[clang] 1760eff - [clang] Implement gcc_struct attribute on Itanium targets (#71148)
via cfe-commits
cfe-commits at lists.llvm.org
Fri Dec 12 09:22:45 PST 2025
Author: Dan Klishch
Date: 2025-12-12T19:22:40+02:00
New Revision: 1760effa337471d9944e26694bdcb7a60139a124
URL: https://github.com/llvm/llvm-project/commit/1760effa337471d9944e26694bdcb7a60139a124
DIFF: https://github.com/llvm/llvm-project/commit/1760effa337471d9944e26694bdcb7a60139a124.diff
LOG: [clang] Implement gcc_struct attribute on Itanium targets (#71148)
This commit implements gcc_struct attribute with the behavior similar to
one in GCC. Current behavior is as follows:
When ItaniumRecordLayoutBuilder is used, [[gcc_struct]] will locally
cancel the effect of -mms-bitfields on a record. If -mms-bitfields is
not supplied and is not a default behavior on a target, [[gcc_struct]]
will be a no-op. This should provide enough compatibility with GCC.
If C++ ABI is "Microsoft", [[gcc_struct]] will currently always produce
a diagnostic, since support for it is not yet implemented in
MicrosoftRecordLayoutBuilder. Note, however, that all the infrastructure
is ready for the future implementation.
In particular, check for default value of -mms-bitfields is moved from a
driver to ASTContext, since it now non-trivially depends on other
supplied flags. This also, unfortunately, makes it impossible to use
usual argument parsing for `-m{no-,}ms-bitfields`.
The patch doesn't introduce any backwards-incompatible changes, except
for situations when cc1 is called directly with `-mms-bitfields` option.
Work towards #24757
---------
Co-authored-by: Martin Storsjö <martin at martin.st>
Added:
clang/test/CodeGen/gcc_struct-msvc-todo-1.c
clang/test/CodeGen/gcc_struct-msvc-todo-2.c
clang/test/CodeGen/gcc_struct.c
clang/test/Sema/gcc_ms_struct.c
Modified:
clang/docs/ReleaseNotes.rst
clang/include/clang/AST/ASTContext.h
clang/include/clang/Basic/Attr.td
clang/include/clang/Basic/AttrDocs.td
clang/include/clang/Basic/DiagnosticASTKinds.td
clang/include/clang/Basic/LangOptions.def
clang/include/clang/Basic/LangOptions.h
clang/include/clang/Options/Options.td
clang/lib/AST/Decl.cpp
clang/lib/AST/RecordLayoutBuilder.cpp
clang/lib/Driver/ToolChains/Clang.cpp
clang/lib/Sema/SemaDecl.cpp
clang/lib/Sema/SemaDeclAttr.cpp
clang/lib/Sema/SemaDeclCXX.cpp
clang/test/CodeGen/mingw-long-double.c
clang/test/CodeGen/mms-bitfields.c
clang/test/Driver/ms-bitfields.c
clang/test/Layout/itanium-union-bitfield.cpp
clang/test/Misc/pragma-attribute-supported-attributes-list.test
clang/test/Sema/mms-bitfields.c
clang/test/SemaCXX/ms_struct-bitfield-padding.cpp
clang/test/SemaCXX/ms_struct.cpp
clang/test/SemaCXX/ms_wide_bitfield.cpp
clang/utils/TableGen/ClangDiagnosticsEmitter.cpp
Removed:
################################################################################
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 22ca79d6adc28..feaf92ad4415f 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -379,6 +379,12 @@ Attribute Changes in Clang
multilibs for printf features. Requires cooperation with the libc
implementation.
+- On targets with Itanium C++ ABI, Clang now supports ``[[gnu:gcc_struct]]``
+ with the behavior similar to one existing in GCC. In particular, whenever
+ ``-mms-bitfields`` command line option is provided (or if Microsoft-compatible
+ structure layout is default on the target), ``[[gnu::gcc_struct]]`` requests
+ the compiler to follow Itanium rules for the layout of an annotated structure.
+
Improvements to Clang's diagnostics
-----------------------------------
- Diagnostics messages now refer to ``structured binding`` instead of ``decomposition``,
diff --git a/clang/include/clang/AST/ASTContext.h b/clang/include/clang/AST/ASTContext.h
index e2e78c4a1c4b0..05926fbd5bd71 100644
--- a/clang/include/clang/AST/ASTContext.h
+++ b/clang/include/clang/AST/ASTContext.h
@@ -2810,6 +2810,10 @@ class ASTContext : public RefCountedBase<ASTContext> {
/// runtime, such as those using the Itanium C++ ABI.
