[clang] Predefine _CRT_USE_BUILTIN_OFFSETOF in MS-compatible modes

[frederick-vs-ja]

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.

[llvmbot]

@llvm/pr-subscribers-clang

@llvm/pr-subscribers-platform-windows

Author: A. Jiang (frederick-vs-ja)

Changes

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.

---

Full diff: https://github.com/llvm/llvm-project/pull/127568.diff

3 Files Affected:

• (modified) clang/lib/Basic/Targets/OSTargets.cpp (+2)
• (added) clang/test/Sema/offsetof-ucrt.c (+16)
• (added) clang/test/SemaCXX/offsetof-ucrt.cpp (+16)

diff --git a/clang/lib/Basic/Targets/OSTargets.cpp b/clang/lib/Basic/Targets/OSTargets.cpp
index 8af6623e5cb15..2e57c286c9a16 100644
--- a/clang/lib/Basic/Targets/OSTargets.cpp
+++ b/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));
+    // https://github.com/llvm/llvm-project/issues/59689
+    Builder.defineMacro("_CRT_USE_BUILTIN_OFFSETOF", Twine(1));
 
     if (Opts.CPlusPlus11 && Opts.isCompatibleWithMSVC(LangOptions::MSVC2015))
       Builder.defineMacro("_HAS_CHAR16_T_LANGUAGE_SUPPORT", Twine(1));
diff --git a/clang/test/Sema/offsetof-ucrt.c b/clang/test/Sema/offsetof-ucrt.c
new file mode 100644
index 0000000000000..0f21b317c1ff2
--- /dev/null
+++ b/clang/test/Sema/offsetof-ucrt.c
@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 %s -triple i686-pc-win32 -fsyntax-only -verify -fms-compatibility
+
+typedef __typeof__(sizeof(0)) size_t;
+
+#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, "");
diff --git a/clang/test/SemaCXX/offsetof-ucrt.cpp b/clang/test/SemaCXX/offsetof-ucrt.cpp
new file mode 100644
index 0000000000000..425e0e7f88f37
--- /dev/null
+++ b/clang/test/SemaCXX/offsetof-ucrt.cpp
@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 %s -triple i686-pc-win32 -fsyntax-only -verify -fms-compatibility
+
+typedef __typeof__(sizeof(0)) size_t;
+
+#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, "");

[AaronBallman]

LGTM! When landing, please add a release note to clang/docs/ReleaseNotes.rst so users know about the change.