[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

clang/include/clang/AST/ASTConsumer.h

@@ -102,6 +102,11 @@ public:
   /// modified by the introduction of an implicit zero initializer.
   virtual void CompleteTentativeDefinition(VarDecl *D) {}
 
+  /// 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) {}
+
   /// Callback invoked when an MSInheritanceAttr has been attached to a
   /// CXXRecordDecl.
   virtual void AssignInheritanceModel(CXXRecordDecl *RD) {}

clang/include/clang/Basic/TargetInfo.h

@@ -1389,6 +1389,9 @@ public:
 
   virtual void setAuxTarget(const TargetInfo *Aux) {}
 
+  /// Whether target allows debuginfo types for decl only variables.
+  virtual bool allowDebugInfoForExternalVar() const { return false; }
+
 protected:
   /// Copy type and layout related info.
   void copyAuxTarget(const TargetInfo *Aux);

clang/include/clang/Sema/Sema.h

@@ -667,6 +667,9 @@ public:
   /// All the tentative definitions encountered in the TU.
   TentativeDefinitionsType TentativeDefinitions;
 
+  /// All the external declarations encoutered and used in the TU.
+  SmallVector<VarDecl *, 4> ExternalDeclarations;
+
   typedef LazyVector<const DeclaratorDecl *, ExternalSemaSource,
                      &ExternalSemaSource::ReadUnusedFileScopedDecls, 2, 2>
     UnusedFileScopedDeclsType;

clang/lib/Basic/Targets/BPF.h

@@ -76,6 +76,8 @@ public:
     return None;
   }
 
+  bool allowDebugInfoForExternalVar() const override { return true; }
+
   CallingConvCheckResult checkCallingConvention(CallingConv CC) const override {
     switch (CC) {
     default:

clang/lib/CodeGen/CGDebugInfo.cpp

@@ -4482,7 +4482,7 @@ void CGDebugInfo::EmitGlobalVariable(llvm::GlobalVariable *Var,
 
     GVE = DBuilder.createGlobalVariableExpression(
         DContext, DeclName, LinkageName, Unit, LineNo, getOrCreateType(T, Unit),
-        Var->hasLocalLinkage(),
+        Var->hasLocalLinkage(), true,
         Expr.empty() ? nullptr : DBuilder.createExpression(Expr),
         getOrCreateStaticDataMemberDeclarationOrNull(D), TemplateParameters,
         Align);
@@ -4585,10 +4585,29 @@ void CGDebugInfo::EmitGlobalVariable(const ValueDecl *VD, const APValue &Init) {
 
   GV.reset(DBuilder.createGlobalVariableExpression(
       DContext, Name, StringRef(), Unit, getLineNumber(VD->getLocation()), Ty,
-      true, InitExpr, getOrCreateStaticDataMemberDeclarationOrNull(VarD),
+      true, true, InitExpr, getOrCreateStaticDataMemberDeclarationOrNull(VarD),
       TemplateParameters, Align));
 }
 
+void CGDebugInfo::EmitExternalVariable(llvm::GlobalVariable *Var,
+                                       const VarDecl *D) {
+  assert(DebugKind >= codegenoptions::LimitedDebugInfo);
+  if (D->hasAttr<NoDebugAttr>())
+    return;
+
+  auto Align = getDeclAlignIfRequired(D, CGM.getContext());
+  llvm::DIFile *Unit = getOrCreateFile(D->getLocation());
+  StringRef Name = D->getName();
+  llvm::DIType *Ty = getOrCreateType(D->getType(), Unit);
+
+  llvm::DIScope *DContext = getDeclContextDescriptor(D);
+  llvm::DIGlobalVariableExpression *GVE =
+      DBuilder.createGlobalVariableExpression(
+          DContext, Name, StringRef(), Unit, getLineNumber(D->getLocation()),
+          Ty, false, false, nullptr, nullptr, nullptr, Align);
+  Var->addDebugInfo(GVE);
+}
+
 llvm::DIScope *CGDebugInfo::getCurrentContextDescriptor(const Decl *D) {
   if (!LexicalBlockStack.empty())
     return LexicalBlockStack.back();

clang/lib/CodeGen/CGDebugInfo.h

@@ -478,6 +478,9 @@ public:
   /// Emit a constant global variable's debug info.
   void EmitGlobalVariable(const ValueDecl *VD, const APValue &Init);
 