CharUnits getExnObjectAlignment() const;
+ /// Return whether unannotated records are treated as if they have
+ /// [[gnu::ms_struct]].
+ bool defaultsToMsStruct() const;
+
/// Get or compute information about the layout of the specified
/// record (struct/union/class) \p D, which indicates its size and field
/// position information.
diff --git a/clang/include/clang/Basic/Attr.td b/clang/include/clang/Basic/Attr.td
index d7e90a28347e5..0b006142cbb74 100644
--- a/clang/include/clang/Basic/Attr.td
+++ b/clang/include/clang/Basic/Attr.td
@@ -4387,8 +4387,13 @@ def CFGuard : InheritableAttr, TargetSpecificAttr<TargetWindows> {
def MSStruct : InheritableAttr {
let Spellings = [GCC<"ms_struct">];
let Subjects = SubjectList<[Record]>;
- let Documentation = [Undocumented];
- let SimpleHandler = 1;
+ let Documentation = [MSStructDocs];
+}
+
+def GCCStruct : InheritableAttr {
+ let Spellings = [GCC<"gcc_struct">];
+ let Subjects = SubjectList<[Record]>;
+ let Documentation = [MSStructDocs]; // Covers this attribute too.
}
def DLLExport : InheritableAttr, TargetSpecificAttr<TargetHasDLLImportExport> {
diff --git a/clang/include/clang/Basic/AttrDocs.td b/clang/include/clang/Basic/AttrDocs.td
index ae929c7dea37d..ec20f7ed93eb8 100644
--- a/clang/include/clang/Basic/AttrDocs.td
+++ b/clang/include/clang/Basic/AttrDocs.td
@@ -9682,3 +9682,24 @@ The following aspects are currently supported:
- ``float``: The call has a floating point argument
}];
}
+
+def MSStructDocs : Documentation {
+ let Category = DocCatDecl;
+ let Content = [{
+The ``ms_struct`` and ``gcc_struct`` attributes request the compiler to enter a
+special record layout compatibility mode which mimics the layout of Microsoft or
+Itanium C++ ABI respectively. Obviously, if the current C++ ABI matches the
+requested ABI, the attribute does nothing. However, if it does not, annotated
+structure or class is laid out in a special compatibility mode, which slightly
+changes offsets for fields and bit-fields. The intention is to match the layout
+of the requested ABI for structures which only use C features.
+
+Note that the default behavior can be controlled by ``-mms-bitfields`` and
+``-mno-ms-bitfields`` switches and via ``#pragma ms_struct``.
+
+The primary
diff erence is for bitfields, where the MS variant only packs
+adjacent fields into the same allocation unit if they have integral types
+of the same size, while the GCC/Itanium variant packs all fields in a bitfield
+tightly.
+ }];
+}
diff --git a/clang/include/clang/Basic/DiagnosticASTKinds.td b/clang/include/clang/Basic/DiagnosticASTKinds.td
index 5c462f9646b3b..e90ee9d376e07 100644
--- a/clang/include/clang/Basic/DiagnosticASTKinds.td
+++ b/clang/include/clang/Basic/DiagnosticASTKinds.td
@@ -1040,6 +1040,9 @@ def warn_npot_ms_struct : Warning<
"data types with sizes that aren't a power of two">,
DefaultError, InGroup<IncompatibleMSStruct>;
+def err_itanium_layout_unimplemented : Error<
+ "Itanium-compatible layout for the Microsoft C++ ABI is not yet supported">;
+
// -Wpadded-bitfield
def warn_padded_struct_bitfield : Warning<
"padding %select{struct|interface|class}0 %1 with %2 "
diff --git a/clang/include/clang/Basic/LangOptions.def b/clang/include/clang/Basic/LangOptions.def
index 891c9c05afc87..8cba1dbaee24e 100644
--- a/clang/include/clang/Basic/LangOptions.def
+++ b/clang/include/clang/Basic/LangOptions.def
@@ -104,7 +104,9 @@ LANGOPT(AssumeNothrowExceptionDtor , 1, 0, NotCompatible, "Assume exception obje
LANGOPT(TraditionalCPP , 1, 0, NotCompatible, "traditional CPP emulation")
LANGOPT(RTTI , 1, 1, NotCompatible, "run-time type information")
LANGOPT(RTTIData , 1, 1, NotCompatible, "emit run-time type information data")
-LANGOPT(MSBitfields , 1, 0, NotCompatible, "Microsoft-compatible structure layout")
+ENUM_LANGOPT(LayoutCompatibility, LayoutCompatibilityKind, 2,
+ LayoutCompatibilityKind::Default, NotCompatible,
+ "Microsoft-compatible structure layout")
LANGOPT(MSVolatile , 1, 0, NotCompatible, "Microsoft-compatible volatile loads and stores")
LANGOPT(Freestanding , 1, 0, NotCompatible, "freestanding implementation")
LANGOPT(NoBuiltin , 1, 0, NotCompatible, "disable builtin functions")
diff --git a/clang/include/clang/Basic/LangOptions.h b/clang/include/clang/Basic/LangOptions.h
index 859ed57996983..4fa2dcffc75b6 100644
--- a/clang/include/clang/Basic/LangOptions.h
+++ b/clang/include/clang/Basic/LangOptions.h
@@ -421,6 +421,16 @@ class LangOptionsBase {
None,
};
+ enum class LayoutCompatibilityKind {
+ /// Use default layout rules of the target.
