[clang] Add "transparent_stepping" attribute

The `transparent_stepping` attribute is intended as a hint for
debuggers that this function itself is not interesting, but it calls a
function that might be.  So, when stepping in arrives at a function
with this attribute, debuggers should transparently step-in through it
into the functions called by the annotated function (but not by
subsequent calls made by those functions), stopping at the first one its
normal rules for whether to stop says to stop at - or stepping out again
if none qualify. Also, when stepping out arrives at a function with this
attribute, the debugger should continue stepping out to its caller.

Differential Revision: https://reviews.llvm.org/D146595

Author: augusto2112

clang/include/clang/Basic/Attr.td

@@ -771,6 +771,12 @@ def Artificial : InheritableAttr {
   let SimpleHandler = 1;
 }
 
+def TransparentStepping: InheritableAttr {
+  let Spellings = [Clang<"transparent_stepping">];
+  let Subjects = SubjectList<[Function]>;
+  let Documentation = [TransparentSteppingDocs];
+}
+
 def XRayInstrument : InheritableAttr {
   let Spellings = [Clang<"xray_always_instrument">,
                    Clang<"xray_never_instrument">];

clang/include/clang/Basic/AttrDocs.td

@@ -6900,6 +6900,66 @@ As such, this function attribute is currently only supported on X86 targets.
   }];
 }
 
+def TransparentSteppingDocs : Documentation {
+  let Category = DocCatFunction;
+  let Content = [{
+The ``transparent_stepping`` attribute is intended as a hint for debuggers that this 
+function itself is not interesting, but it calls a function that might be.  So, when 
+stepping in arrives at a function with this attribute, debuggers should transparently 
+step-in through it into the functions called by the annotated function (but not by 
+subsequent calls made by those functions), stopping at the first one its normal rules 
+for whether to stop says to stop at - or stepping out again if none qualify. Also, when 
+stepping out arrives at a function with this attribute, the debugger should continue 
+stepping out to its caller.
+
+For example:
+
+.. code-block:: c
+
+  int bar(void) {
+    return 42;
+  }
+
+  __attribute__((transparent_stepping))
+  int foo(void) {
+    return bar();
+  }
+
+  int caller(void) {
+    return foo();
+  }
+
+Stepping into ``foo`` should step directly into ``bar`` instead, and stepping out of ``bar`` 
+should stop in ``caller``.
+
+Functions with the ``transparent_stepping`` attribute can be chained together:
+
+.. code-block:: c
+
+  int baz(void) {
+    return 42;
+  }
+
+  __attribute__((transparent_stepping))
+  int bar(void) {
+    return baz();
+  }
+
+  __attribute__((transparent_stepping))
+  int foo(void) {
+    return bar();
+  }
+  
+  int caller(void) {
+    return foo();
+  }
+
+In this example, stepping into ``foo`` should step directly into ``baz``, and stepping out of 
+``baz`` should stop in ``caller``.
+  }];
+}
+
+
 def ReadOnlyPlacementDocs : Documentation {
   let Category = DocCatType;
   let Content = [{This attribute is attached to a structure, class or union declaration.

clang/lib/CodeGen/CGDebugInfo.cpp

@@ -69,6 +69,12 @@ static uint32_t getDeclAlignIfRequired(const Decl *D, const ASTContext &Ctx) {
   return D->hasAttr<AlignedAttr>() ? D->getMaxAlignment() : 0;
 }
 
+static bool getIsTransparentStepping(const Decl *D) {
+  if (!D)
+    return false;
+  return D->hasAttr<TransparentSteppingAttr>();
+}
+
 CGDebugInfo::CGDebugInfo(CodeGenModule &CGM)
     : CGM(CGM), DebugKind(CGM.getCodeGenOpts().getDebugInfo()),
       DebugTypeExtRefs(CGM.getCodeGenOpts().DebugTypeExtRefs),
@@ -1987,6 +1993,8 @@ llvm::DISubprogram *CGDebugInfo::CreateCXXMemberFunction(
     SPFlags |= llvm::DISubprogram::SPFlagLocalToUnit;
   if (CGM.getLangOpts().Optimize)
     SPFlags |= llvm::DISubprogram::SPFlagOptimized;
+  if (getIsTransparentStepping(Method))
+    SPFlags |= llvm::DISubprogram::SPFlagIsTransparentStepping;
 