+  /// Emit information about an external variable.
+  void EmitExternalVariable(llvm::GlobalVariable *GV, const VarDecl *Decl);
+
   /// Emit C++ using directive.
   void EmitUsingDirective(const UsingDirectiveDecl &UD);
 

clang/lib/CodeGen/CodeGenAction.cpp

@@ -336,6 +336,10 @@ namespace clang {
       Gen->CompleteTentativeDefinition(D);
     }
 
+    void CompleteExternalDeclaration(VarDecl *D) override {
+      Gen->CompleteExternalDeclaration(D);
+    }
+
     void AssignInheritanceModel(CXXRecordDecl *RD) override {
       Gen->AssignInheritanceModel(RD);
     }

clang/lib/CodeGen/CodeGenModule.cpp

@@ -3715,6 +3715,10 @@ void CodeGenModule::EmitTentativeDefinition(const VarDecl *D) {
   EmitGlobalVarDefinition(D);
 }
 
+void CodeGenModule::EmitExternalDeclaration(const VarDecl *D) {
+  EmitExternalVarDeclaration(D);
+}
+
 CharUnits CodeGenModule::GetTargetTypeStoreSize(llvm::Type *Ty) const {
   return Context.toCharUnitsFromBits(
       getDataLayout().getTypeStoreSizeInBits(Ty));
@@ -4098,6 +4102,19 @@ void CodeGenModule::EmitGlobalVarDefinition(const VarDecl *D,
       DI->EmitGlobalVariable(GV, D);
 }
 
+void CodeGenModule::EmitExternalVarDeclaration(const VarDecl *D) {
+  if (CGDebugInfo *DI = getModuleDebugInfo())
+    if (getCodeGenOpts().getDebugInfo() >= codegenoptions::LimitedDebugInfo) {
+      QualType ASTTy = D->getType();
+      llvm::Type *Ty = getTypes().ConvertTypeForMem(D->getType());
+      llvm::PointerType *PTy =
+          llvm::PointerType::get(Ty, getContext().getTargetAddressSpace(ASTTy));
+      llvm::Constant *GV = GetOrCreateLLVMGlobal(D->getName(), PTy, D);
+      DI->EmitExternalVariable(
+          cast<llvm::GlobalVariable>(GV->stripPointerCasts()), D);
+    }
+}
+
 static bool isVarDeclStrongDefinition(const ASTContext &Context,
                                       CodeGenModule &CGM, const VarDecl *D,
                                       bool NoCommon) {

clang/lib/CodeGen/CodeGenModule.h

@@ -1165,6 +1165,8 @@ public:
 
   void EmitTentativeDefinition(const VarDecl *D);
 
+  void EmitExternalDeclaration(const VarDecl *D);
+
   void EmitVTable(CXXRecordDecl *Class);
 
   void RefreshTypeCacheForClass(const CXXRecordDecl *Class);
@@ -1400,6 +1402,7 @@ private:
   void EmitMultiVersionFunctionDefinition(GlobalDecl GD, llvm::GlobalValue *GV);
 
   void EmitGlobalVarDefinition(const VarDecl *D, bool IsTentative = false);
+  void EmitExternalVarDeclaration(const VarDecl *D);
   void EmitAliasDefinition(GlobalDecl GD);
   void emitIFuncDefinition(GlobalDecl GD);
   void emitCPUDispatchDefinition(GlobalDecl GD);

clang/lib/CodeGen/ModuleBuilder.cpp

@@ -290,6 +290,10 @@ namespace {
       Builder->EmitTentativeDefinition(D);
     }
 
+    void CompleteExternalDeclaration(VarDecl *D) override {
+      Builder->EmitExternalDeclaration(D);
+    }
+
     void HandleVTable(CXXRecordDecl *RD) override {
       if (Diags.hasErrorOccurred())
         return;

clang/lib/Sema/Sema.cpp

@@ -1137,6 +1137,13 @@ void Sema::ActOnEndOfTranslationUnit() {
       Consumer.CompleteTentativeDefinition(VD);
   }
 
+  for (auto D : ExternalDeclarations) {
+    if (!D || D->isInvalidDecl() || D->getPreviousDecl() || !D->isUsed())
+      continue;
+
+    Consumer.CompleteExternalDeclaration(D);
+  }
+
   // If there were errors, disable 'unused' warnings since they will mostly be
   // noise. Don't warn for a use from a module: either we should warn on all
   // file-scope declarations in modules or not at all, but whether the

clang/lib/Sema/SemaDecl.cpp

@@ -12200,6 +12200,10 @@ void Sema::ActOnUninitializedDecl(Decl *RealDecl) {
         Diag(Var->getLocation(), diag::note_private_extern);
       }
 