+ Default = 0,
+ /// Use Itanium rules for bit-field layout and fundamental types alignment.
+ Itanium = 1,
+ /// Use Microsoft C++ ABI rules for bit-field layout and fundamental types
+ /// alignment.
+ Microsoft = 2,
+ };
+
// Define simple language options (with no accessors).
#define LANGOPT(Name, Bits, Default, Compatibility, Description) \
unsigned Name : Bits;
diff --git a/clang/include/clang/Options/Options.td b/clang/include/clang/Options/Options.td
index 0c99ca54c2274..45c5322351a17 100644
--- a/clang/include/clang/Options/Options.td
+++ b/clang/include/clang/Options/Options.td
@@ -5219,9 +5219,7 @@ def : Joined<["-"], "mmacosx-version-min=">,
Visibility<[ClangOption, CC1Option, FC1Option, FlangOption]>,
Group<m_Group>, Alias<mmacos_version_min_EQ>;
def mms_bitfields : Flag<["-"], "mms-bitfields">, Group<m_Group>,
- Visibility<[ClangOption, CC1Option]>,
- HelpText<"Set the default structure layout to be compatible with the Microsoft compiler standard">,
- MarshallingInfoFlag<LangOpts<"MSBitfields">>;
+ HelpText<"Set the default structure layout to be compatible with the Microsoft compiler standard">;
def moutline : Flag<["-"], "moutline">, Group<f_clang_Group>,
Visibility<[ClangOption, CC1Option]>,
HelpText<"Enable function outlining (AArch64 only)">;
@@ -5230,6 +5228,12 @@ def mno_outline : Flag<["-"], "mno-outline">, Group<f_clang_Group>,
HelpText<"Disable function outlining (AArch64 only)">;
def mno_ms_bitfields : Flag<["-"], "mno-ms-bitfields">, Group<m_Group>,
HelpText<"Do not set the default structure layout to be compatible with the Microsoft compiler standard">;
+def fms_layout_compatibility_EQ : Joined<["-"], "fms-layout-compatibility=">,
+ Visibility<[CC1Option]>,
+ HelpText<"Structure layout compatibility with Microsoft C++ ABI">,
+ Values<"default,itanium,microsoft">,
+ NormalizedValues<["Default", "Itanium", "Microsoft"]>, NormalizedValuesScope<"LangOptions::LayoutCompatibilityKind">,
+ MarshallingInfoEnum<LangOpts<"LayoutCompatibility">, "Default">;
def mskip_rax_setup : Flag<["-"], "mskip-rax-setup">, Group<m_Group>,
Visibility<[ClangOption, CC1Option]>,
HelpText<"Skip setting up RAX register when passing variable arguments (x86 only)">,
diff --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
index 4444b10db8cc0..b12f042646bbb 100644
--- a/clang/lib/AST/Decl.cpp
+++ b/clang/lib/AST/Decl.cpp
@@ -5244,7 +5244,14 @@ void RecordDecl::completeDefinition() {
/// This which can be turned on with an attribute, pragma, or the
/// -mms-bitfields command-line option.