   // In this debug mode, emit type info for a class when its constructor type
   // info is emitted.
@@ -3928,6 +3936,8 @@ llvm::DISubprogram *CGDebugInfo::getFunctionFwdDeclOrStub(GlobalDecl GD,
   if (Stub) {
     Flags |= getCallSiteRelatedAttrs();
     SPFlags |= llvm::DISubprogram::SPFlagDefinition;
+    if (getIsTransparentStepping(FD))
+      SPFlags |= llvm::DISubprogram::SPFlagIsTransparentStepping;
     return DBuilder.createFunction(
         DContext, Name, LinkageName, Unit, Line,
         getOrCreateFunctionType(GD.getDecl(), FnType, Unit), 0, Flags, SPFlags,
@@ -4077,6 +4087,8 @@ llvm::DISubprogram *CGDebugInfo::getObjCMethodDeclaration(
   if (It == TypeCache.end())
     return nullptr;
   auto *InterfaceType = cast<llvm::DICompositeType>(It->second);
+  if (getIsTransparentStepping(D))
+    SPFlags |= llvm::DISubprogram::SPFlagIsTransparentStepping;
   llvm::DISubprogram *FD = DBuilder.createFunction(
       InterfaceType, getObjCMethodName(OMD), StringRef(),
       InterfaceType->getFile(), LineNo, FnType, LineNo, Flags, SPFlags);
@@ -4244,6 +4256,8 @@ void CGDebugInfo::emitFunctionStart(GlobalDecl GD, SourceLocation Loc,
     SPFlags |= llvm::DISubprogram::SPFlagLocalToUnit;
   if (CGM.getLangOpts().Optimize)
     SPFlags |= llvm::DISubprogram::SPFlagOptimized;
+  if (getIsTransparentStepping(D))
+    SPFlags |= llvm::DISubprogram::SPFlagIsTransparentStepping;
 
   llvm::DINode::DIFlags FlagsForDef = Flags | getCallSiteRelatedAttrs();
   llvm::DISubprogram::DISPFlags SPFlagsForDef =
@@ -4330,6 +4344,9 @@ void CGDebugInfo::EmitFunctionDecl(GlobalDecl GD, SourceLocation Loc,
 
   llvm::DINodeArray Annotations = CollectBTFDeclTagAnnotations(D);
   llvm::DISubroutineType *STy = getOrCreateFunctionType(D, FnType, Unit);
+  if (getIsTransparentStepping(D))
+    SPFlags |= llvm::DISubprogram::SPFlagIsTransparentStepping;
+
   llvm::DISubprogram *SP = DBuilder.createFunction(
       FDContext, Name, LinkageName, Unit, LineNo, STy, ScopeLine, Flags,
       SPFlags, TParamsArray.get(), nullptr, nullptr, Annotations);

clang/lib/Sema/SemaDeclAttr.cpp

@@ -6773,6 +6773,12 @@ static void handleSwiftAsyncName(Sema &S, Decl *D, const ParsedAttr &AL) {
   D->addAttr(::new (S.Context) SwiftAsyncNameAttr(S.Context, AL, Name));
 }
 
