[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);
 }