bool RecordDecl::isMsStruct(const ASTContext &C) const {
- return hasAttr<MSStructAttr>() || C.getLangOpts().MSBitfields == 1;
+ if (hasAttr<GCCStructAttr>())
+ return false;
+ if (hasAttr<MSStructAttr>())
+ return true;
+ auto LayoutCompatibility = C.getLangOpts().getLayoutCompatibility();
+ if (LayoutCompatibility == LangOptions::LayoutCompatibilityKind::Default)
+ return C.defaultsToMsStruct();
+ return LayoutCompatibility == LangOptions::LayoutCompatibilityKind::Microsoft;
}
void RecordDecl::reorderDecls(const SmallVectorImpl<Decl *> &Decls) {
diff --git a/clang/lib/AST/RecordLayoutBuilder.cpp b/clang/lib/AST/RecordLayoutBuilder.cpp
index 5d8f54f111ab2..36b8eea92c0a4 100644
--- a/clang/lib/AST/RecordLayoutBuilder.cpp
+++ b/clang/lib/AST/RecordLayoutBuilder.cpp
@@ -2794,6 +2794,13 @@ void MicrosoftRecordLayoutBuilder::initializeLayout(const RecordDecl *RD) {
UseExternalLayout = Source->layoutRecordType(
RD, External.Size, External.Align, External.FieldOffsets,
External.BaseOffsets, External.VirtualBaseOffsets);
+
+ if (!RD->isMsStruct(Context)) {
+ auto Location = RD->getLocation();
+ if (Location.isValid())
+ Context.getDiagnostics().Report(Location,
+ diag::err_itanium_layout_unimplemented);
+ }
}
void
@@ -3358,6 +3365,11 @@ void MicrosoftRecordLayoutBuilder::computeVtorDispSet(
}
}
+bool ASTContext::defaultsToMsStruct() const {
+ return getTargetInfo().hasMicrosoftRecordLayout() ||
+ getTargetInfo().getTriple().isWindowsGNUEnvironment();
+}
+
/// getASTRecordLayout - Get or compute information about the layout of the
/// specified record (struct/union/class), which indicates its size and field
/// position information.
diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index 687b110b53bb0..310f3b58a211e 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -5889,9 +5889,12 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
if (KernelOrKext && RawTriple.isOSDarwin())
CmdArgs.push_back("-fforbid-guard-variables");
- if (Args.hasFlag(options::OPT_mms_bitfields, options::OPT_mno_ms_bitfields,
- Triple.isWindowsGNUEnvironment())) {
- CmdArgs.push_back("-mms-bitfields");
+ if (Arg *A = Args.getLastArg(options::OPT_mms_bitfields,
+ options::OPT_mno_ms_bitfields)) {
+ if (A->getOption().matches(options::OPT_mms_bitfields))
+ CmdArgs.push_back("-fms-layout-compatibility=microsoft");
+ else
+ CmdArgs.push_back("-fms-layout-compatibility=itanium");
}
if (Triple.isOSCygMing()) {
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 47bd7295e93f6..db711ee08c8da 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -18992,9 +18992,7 @@ ExprResult Sema::VerifyBitField(SourceLocation FieldLoc,
// ABI.
bool CStdConstraintViolation =
BitfieldIsOverwide && !getLangOpts().CPlusPlus;
- bool MSBitfieldViolation =
- Value.ugt(TypeStorageSize) &&
- (IsMsStruct || Context.getTargetInfo().getCXXABI().isMicrosoft());
+ bool MSBitfieldViolation = Value.ugt(TypeStorageSize) && IsMsStruct;
if (CStdConstraintViolation || MSBitfieldViolation) {
unsigned DiagWidth =
CStdConstraintViolation ? TypeWidth : TypeStorageSize;
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
index 43f11e5595844..df4aa0e6189c4 100644
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -6173,6 +6173,36 @@ static void handleMSConstexprAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
D->addAttr(::new (S.Context) MSConstexprAttr(S.Context, AL));
}
+static void handleMSStructAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
+ if (const auto *First = D->getAttr<GCCStructAttr>()) {
+ S.Diag(AL.getLoc(), diag::err_attributes_are_not_compatible)
+ << AL << First << 0;
+ S.Diag(First->getLocation(), diag::note_conflicting_attribute);
+ return;
+ }
+ if (const auto *Preexisting = D->getAttr<MSStructAttr>()) {
+ if (Preexisting->isImplicit())
+ D->dropAttr<MSStructAttr>();
+ }
+
+ D->addAttr(::new (S.Context) MSStructAttr(S.Context, AL));
+}
+
+static void handleGCCStructAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
+ if (const auto *First = D->getAttr<MSStructAttr>()) {
+ if (First->isImplicit()) {
+ D->dropAttr<MSStructAttr>();
+ } else {
+ S.Diag(AL.getLoc(), diag::err_attributes_are_not_compatible)
+ << AL << First << 0;
+ S.Diag(First->getLocation(), diag::note_conflicting_attribute);
+ return;
+ }
+ }
+
+ D->addAttr(::new (S.Context) GCCStructAttr(S.Context, AL));
+}
+
static void handleAbiTagAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
SmallVector<StringRef, 4> Tags;
for (unsigned I = 0, E = AL.getNumArgs(); I != E; ++I) {
@@ -7983,6 +8013,14 @@ ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D, const ParsedAttr &AL,
case ParsedAttr::AT_ModularFormat:
handleModularFormat(S, D, AL);
break;
+
+ case ParsedAttr::AT_MSStruct:
+ handleMSStructAttr(S, D, AL);
+ break;
+
+ case ParsedAttr::AT_GCCStruct:
+ handleGCCStructAttr(S, D, AL);
+ break;
}
}
diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index 033cca332d788..001d15a51a58e 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -7288,20 +7288,27 @@ void Sema::CheckCompletedCXXClass(Scope *S, CXXRecordDecl *Record) {
CheckCompletedMemberFunction(MD);
}
- // ms_struct is a request to use the same ABI rules as MSVC. Check
- // whether this class uses any C++ features that are implemented
- // completely
diff erently in MSVC, and if so, emit a diagnostic.