+      if (Context.getTargetInfo().allowDebugInfoForExternalVar() &&
+          !Var->isInvalidDecl() && !getLangOpts().CPlusPlus)
+        ExternalDeclarations.push_back(Var);
+
       return;
 
     case VarDecl::TentativeDefinition:

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

@@ -0,0 +1,27 @@
+// REQUIRES: bpf-registered-target
+// RUN: %clang -target bpf -emit-llvm -S -g %s -o - | FileCheck %s
+
+extern char ch;
+int test() {
+  return ch;
+}
+
+int test2() {
+  extern char ch2;
+  return ch2;
+}
+
+extern int (*foo)(int);
+int test3() {
+  return foo(0);
+}
+
+// CHECK: distinct !DIGlobalVariable(name: "ch",{{.*}} type: ![[CHART:[0-9]+]], isLocal: false, isDefinition: false
+// CHECK: distinct !DIGlobalVariable(name: "ch2",{{.*}} type: ![[CHART]], isLocal: false, isDefinition: false
+// CHECK: ![[CHART]] = !DIBasicType(name: "char", size: 8, encoding: DW_ATE_signed_char)
+
+// CHECK: distinct !DIGlobalVariable(name: "foo",{{.*}} type: ![[FUNC:[0-9]+]], isLocal: false, isDefinition: false)
+// CHECK: ![[FUNC]] = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: ![[SUB:[0-9]+]], size: 64)
+// CHECK: ![[SUB]] = !DISubroutineType(types: ![[TYPES:[0-9]+]])
+// CHECK: ![[TYPES]] = !{![[BASET:[0-9]+]], ![[BASET]]}
+// CHECK: ![[BASET]] = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)

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

@@ -0,0 +1,11 @@
+// REQUIRES: bpf-registered-target
+// RUN: %clang -target bpf -emit-llvm -S -g %s -o - | FileCheck %s
+
+extern char ch;
+extern char ch;
+int test() {
+  return ch;
+}
+
+// CHECK: distinct !DIGlobalVariable(name: "ch",{{.*}} type: ![[T:[0-9]+]], isLocal: false, isDefinition: false
+// CHECK-NOT: distinct !DIGlobalVariable(name: "ch"

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

@@ -0,0 +1,23 @@
+// REQUIRES: bpf-registered-target
+// RUN: %clang -target bpf -emit-llvm -S -g %s -o - | FileCheck %s
+
+extern char ch;
+int test() {
+  extern short sh;
+  return ch + sh;
+}
+
+extern char (*foo)(char);
+int test2() {
+  return foo(0) + ch;
+}
+
+// CHECK: distinct !DIGlobalVariable(name: "ch",{{.*}} type: ![[Tch:[0-9]+]], isLocal: false, isDefinition: false
+// CHECK: distinct !DIGlobalVariable(name: "sh",{{.*}} type: ![[Tsh:[0-9]+]], isLocal: false, isDefinition: false
+// CHECK: ![[Tsh]] = !DIBasicType(name: "short", size: 16, encoding: DW_ATE_signed)
+
+// CHECK: distinct !DIGlobalVariable(name: "foo",{{.*}} type: ![[Tptr:[0-9]+]], isLocal: false, isDefinition: false
+// ![[Tptr]] = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: ![[Tsub:[0-9]+]], size: 64)
+// ![[Tsub]] = !DISubroutineType(types: ![[Tproto:[0-9]+]])
+// ![[Tproto]] = !{![[Tch]], ![[Tch]]}
+// CHECK: ![[Tch]] = !DIBasicType(name: "char", size: 8, encoding: DW_ATE_signed_char)

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

@@ -0,0 +1,27 @@
+// REQUIRES: bpf-registered-target
+// RUN: %clang -target bpf -emit-llvm -S -g %s -o - | FileCheck %s
+
+extern char ch;
+int test() {
+  return 0;
+}
+
+int test2() {
+  extern char ch2;
+  return 0;
+}
+
+extern int (*foo)(int);
+int test3() {
+  return 0;
+}
+
+int test4() {
+  extern int (*foo2)(int);
+  return 0;
+}
+
+// CHECK-NOT: distinct !DIGlobalVariable(name: "ch"
+// CHECK-NOT: distinct !DIGlobalVariable(name: "ch2"
+// CHECK-NOT: distinct !DIGlobalVariable(name: "foo"
+// CHECK-NOT: distinct !DIGlobalVariable(name: "foo2"

llvm/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);
 

llvm/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();

llvm/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));
 }
 

llvm/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);