495,6 +1495,29 @@ void BTFDebug::processGlobals(bool ProcessingMapDef) {
 
     DataSecEntries[std::string(SecName)]->addDataSecEntry(VarId,
         Asm->getSymbol(&Global), Size);
+
+    if (Global.hasInitializer())
+      processGlobalInitializer(Global.getInitializer());
+  }
+}
+
+/// Process global variable initializer in pursuit for function
+/// pointers. Add discovered (extern) functions to BTF. Some (extern)
+/// functions might have been missed otherwise. Every symbol needs BTF
+/// info when linking with bpftool. Primary use case: "static"
+/// initialization of BPF maps.
+///
+/// struct {
+///   __uint(type, BPF_MAP_TYPE_PROG_ARRAY);
+///   ...
+/// } prog_map SEC(".maps") = { .values = { extern_func } };
+///
+void BTFDebug::processGlobalInitializer(const Constant *C) {
+  if (auto *Fn = dyn_cast<Function>(C))
+    processFuncPrototypes(Fn);
+  if (auto *CA = dyn_cast<ConstantAggregate>(C)) {
+    for (unsigned I = 0, N = CA->getNumOperands(); I < N; ++I)
+      processGlobalInitializer(CA->getOperand(I));
   }
 }
 
diff --git a/llvm/lib/Target/BPF/BTFDebug.h b/llvm/lib/Target/BPF/BTFDebug.h
index 3ef4a85..b24a79d 100644
--- a/llvm/lib/Target/BPF/BTFDebug.h
+++ b/llvm/lib/Target/BPF/BTFDebug.h
@@ -352,6 +352,10 @@ class BTFDebug : public DebugHandlerBase {
   /// Generate types and variables for globals.
   void processGlobals(bool ProcessingMapDef);
 
+  /// Process global variable initializer in pursuit for function
+  /// pointers.
+  void processGlobalInitializer(const Constant *C);
+
   /// Generate types for function prototypes.
   void processFuncPrototypes(const Function *);
 
diff --git a/llvm/test/CodeGen/BPF/BTF/extern-var-func2.ll b/llvm/test/CodeGen/BPF/BTF/extern-var-func2.ll
new file mode 100644
index 0000000..700486d9
--- /dev/null
+++ b/llvm/test/CodeGen/BPF/BTF/extern-var-func2.ll
@@ -0,0 +1,54 @@
+; RUN: llc -march=bpfel -filetype=asm -o - %s | FileCheck -check-prefixes=CHECK %s
+; RUN: llc -march=bpfeb -filetype=asm -o - %s | FileCheck -check-prefixes=CHECK %s
+;
+; Source code:
+;   extern int elsewhere(void);
+;   struct {
+;     void *values[];
+;   } prog_map = { .values = { elsewhere } };
+; Compilation flag:
+;   clang -target bpf -O2 -g -S -emit-llvm test.c
+; ModuleID = 'b.c'
+
+@prog_map = dso_local local_unnamed_addr global { [1 x ptr] } { [1 x ptr] [ptr @elsewhere] }, align 8, !dbg !0
+
+declare !dbg !17 dso_local i32 @elsewhere() #0
+
+; CHECK:             .long   0                               # BTF_KIND_FUNC_PROTO(id = 6)
+; CHECK-NEXT:        .long   218103808                       # 0xd000000
+; CHECK-NEXT:        .long   7
+; CHECK-NEXT:        .long   37                              # BTF_KIND_INT(id = 7)
+; CHECK-NEXT:        .long   16777216                        # 0x1000000
+; CHECK-NEXT:        .long   4
+; CHECK-NEXT:        .long   16777248                        # 0x1000020
+; CHECK-NEXT:        .long   41                              # BTF_KIND_FUNC(id = 8)
+; CHECK-NEXT:        .long   201326594                       # 0xc000002
+; CHECK-NEXT:        .long   6
+
+attributes #0 = { "frame-pointer"="all" "no-trapping-math"="true" "stack-protector-buffer-size"="8" }
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!12, !13, !14, !15}
+!llvm.ident = !{!16}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "prog_map", scope: !2, file: !3, line: 4, type: !5, isLocal: false, isDefinition: true)
+!2 = distinct !DICompileUnit(language: DW_LANG_C11, file: !3, producer: "clang version 19.0.0git (git@github.com:llvm/llvm-project.git 0390a6803608e3a5314315b73740c2d3f5a5723f)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, globals: !4, splitDebugInlining: false, nameTableKind: None)
+!3 = !DIFile(filename: "b.c", directory: "/home/nickz/llvm-project.git", checksumkind: CSK_MD5, checksum: "41cc17375f1261a0e072590833492553")
+!4 = !{!0}
+!5 = distinct !DICompositeType(tag: DW_TAG_structure_type, file: !3, line: 2, elements: !6)
+!6 = !{!7}
+!7 = !DIDerivedType(tag: DW_TAG_member, name: "values", scope: !5, file: !3, line: 3, baseType: !8)
+!8 = !DICompositeType(tag: DW_TAG_array_type, baseType: !9, elements: !10)
+!9 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: null, size: 64)
+!10 = !{!11}
+!11 = !DISubrange(count: -1)
+!12 = !{i32 7, !"Dwarf Version", i32 5}
+!13 = !{i32 2, !"Debug Info Version", i32 3}
+!14 = !{i32 1, !"wchar_size", i32 4}
+!15 = !{i32 7, !"frame-pointer", i32 2}
+!16 = !{!"clang version 19.0.0git (git@github.com:llvm/llvm-project.git 0390a6803608e3a5314315b73740c2d3f5a5723f)"}
+!17 = !DISubprogram(name: "elsewhere", scope: !3, file: !3, line: 1, type: !18, flags: DIFlagPrototyped, spFlags: DISPFlagOptimized)
+!18 = !DISubroutineType(types: !19)
+!19 = !{!20}
+!20 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)