- // That diagnostic defaults to an error, but we allow projects to
- // map it down to a warning (or ignore it). It's a fairly common
- // practice among users of the ms_struct pragma to mass-annotate
- // headers, sweeping up a bunch of types that the project doesn't
- // really rely on MSVC-compatible layout for. We must therefore
- // support "ms_struct except for C++ stuff" as a secondary ABI.
+ // {ms,gcc}_struct is a request to change ABI rules to either follow
+ // Microsoft or Itanium C++ ABI. However, even if these attributes are
+ // present, we do not layout classes following foreign ABI rules, but
+ // instead enter a special "compatibility mode", which only changes
+ // alignments of fundamental types and layout of bit fields.
+ // Check whether this class uses any C++ features that are implemented
+ // completely
diff erently in the requested ABI, and if so, emit a
+ // diagnostic. That diagnostic defaults to an error, but we allow
+ // projects to map it down to a warning (or ignore it). It's a fairly
+ // common practice among users of the ms_struct pragma to
+ // mass-annotate headers, sweeping up a bunch of types that the
+ // project doesn't really rely on MSVC-compatible layout for. We must
+ // therefore support "ms_struct except for C++ stuff" as a secondary
+ // ABI.
// Don't emit this diagnostic if the feature was enabled as a
// language option (as opposed to via a pragma or attribute), as
// the option -mms-bitfields otherwise essentially makes it impossible
// to build C++ code, unless this diagnostic is turned off.
- if (Record->isMsStruct(Context) && !Context.getLangOpts().MSBitfields &&
+ if (Context.getLangOpts().getLayoutCompatibility() ==
+ LangOptions::LayoutCompatibilityKind::Default &&
+ Record->isMsStruct(Context) != Context.defaultsToMsStruct() &&
(Record->isPolymorphic() || Record->getNumBases())) {
Diag(Record->getLocation(), diag::warn_cxx_ms_struct);
}
diff --git a/clang/test/CodeGen/gcc_struct-msvc-todo-1.c b/clang/test/CodeGen/gcc_struct-msvc-todo-1.c
new file mode 100644
index 0000000000000..0bd097f0765cc
--- /dev/null
+++ b/clang/test/CodeGen/gcc_struct-msvc-todo-1.c
@@ -0,0 +1,6 @@
+// RUN: %clang_cc1 -emit-llvm-only -triple x86_64-pc-windows-msvc -verify %s
+
+// expected-error at +1 {{Itanium-compatible layout for the Microsoft C++ ABI is not yet supported}}
+struct {
+ int a;
+} __attribute__((gcc_struct)) t1;
diff --git a/clang/test/CodeGen/gcc_struct-msvc-todo-2.c b/clang/test/CodeGen/gcc_struct-msvc-todo-2.c
new file mode 100644
index 0000000000000..048bf43d31c2a
--- /dev/null
+++ b/clang/test/CodeGen/gcc_struct-msvc-todo-2.c
@@ -0,0 +1,6 @@
+// RUN: %clang_cc1 -emit-llvm-only -triple x86_64-pc-windows-msvc -fms-layout-compatibility=itanium -verify %s
+
+// expected-error at +1 {{Itanium-compatible layout for the Microsoft C++ ABI is not yet supported}}
+struct {
+ int a;
+} t1;
diff --git a/clang/test/CodeGen/gcc_struct.c b/clang/test/CodeGen/gcc_struct.c
