BPF: emit debuginfo for Function of DeclRefExpr if requested

Commit e3d8ee3 ("reland "[DebugInfo] Support to emit debugInfo
for extern variables"") added support to emit debugInfo for
extern variables if requested by the target. Currently, only
BPF target enables this feature by default.

As BPF ecosystem grows, callback function started to get
support, e.g., recently bpf_for_each_map_elem() is introduced
(https://lwn.net/Articles/846504/) with a callback function as an
argument. In the future we may have something like below as
a demonstration of use case :
    extern int do_work(int);
    long bpf_helper(void *callback_fn, void *callback_ctx, ...);
    long prog_main() {
        struct { ... } ctx = { ... };
        return bpf_helper(&do_work, &ctx, ...);
    }
Basically bpf helper may have a callback function and the
callback function is defined in another file or in the kernel.
In this case, we would like to know the debuginfo types for
do_work(), so the verifier can proper verify the safety of
bpf_helper() call.

For the following example,
    extern int do_work(int);
    long bpf_helper(void *callback_fn);
    long prog() {
        return bpf_helper(&do_work);
    }

Currently, there is no debuginfo generated for extern function do_work().
In the IR, we have,
    ...
    define dso_local i64 @prog() local_unnamed_addr #0 !dbg !7 {
    entry:
      %call = tail call i64 @bpf_helper(i8* bitcast (i32 (i32)* @do_work to i8*)) #2, !dbg !11
      ret i64 %call, !dbg !12
    }
    ...
    declare dso_local i32 @do_work(i32) #1
    ...

This patch added support for the above callback function use case, and
the generated IR looks like below:
    ...
    declare !dbg !17 dso_local i32 @do_work(i32) #1
    ...
    !17 = !DISubprogram(name: "do_work", scope: !1, file: !1, line: 1, type: !18, flags: DIFlagPrototyped, spFlags: DISPFlagOptimized, retainedNodes: !2)
    !18 = !DISubroutineType(types: !19)
    !19 = !{!20, !20}
    !20 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)

The TargetInfo.allowDebugInfoForExternalVar is renamed to
TargetInfo.allowDebugInfoForExternalRef as now it guards
both extern variable and extern function debuginfo generation.

Differential Revision: https://reviews.llvm.org/D100567

clang/include/clang/Basic/TargetInfo.h

@@ -1538,8 +1538,8 @@ class TargetInfo : public virtual TransferrableTargetInfo,
 
   virtual void setAuxTarget(const TargetInfo *Aux) {}
 
-  /// Whether target allows debuginfo types for decl only variables.
-  virtual bool allowDebugInfoForExternalVar() const { return false; }
+  /// Whether target allows debuginfo types for decl only variables/functions.
+  virtual bool allowDebugInfoForExternalRef() const { return false; }
 
 protected:
   /// Copy type and layout related info.

clang/lib/Basic/Targets/BPF.h

@@ -76,7 +76,7 @@ class LLVM_LIBRARY_VISIBILITY BPFTargetInfo : public TargetInfo {
     return None;
   }
 
-  bool allowDebugInfoForExternalVar() const override { return true; }
+  bool allowDebugInfoForExternalRef() const override { return true; }
 
   CallingConvCheckResult checkCallingConvention(CallingConv CC) const override {
     switch (CC) {

clang/lib/CodeGen/CGExpr.cpp

@@ -2832,8 +2832,21 @@ LValue CodeGenFunction::EmitDeclRefLValue(const DeclRefExpr *E) {
     return LV;
   }
 
-  if (const auto *FD = dyn_cast<FunctionDecl>(ND))
-    return EmitFunctionDeclLValue(*this, E, FD);
+  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;
+  }
 
   // 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/Sema/SemaDecl.cpp

@@ -12688,7 +12688,7 @@ void Sema::ActOnUninitializedDecl(Decl *RealDecl) {
         Diag(Var->getLocation(), diag::note_private_extern);
       }
 
-      if (Context.getTargetInfo().allowDebugInfoForExternalVar() &&
+      if (Context.getTargetInfo().allowDebugInfoForExternalRef() &&
           !Var->isInvalidDecl() && !getLangOpts().CPlusPlus)
         ExternalDeclarations.push_back(Var);
 

clang/test/CodeGen/debug-info-extern-callback.c

@@ -0,0 +1,22 @@
+// RUN: %clang_cc1 -x c -debug-info-kind=limited -triple bpf-linux-gnu -emit-llvm %s -o - | FileCheck %s
+
+extern int do_work1(int);
+long bpf_helper1(void *callback_fn);
+long prog1() {
+  return bpf_helper1(&do_work1);
+}
+
+extern int do_work2();
+long prog2_1() {
+  return (long)&do_work2;
+}
+int do_work2() { return 0; }
+long prog2_2() {
+  return (long)&do_work2;
+}
+
+// CHECK: declare !dbg ![[FUNC1:[0-9]+]] i32 @do_work1
+// CHECK: define dso_local i32 @do_work2() #{{[0-9]+}} !dbg ![[FUNC2:[0-9]+]]
+
+// CHECK: ![[FUNC1]] = !DISubprogram(name: "do_work1"
+// CHECK: ![[FUNC2]] = distinct !DISubprogram(name: "do_work2"