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

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.

Author: frederick-vs-ja

clang/docs/ReleaseNotes.rst

@@ -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
 ^^^^^^^^^^^^^^^^^
 

clang/lib/Basic/Targets/OSTargets.cpp

@@ -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));

clang/test/Sema/offsetof-ucrt.c

@@ -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, "");

clang/test/SemaCXX/offsetof-ucrt.cpp

@@ -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, "");