new file mode 100644
index 0000000000000..57ea58de7cfe2
--- /dev/null
+++ b/clang/test/CodeGen/gcc_struct.c
@@ -0,0 +1,18 @@
+// RUN: %clang_cc1 -emit-llvm-only -triple x86_64-pc-linux-gnu %s
+// RUN: %clang_cc1 -emit-llvm-only -triple x86_64-pc-linux-gnu -fms-layout-compatibility=microsoft %s
+// RUN: %clang_cc1 -emit-llvm-only -triple x86_64-pc-windows-gnu %s
+// RUN: %clang_cc1 -emit-llvm-only -triple x86_64-pc-windows-gnu -fms-layout-compatibility=itanium %s
+
+struct {
+ int a : 24;
+ char b : 8;
+} __attribute__((gcc_struct)) t1;
+_Static_assert(sizeof(t1) == 4, "");
+
+#pragma ms_struct on
+struct {
+ int a : 24;
+ char b : 8;
+} __attribute__((gcc_struct)) t2;
+_Static_assert(sizeof(t2) == 4, "");
+#pragma ms_struct off
diff --git a/clang/test/CodeGen/mingw-long-double.c b/clang/test/CodeGen/mingw-long-double.c
index b98929701bc62..4050cd159136e 100644
--- a/clang/test/CodeGen/mingw-long-double.c
+++ b/clang/test/CodeGen/mingw-long-double.c
@@ -1,6 +1,6 @@
// RUN: %clang_cc1 -triple i686-windows-gnu -emit-llvm -o - %s \
// RUN: | FileCheck %s --check-prefix=GNU32
-// RUN: %clang_cc1 -triple i686-windows-gnu -emit-llvm -o - %s -mms-bitfields \
+// RUN: %clang_cc1 -triple i686-windows-gnu -emit-llvm -o - %s -fms-layout-compatibility=microsoft \
// RUN: | FileCheck %s --check-prefix=GNU32
// RUN: %clang_cc1 -triple x86_64-windows-gnu -emit-llvm -o - %s \
// RUN: | FileCheck %s --check-prefix=GNU64
diff --git a/clang/test/CodeGen/mms-bitfields.c b/clang/test/CodeGen/mms-bitfields.c
index 2ccce326c7131..8d3b403b7e7be 100644
--- a/clang/test/CodeGen/mms-bitfields.c
+++ b/clang/test/CodeGen/mms-bitfields.c
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -triple i386-apple-darwin10 -mms-bitfields -emit-llvm %s -o - | FileCheck %s
+// RUN: %clang_cc1 -triple i386-apple-darwin10 -fms-layout-compatibility=microsoft -emit-llvm %s -o - | FileCheck %s
struct s1 {
int f32;
diff --git a/clang/test/Driver/ms-bitfields.c b/clang/test/Driver/ms-bitfields.c
index d5a3656b3d110..acd1bdc9c0ac9 100644
--- a/clang/test/Driver/ms-bitfields.c
+++ b/clang/test/Driver/ms-bitfields.c
@@ -1,8 +1,15 @@
-// RUN: %clang -### --target=x86_64-linux-gnu %s 2>&1 | FileCheck %s -check-prefix=NO-MSBITFIELDS
-// RUN: %clang -### --target=x86_64-windows-gnu %s 2>&1 | FileCheck %s -check-prefix=MSBITFIELDS
-// RUN: %clang -### -mno-ms-bitfields -mms-bitfields %s 2>&1 | FileCheck %s -check-prefix=MSBITFIELDS
-// RUN: %clang -### -mms-bitfields -mno-ms-bitfields %s 2>&1 | FileCheck %s -check-prefix=NO-MSBITFIELDS
+// RUN: %clang -### --target=x86_64-linux-gnu %s 2>&1 | FileCheck %s -check-prefix=DEFAULT-LAYOUT
+// RUN: %clang -### --target=x86_64-windows-gnu %s 2>&1 | FileCheck %s -check-prefix=DEFAULT-LAYOUT
+// RUN: %clang -### --target=x86_64-windows-msvc %s 2>&1 | FileCheck %s -check-prefix=DEFAULT-LAYOUT
+// RUN: %clang -### -mms-bitfields %s 2>&1 | FileCheck %s -check-prefix=MICROSOFT-LAYOUT
+// RUN: %clang -### -mno-ms-bitfields %s 2>&1 | FileCheck %s -check-prefix=ITANIUM-LAYOUT
