[DebugInfo] Support to emit debugInfo for extern variables

Extern variable usage in BPF is different from traditional
pure user space application. Recent discussion in linux bpf
mailing list has two use cases where debug info types are
required to use extern variables:
  - extern types are required to have a suitable interface
    in libbpf (bpf loader) to provide kernel config parameters
    to bpf programs.
    https://lore.kernel.org/bpf/CAEf4BzYCNo5GeVGMhp3fhysQ=_axAf=23PtwaZs-yAyafmXC9g@mail.gmail.com/T/#t
  - extern types are required so kernel bpf verifier can
    verify program which uses external functions more precisely.
    This will make later link with actual external function no
    need to reverify.
    https://lore.kernel.org/bpf/87eez4odqp.fsf@toke.dk/T/#m8d5c3e87ffe7f2764e02d722cb0d8cbc136880ed

This patch added clang support to emit debuginfo for extern variables
with a TargetInfo hook to enable it. The debuginfo for the
extern variable is emitted only if that extern variable is
referenced in the current compilation unit.

Currently, only BPF target enables to generate debug info for
extern variables. The emission of such debuginfo is disabled for C++
 at this moment since BPF only supports a subset of C language.
Emission with C++ can be enabled later if an appropriate use case
is identified.

-fstandalone-debug permits us to see more debuginfo with the cost
of bloated binary size. This patch did not add emission of extern
variable debug info with -fstandalone-debug. This can be
re-evaluated if there is a real need.

Differential Revision: https://reviews.llvm.org/D70696

include/llvm/IR/DIBuilder.h

@@ -583,7 +583,7 @@ namespace llvm {
     ///                    specified)
     DIGlobalVariableExpression *createGlobalVariableExpression(
         DIScope *Context, StringRef Name, StringRef LinkageName, DIFile *File,
-        unsigned LineNo, DIType *Ty, bool isLocalToUnit,
+        unsigned LineNo, DIType *Ty, bool isLocalToUnit, bool isDefined = true,
         DIExpression *Expr = nullptr, MDNode *Decl = nullptr,
         MDTuple *templateParams = nullptr, uint32_t AlignInBits = 0);
 

lib/IR/DIBuilder.cpp

@@ -640,13 +640,14 @@ static void checkGlobalVariableScope(DIScope *Context) {
 
 DIGlobalVariableExpression *DIBuilder::createGlobalVariableExpression(
     DIScope *Context, StringRef Name, StringRef LinkageName, DIFile *F,
-    unsigned LineNumber, DIType *Ty, bool isLocalToUnit, DIExpression *Expr,
+    unsigned LineNumber, DIType *Ty, bool isLocalToUnit,
+    bool isDefined, DIExpression *Expr,
     MDNode *Decl, MDTuple *templateParams, uint32_t AlignInBits) {
   checkGlobalVariableScope(Context);
 
   auto *GV = DIGlobalVariable::getDistinct(
       VMContext, cast_or_null<DIScope>(Context), Name, LinkageName, F,
-      LineNumber, Ty, isLocalToUnit, true, cast_or_null<DIDerivedType>(Decl),
+      LineNumber, Ty, isLocalToUnit, isDefined, cast_or_null<DIDerivedType>(Decl),
       templateParams, AlignInBits);
   if (!Expr)
     Expr = createExpression();

lib/IR/DebugInfo.cpp

@@ -1289,7 +1289,7 @@ LLVMMetadataRef LLVMDIBuilderCreateGlobalVariableExpression(
   return wrap(unwrap(Builder)->createGlobalVariableExpression(
       unwrapDI<DIScope>(Scope), {Name, NameLen}, {Linkage, LinkLen},
       unwrapDI<DIFile>(File), LineNo, unwrapDI<DIType>(Ty), LocalToUnit,
-      unwrap<DIExpression>(Expr), unwrapDI<MDNode>(Decl),
+      true, unwrap<DIExpression>(Expr), unwrapDI<MDNode>(Decl),
       nullptr, AlignInBits));
 }
 

unittests/Transforms/Utils/CloningTest.cpp

@@ -764,7 +764,7 @@ protected:
 
     DBuilder.createGlobalVariableExpression(
         Subprogram, "unattached", "unattached", File, 1,
-        DBuilder.createNullPtrType(), false, Expr);
+        DBuilder.createNullPtrType(), false, true, Expr);
 
     auto *Entry = BasicBlock::Create(C, "", F);
     IBuilder.SetInsertPoint(Entry);