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[clang] Predefine _CRT_USE_BUILTIN_OFFSETOF in MS-compatible modes (#127568)

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This patch makes Clang predefine `_CRT_USE_BUILTIN_OFFSETOF` in
MS-compatible modes. The macro can make the `offsetof` provided by MS
UCRT's `<stddef.h>` to select the `__builtin_offsetof` version, so with
it Clang (Clang-cl) can directly consume UCRT's `offsetof`.

MSVC predefines the macro as `1` since at least VS 2017 19.14, but I
think it's also OK to define it in "older" compatible modes.

Fixes #59689.
This commit is contained in:
A. Jiang 2025-03-13 14:02:44 +08:00 committed by GitHub
parent 5d5f16204f
commit 6abe19ac58
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4
clang/docs/ReleaseNotes.rst
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@ -358,6 +358,10 @@ Android Support
Windows Support
^^^^^^^^^^^^^^^
- Clang now defines ``_CRT_USE_BUILTIN_OFFSETOF`` macro in MSVC-compatible mode,
which makes ``offsetof`` provided by Microsoft's ``<stddef.h>`` to be defined
correctly. (#GH59689)
LoongArch Support
^^^^^^^^^^^^^^^^^

2
clang/lib/Basic/Targets/OSTargets.cpp
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@ -220,6 +220,8 @@ static void addVisualCDefines(const LangOptions &Opts, MacroBuilder &Builder) {
Builder.defineMacro("_MSC_FULL_VER", Twine(Opts.MSCompatibilityVersion));
// FIXME We cannot encode the revision information into 32-bits
Builder.defineMacro("_MSC_BUILD", Twine(1));
// Exposed by MSVC, used in their stddef.h.
Builder.defineMacro("_CRT_USE_BUILTIN_OFFSETOF", Twine(1));
if (Opts.CPlusPlus11 && Opts.isCompatibleWithMSVC(LangOptions::MSVC2015))
Builder.defineMacro("_HAS_CHAR16_T_LANGUAGE_SUPPORT", Twine(1));

28
clang/test/Sema/offsetof-ucrt.c Normal file
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@ -0,0 +1,28 @@
// RUN: %clang_cc1 %s -triple i686-pc-win32 -fsyntax-only -verify -fms-compatibility
// RUN: %clang_cc1 %s -triple i686-pc-win32 -fsyntax-only -verify
// expected-no-diagnostics
typedef __typeof__(sizeof(0)) size_t;
#ifdef _MSC_VER
#ifndef _CRT_USE_BUILTIN_OFFSETOF
#error _CRT_USE_BUILTIN_OFFSETOF should be predefined in MSVC-compatible modes.
#endif
#else
#ifdef _CRT_USE_BUILTIN_OFFSETOF
#error _CRT_USE_BUILTIN_OFFSETOF should not be predefined in non-MSVC-compatible modes.
#endif
#endif
#if defined _MSC_VER && !defined _CRT_USE_BUILTIN_OFFSETOF
#ifdef __cplusplus
#define offsetof(s,m) ((::size_t)&reinterpret_cast<char const volatile&>((((s*)0)->m)))
#else
#define offsetof(s,m) ((size_t)&(((s*)0)->m))
#endif
#else
#define offsetof(s,m) __builtin_offsetof(s,m)
#endif
struct S { int a; };
_Static_assert(offsetof(struct S, a) == 0, "");

28
clang/test/SemaCXX/offsetof-ucrt.cpp Normal file
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@ -0,0 +1,28 @@
// RUN: %clang_cc1 %s -triple i686-pc-win32 -fsyntax-only -verify -fms-compatibility
// RUN: %clang_cc1 %s -triple i686-pc-win32 -fsyntax-only -verify
// expected-no-diagnostics
typedef __typeof__(sizeof(0)) size_t;
#ifdef _MSC_VER
#ifndef _CRT_USE_BUILTIN_OFFSETOF
#error _CRT_USE_BUILTIN_OFFSETOF should be predefined in MSVC-compatible modes.
#endif
#else
#ifdef _CRT_USE_BUILTIN_OFFSETOF
#error _CRT_USE_BUILTIN_OFFSETOF should not be predefined in non-MSVC-compatible modes.
#endif
#endif
#if defined _MSC_VER && !defined _CRT_USE_BUILTIN_OFFSETOF
#ifdef __cplusplus
#define offsetof(s,m) ((::size_t)&reinterpret_cast<char const volatile&>((((s*)0)->m)))
#else
#define offsetof(s,m) ((size_t)&(((s*)0)->m))
#endif
#else
#define offsetof(s,m) __builtin_offsetof(s,m)
#endif
struct S { int a; };
_Static_assert(offsetof(S, a) == 0, "");