+// RUN: %clang -### -mno-ms-bitfields -mms-bitfields %s 2>&1 | FileCheck %s -check-prefix=MICROSOFT-LAYOUT
+// RUN: %clang -### -mms-bitfields -mno-ms-bitfields %s 2>&1 | FileCheck %s -check-prefix=ITANIUM-LAYOUT
-// MSBITFIELDS: -mms-bitfields
-// NO-MSBITFIELDS-NOT: -mms-bitfields
+// DEFAULT-LAYOUT-NOT: -fms-layout-compatibility=itanium
+// DEFAULT-LAYOUT-NOT: -fms-layout-compatibility=microsoft
+// MICROSOFT-LAYOUT: -fms-layout-compatibility=microsoft
+// MICROSOFT-LAYOUT-NOT: -fms-layout-compatibility=itanium
+// ITANIUM-LAYOUT: -fms-layout-compatibility=itanium
+// ITANIUM-LAYOUT-NOT: -fms-layout-compatibility=microsoft
diff --git a/clang/test/Layout/itanium-union-bitfield.cpp b/clang/test/Layout/itanium-union-bitfield.cpp
index febfc46dfee54..34f501f4b569a 100644
--- a/clang/test/Layout/itanium-union-bitfield.cpp
+++ b/clang/test/Layout/itanium-union-bitfield.cpp
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -emit-llvm-only -triple %itanium_abi_triple -fdump-record-layouts %s 2>/dev/null \
+// RUN: %clang_cc1 -emit-llvm-only -triple %itanium_abi_triple -fms-layout-compatibility=itanium -fdump-record-layouts %s 2>/dev/null \
// RUN: | FileCheck %s
// On z/OS, a bit-field has single byte alignment. Add aligned(4) on z/OS so the union has
diff --git a/clang/test/Misc/pragma-attribute-supported-attributes-list.test b/clang/test/Misc/pragma-attribute-supported-attributes-list.test
index 081ea8d5c821c..82072ce9f69f7 100644
--- a/clang/test/Misc/pragma-attribute-supported-attributes-list.test
+++ b/clang/test/Misc/pragma-attribute-supported-attributes-list.test
@@ -87,6 +87,7 @@
// CHECK-NEXT: FlagEnum (SubjectMatchRule_enum)
// CHECK-NEXT: Flatten (SubjectMatchRule_function)
// CHECK-NEXT: FunctionReturnThunks (SubjectMatchRule_function)
+// CHECK-NEXT: GCCStruct (SubjectMatchRule_record)
// CHECK-NEXT: GNUInline (SubjectMatchRule_function)
// CHECK-NEXT: HIPManaged (SubjectMatchRule_variable)
// CHECK-NEXT: HLSLVkLocation (SubjectMatchRule_variable_is_parameter, SubjectMatchRule_field, SubjectMatchRule_function)
diff --git a/clang/test/Sema/gcc_ms_struct.c b/clang/test/Sema/gcc_ms_struct.c
new file mode 100644
index 0000000000000..4e70e60fb63bd
--- /dev/null
+++ b/clang/test/Sema/gcc_ms_struct.c
@@ -0,0 +1,37 @@
+// RUN: %clang_cc1 -emit-llvm-only -triple x86_64-pc-windows-gnu -verify %s
+
+struct {
+ int a;
+}
+// expected-note at +1 {{conflicting attribute is here}}
+__attribute__((gcc_struct))
+// expected-error at +1 {{'ms_struct' and 'gcc_struct' attributes are not compatible}}
+__attribute__((ms_struct))
+t1;
+
+struct {
+ int a;
+}
+// expected-note at +1 {{conflicting attribute is here}}
+__attribute__((ms_struct))
+// expected-error at +1 {{'gcc_struct' and 'ms_struct' attributes are not compatible}}
+__attribute__((gcc_struct))
+t2;
+
+#pragma ms_struct on
+struct {
+ int a;
+}
+// No diagnostic for an attribute, unambiguously overriding the pragma.
