[clang] [clang] Add clang::debug_info_type attribute for bitfields (PR #69104)
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Sun Oct 15 03:52:10 PDT 2023
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-debuginfo
Author: Vlad Serebrennikov (Endilll)
<details>
<summary>Changes</summary>
This attribute allows user to specify type of the bitfield that will be emitted to debug info without affecting semantics of the program. Since it doesn't affect semantics, this attribute can be safely ignored by other compilers.
This is useful when user is forced to use the same type for all bitfields in a class to get better [layout](https://godbolt.org/z/ovWqzqv9x) and [codegen](https://godbolt.org/z/bdoqvz9e6) from MSVC, because it allows debuggers to interpret the value of bitfield in the most human-friendly way (e.g. when value actually comes from an enum). This is driven by my work on LLDB formatters for Clang. I have two use cases for this:
```cpp
namespace Clang {
class Type {
enum TypeClass { ... };
struct TypeBitfields {
[[clang::debug_info_type(clang::Type::TypeClass)]] unsigned TC: 8;
[[clang::debug_info_type(bool)]] mutable unsigned FromAST : 1;
};
};
}
```
---
Full diff: https://github.com/llvm/llvm-project/pull/69104.diff
6 Files Affected:
- (modified) clang/include/clang/Basic/Attr.td (+11)
- (modified) clang/include/clang/Basic/AttrDocs.td (+12)
- (modified) clang/include/clang/Basic/DiagnosticSemaKinds.td (+2)
- (modified) clang/lib/CodeGen/CGDebugInfo.cpp (+2)
- (modified) clang/lib/Sema/SemaDeclAttr.cpp (+26)
- (added) clang/test/CodeGen/debug-info-debug-info-type.cpp (+14)
``````````diff
diff --git a/clang/include/clang/Basic/Attr.td b/clang/include/clang/Basic/Attr.td
index 5c9eb7b8a981037..024421c0583c019 100644
--- a/clang/include/clang/Basic/Attr.td
+++ b/clang/include/clang/Basic/Attr.td
@@ -107,6 +107,10 @@ def NonBitField : SubsetSubject<Field,
[{!S->isBitField()}],
"non-bit-field non-static data members">;
+def BitField : SubsetSubject<Field,
+ [{S->isBitField()}],
+ "bit-field non-static data members">;
+
def NonStaticCXXMethod : SubsetSubject<CXXMethod,
[{!S->isStatic()}],
"non-static member functions">;
@@ -4264,3 +4268,10 @@ def CountedBy : InheritableAttr {
void setCountedByFieldLoc(SourceRange Loc) { CountedByFieldLoc = Loc; }
}];
}
+
+def DebugInfoType: InheritableAttr {
+ let Spellings = [Clang<"debug_info_type">];
+ let Subjects = SubjectList<[BitField], ErrorDiag>;
+ let Args = [TypeArgument<"Type", 1>];
+ let Documentation = [DebugInfoTypeDocumentation];
+}
diff --git a/clang/include/clang/Basic/AttrDocs.td b/clang/include/clang/Basic/AttrDocs.td
index 9f9991bdae36155..6cceba1e0e0ad01 100644
--- a/clang/include/clang/Basic/AttrDocs.td
+++ b/clang/include/clang/Basic/AttrDocs.td
@@ -7219,6 +7219,18 @@ its underlying representation to be a WebAssembly ``funcref``.
}];
}
+def DebugInfoTypeDocumentation : Documentation {
+ let Category = DocCatField;
+ let Content = [{
+This attribute allows to alter type of a bitfield in debug information.
+Such a need might arise when bitfield is intended to store an enumeration value,
+but has to be specified as having enumeration's underlying type, in order to
+facilitate compiler optimizations. But this also causes underlying type to be
+emitted in debug information, making it hard for debuggers to map bitfield's
+value back to enumeration. This attribute helps with this.
