[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.

1 files changed, 17 insertions, 2 deletions

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) {