+__attribute__((gcc_struct))
+t3;
+
+struct {
+ int a;
+}
+// expected-note at +1 {{conflicting attribute is here}}
+__attribute__((ms_struct))
+// expected-error at +1 {{'gcc_struct' and 'ms_struct' attributes are not compatible}}
+__attribute__((gcc_struct))
+t4;
+#pragma ms_struct off
diff --git a/clang/test/Sema/mms-bitfields.c b/clang/test/Sema/mms-bitfields.c
index a976578845229..d7c8b0aa2cb9a 100644
--- a/clang/test/Sema/mms-bitfields.c
+++ b/clang/test/Sema/mms-bitfields.c
@@ -1,6 +1,5 @@
-// RUN: %clang_cc1 -mms-bitfields -fsyntax-only -verify -triple x86_64-apple-darwin9 %s
-// RUN: %clang_cc1 -mms-bitfields -fsyntax-only -Wms-bitfield-padding -verify=checkms -triple x86_64-apple-darwin9 %s
-
+// RUN: %clang_cc1 -fms-layout-compatibility=microsoft -fsyntax-only -verify -triple x86_64-apple-darwin9 %s
+// RUN: %clang_cc1 -fms-layout-compatibility=microsoft -fsyntax-only -Wms-bitfield-padding -verify=checkms -triple x86_64-apple-darwin9 %s
// expected-no-diagnostics
// The -mms-bitfields commandline parameter should behave the same
diff --git a/clang/test/SemaCXX/ms_struct-bitfield-padding.cpp b/clang/test/SemaCXX/ms_struct-bitfield-padding.cpp
index c0f90f798118a..2499d9e392c19 100644
--- a/clang/test/SemaCXX/ms_struct-bitfield-padding.cpp
+++ b/clang/test/SemaCXX/ms_struct-bitfield-padding.cpp
@@ -1,6 +1,6 @@
// RUN: %clang_cc1 -fsyntax-only -Wms-bitfield-padding -verify -triple armv8 -std=c++23 %s
-// RUN: %clang_cc1 -fsyntax-only -DMS_BITFIELDS -mms-bitfields -verify=msbitfields -triple armv8-apple-macos10.15 -std=c++23 %s
+// RUN: %clang_cc1 -fsyntax-only -DMS_BITFIELDS -fms-layout-compatibility=microsoft -verify=msbitfields -triple armv8-apple-macos10.15 -std=c++23 %s
// msbitfields-no-diagnostics
diff --git a/clang/test/SemaCXX/ms_struct.cpp b/clang/test/SemaCXX/ms_struct.cpp
index 409350f2606a9..8ba4631e2d1a3 100644
--- a/clang/test/SemaCXX/ms_struct.cpp
+++ b/clang/test/SemaCXX/ms_struct.cpp
@@ -1,7 +1,7 @@
// RUN: %clang_cc1 -fsyntax-only -DTEST_FOR_WARNING -Wno-error=incompatible-ms-struct -verify -triple i686-apple-darwin9 -std=c++11 %s
// RUN: %clang_cc1 -fsyntax-only -DTEST_FOR_WARNING -Wno-error=incompatible-ms-struct -verify -triple armv7-apple-darwin9 -std=c++11 %s
// RUN: %clang_cc1 -fsyntax-only -DTEST_FOR_ERROR -verify -triple armv7-apple-darwin9 -std=c++11 %s
-// RUN: %clang_cc1 -fsyntax-only -DNO_PRAGMA -mms-bitfields -verify -triple armv7-apple-darwin9 -std=c++11 %s
+// RUN: %clang_cc1 -fsyntax-only -DNO_PRAGMA -fms-layout-compatibility=microsoft -verify -triple armv7-apple-darwin9 -std=c++11 %s
#ifndef NO_PRAGMA
#pragma ms_struct on
diff --git a/clang/test/SemaCXX/ms_wide_bitfield.cpp b/clang/test/SemaCXX/ms_wide_bitfield.cpp
index a5b7a433c7c4c..eccfd90f0faf0 100644
--- a/clang/test/SemaCXX/ms_wide_bitfield.cpp
+++ b/clang/test/SemaCXX/ms_wide_bitfield.cpp
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fno-rtti -triple i686-pc-win32 -fdump-record-layouts -fsyntax-only -mms-bitfields -verify %s 2>&1
+// RUN: %clang_cc1 -fno-rtti -triple i686-pc-win32 -fdump-record-layouts -fsyntax-only -fms-layout-compatibility=microsoft -verify %s 2>&1
struct A {
char a : 9; // expected-error{{width of bit-field 'a' (9 bits) exceeds the size of its type (8 bits)}}
diff --git a/clang/utils/TableGen/ClangDiagnosticsEmitter.cpp b/clang/utils/TableGen/ClangDiagnosticsEmitter.cpp
index 17078e2bc1505..f4d5023d1cb92 100644
--- a/clang/utils/TableGen/ClangDiagnosticsEmitter.cpp
+++ b/clang/utils/TableGen/ClangDiagnosticsEmitter.cpp
@@ -1357,6 +1357,7 @@ static bool isExemptAtStart(StringRef Text) {
.Case("Fuchsia", true)
.Case("GNUstep", true)
.Case("IBOutletCollection", true)
+ .Case("Itanium", true)
.Case("Microsoft", true)
.Case("Neon", true)
.StartsWith("NSInvocation", true) // NSInvocation, NSInvocation's
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