+ }];
+}
+
def CleanupDocs : Documentation {
let Category = DocCatType;
let Content = [{
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index e85cd4d1a1ddc0d..b5c73494df367a6 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -3153,6 +3153,8 @@ def err_invalid_branch_protection_spec : Error<
"invalid or misplaced branch protection specification '%0'">;
def warn_unsupported_branch_protection_spec : Warning<
"unsupported branch protection specification '%0'">, InGroup<BranchProtection>;
+def warn_attribute_underlying_type_mismatch : Warning<
+ "underlying type %0 of enumeration %1 doesn't match bitfield type %2">;
def warn_unsupported_target_attribute
: Warning<"%select{unsupported|duplicate|unknown}0%select{| CPU|"
diff --git a/clang/lib/CodeGen/CGDebugInfo.cpp b/clang/lib/CodeGen/CGDebugInfo.cpp
index c73a63e12f03aab..85aedd87b21d41e 100644
--- a/clang/lib/CodeGen/CGDebugInfo.cpp
+++ b/clang/lib/CodeGen/CGDebugInfo.cpp
@@ -1497,6 +1497,8 @@ CGDebugInfo::createBitFieldType(const FieldDecl *BitFieldDecl,
llvm::DIScope *RecordTy, const RecordDecl *RD) {
StringRef Name = BitFieldDecl->getName();
QualType Ty = BitFieldDecl->getType();
+ if (BitFieldDecl->hasAttr<DebugInfoTypeAttr>())
+ Ty = BitFieldDecl->getAttr<DebugInfoTypeAttr>()->getType();
SourceLocation Loc = BitFieldDecl->getLocation();
llvm::DIFile *VUnit = getOrCreateFile(Loc);
llvm::DIType *DebugType = getOrCreateType(Ty, VUnit);
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
index feb02cad9080e3e..8d58968b7f985c8 100644
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -5910,6 +5910,28 @@ static void handleBuiltinAliasAttr(Sema &S, Decl *D,
D->addAttr(::new (S.Context) BuiltinAliasAttr(S.Context, AL, Ident));
}
+static void handleDebugInfoTypeAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
+ if (!AL.hasParsedType()) {
+ S.Diag(AL.getLoc(), diag::err_attribute_wrong_number_arguments) << AL << 1;
+ return;
+ }
+
+ TypeSourceInfo *ParmTSI = nullptr;
+ QualType type = S.GetTypeFromParser(AL.getTypeArg(), &ParmTSI);
+ assert(ParmTSI && "no type source info for attribute argument");
+
+ if (type->isEnumeralType()) {
+ QualType BitfieldType = llvm::cast<FieldDecl>(D)->getType();
+ QualType EnumUnderlyingType = type->getAs<EnumType>()->getDecl()->getIntegerType();
+ if (EnumUnderlyingType != BitfieldType) {
+ S.Diag(AL.getLoc(), diag::warn_attribute_underlying_type_mismatch) << EnumUnderlyingType << type << BitfieldType;
+ return;
+ }
+ }
+
+ D->addAttr(::new (S.Context) DebugInfoTypeAttr(S.Context, AL, ParmTSI));
+}
+
//===----------------------------------------------------------------------===//
// Checker-specific attribute handlers.
//===----------------------------------------------------------------------===//
@@ -9629,6 +9651,10 @@ ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D, const ParsedAttr &AL,
handleBuiltinAliasAttr(S, D, AL);
break;
+ case ParsedAttr::AT_DebugInfoType:
+ handleDebugInfoTypeAttr(S, D, AL);
+ break;
+
case ParsedAttr::AT_UsingIfExists:
handleSimpleAttribute<UsingIfExistsAttr>(S, D, AL);
break;
diff --git a/clang/test/CodeGen/debug-info-debug-info-type.cpp b/clang/test/CodeGen/debug-info-debug-info-type.cpp
new file mode 100644
index 000000000000000..4b60d1b64be239f
--- /dev/null
+++ b/clang/test/CodeGen/debug-info-debug-info-type.cpp
@@ -0,0 +1,14 @@
+// RUN: %clang -target x86_64-linux -g -S -emit-llvm -o - %s | FileCheck %s
+// RUN: %clang_cc1 -verify -DMISMATCH %s
+
+struct A {
+ enum E : unsigned {};
+ [[clang::debug_info_type(E)]] unsigned b : 2;
+#ifdef MISMATCH
+ [[clang::debug_info_type(E)]] int b2 : 2;
+ // expected-warning at -1 {{underlying type 'unsigned int' of enumeration 'E' doesn't match bitfield type 'int'}}
+#endif
+} a;
+
+// CHECK-DAG: [[ENUM:![0-9]+]] = !DICompositeType(tag: DW_TAG_enumeration_type, name: "E"{{.*}}
+// CHECK-DAG: !DIDerivedType(tag: DW_TAG_member, name: "b",{{.*}} baseType: [[ENUM]]
``````````
</details>
https://github.com/llvm/llvm-project/pull/69104
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