[BPF] Fix linking issues in static map initializers (#91310)

When BPF object files are linked with bpftool, every symbol must be
accompanied by BTF info. Ensure that extern functions referenced by
global variable initializers are included in BTF.

The primary motivation is "static" initialization of PROG maps:

```c
extern int elsewhere(struct xdp_md *);

struct {
  __uint(type, BPF_MAP_TYPE_PROG_ARRAY);
  __uint(max_entries, 1);
  __type(key, int);
  __type(value, int);
  __array(values, int (struct xdp_md *));
} prog_map SEC(".maps") = { .values = { elsewhere } };
```

BPF backend needs debug info to produce BTF. Debug info is not
normally generated for external variables and functions. Previously, it
was solved differently for variables (collecting variable declarations
    in ExternalDeclarations vector) and functions (logic invoked during
    codegen in CGExpr.cpp).

This patch generalises ExternalDefclarations to include both function
and variable declarations. This change ensures that function references
    are not missed no matter the context. Previously external functions
    referenced in constant expressions lacked debug info.

Author: mejedi

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 @@ class ASTConsumer {
   /// 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.

clang/include/clang/Frontend/MultiplexConsumer.h

@@ -67,7 +67,7 @@ class MultiplexConsumer : public SemaConsumer {
   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;

clang/include/clang/Sema/Sema.h

@@ -3098,7 +3098,7 @@ class Sema final : public SemaBase {
   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.

clang/lib/CodeGen/BackendConsumer.h

@@ -107,7 +107,7 @@ class BackendConsumer : public ASTConsumer {
   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;
 

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

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);
 }
 

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) {

clang/lib/CodeGen/CodeGenModule.h

@@ -1338,7 +1338,7 @@ class CodeGenModule : public CodeGenTypeCache {
 
   void EmitTentativeDefinition(const VarDecl *D);
 
-  void EmitExternalDeclaration(const VarDecl *D);
+  void EmitExternalDeclaration(const DeclaratorDecl *D);
 
   void EmitVTable(CXXRecordDecl *Class);
 
@@ -1690,6 +1690,7 @@ class CodeGenModule : public CodeGenTypeCache {
 
   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);

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);
     }
 

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);
 }

clang/lib/Interpreter/IncrementalParser.cpp

@@ -79,7 +79,7 @@ class IncrementalASTConsumer final : public ASTConsumer {
   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 {

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;
 }
 

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;

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)];

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));
   }
 }
 

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 *);
 

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 ([email protected]: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 ([email protected]: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)