+static void handleTransparentStepping(Sema &S, Decl *D,
+                                          const ParsedAttr &AL) {
+  D->addAttr(::new (S.Context)
+                 TransparentSteppingAttr(S.Context, AL));
+}
+
 static void handleSwiftNewType(Sema &S, Decl *D, const ParsedAttr &AL) {
   // Make sure that there is an identifier as the annotation's single argument.
   if (!AL.checkExactlyNumArgs(S, 1))
@@ -9085,6 +9091,9 @@ ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D, const ParsedAttr &AL,
   case ParsedAttr::AT_NoDebug:
     handleNoDebugAttr(S, D, AL);
     break;
+  case ParsedAttr::AT_TransparentStepping:
+    handleTransparentStepping(S, D, AL);
+    break;
   case ParsedAttr::AT_CmseNSEntry:
     handleCmseNSEntryAttr(S, D, AL);
     break;

clang/test/CodeGen/attr-transparent-stepping-method.cpp

@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 -debug-info-kind=limited -emit-llvm -o - %s | FileCheck %s
+
+void bar(void) {}
+
+struct A {
+[[clang::transparent_stepping()]]
+void foo(void) {
+  bar();
+}
+};
+
+int main() {
+  A().foo();
+}
+
+// CHECK: DISubprogram(name: "foo"{{.*}} DISPFlagIsTransparentStepping

clang/test/CodeGen/attr-transparent-stepping.c

@@ -0,0 +1,10 @@
+// RUN: %clang_cc1 -debug-info-kind=limited -emit-llvm -o - %s | FileCheck %s
+
+void bar(void) {}
+
+__attribute__((transparent_stepping))
+void foo(void) {
+  bar();
+}
+
+// CHECK: DISubprogram(name: "foo"{{.*}} DISPFlagIsTransparentStepping

clang/test/Misc/pragma-attribute-supported-attributes-list.test

@@ -186,6 +186,7 @@
 // CHECK-NEXT: TargetClones (SubjectMatchRule_function)
 // CHECK-NEXT: TargetVersion (SubjectMatchRule_function)
 // CHECK-NEXT: TestTypestate (SubjectMatchRule_function_is_member)
+// CHECK-NEXT: TransparentStepping (SubjectMatchRule_function)
 // CHECK-NEXT: TrivialABI (SubjectMatchRule_record)
 // CHECK-NEXT: Uninitialized (SubjectMatchRule_variable_is_local)
 // CHECK-NEXT: UnsafeBufferUsage (SubjectMatchRule_function)

clang/test/Sema/attr-transparent-stepping.c

@@ -0,0 +1,7 @@
+// RUN: %clang_cc1 %s -verify -fsyntax-only
+
+__attribute__((transparent_stepping))
+void correct(void) {}
+
+__attribute__((transparent_stepping(1))) // expected-error {{'transparent_stepping' attribute takes no arguments}}
+void wrong_arg(void) {}

clang/test/SemaCXX/attr-transparent-stepping-method.cpp

@@ -0,0 +1,11 @@
+// RUN: %clang_cc1 %s -verify -fsyntax-only
+
+
+struct S {
+[[clang::transparent_stepping]]
+void correct(void) {}
+
+[[clang::transparent_stepping(1)]] // expected-error {{'transparent_stepping' attribute takes no arguments}}
+void one_arg(void) {}
+};
+

llvm/include/llvm/IR/DebugInfoFlags.def

@@ -91,11 +91,12 @@ HANDLE_DISP_FLAG((1u << 8), MainSubprogram)
 // for defaulted functions
 HANDLE_DISP_FLAG((1u << 9), Deleted)
 HANDLE_DISP_FLAG((1u << 11), ObjCDirect)
+HANDLE_DISP_FLAG((1u << 12), IsTransparentStepping)
 
 #ifdef DISP_FLAG_LARGEST_NEEDED
 // Intended to be used with ADT/BitmaskEnum.h.
 // NOTE: Always must be equal to largest flag, check this when adding new flags.
-HANDLE_DISP_FLAG((1 << 11), Largest)
+HANDLE_DISP_FLAG((1 << 12), Largest)
 #undef DISP_FLAG_LARGEST_NEEDED
 #endif