Headers: use C++ inline semantics in C++ mode

[compnerd]

Summary

When building with the 17.5.0 preview toolset for MSVC and building with modules, the definition of _addcarry_u64 and _subborrow_u64 seem to cause issues due to the use of GNU inline semantics. Change the headers to prefer C++ inline semantics for C++ compilation, falling back to GNU inlining semantics for C compilation.

This is motivated by https://github.com/microsoft/STL/issues/2520.

[fsb4000]

I also asked at MS STL Discord(https://discord.gg/XWanNww) about the patch.

[compnerd]

@fsb4000 is my reading correct that MSVC will look into trying to handle static inline even though it is a GNUism? I wonder if we should consider limiting the use of static inline to C mode rather than including C++. I also wonder if I can loosen it similarly to avoid the issue I'm running into (which @STL_MSFT correctly identified - modules).

[compnerd]

Seems that this alone is insufficient as some build does run into issues, so I may need to refine this further.

[fsb4000]

@CaseyCarter created https://github.com/microsoft/STL/pull/3285
So MSVC STL don't use <intrin0.h> with Clang now. It should fix the issues too because Clang overrides <intrin.h> (and doesn't override<intrin0.h>)

[compnerd]

Thanks @fsb4000 (and @CaseyCarter)! I think that due to the shipped version, it makes sense to do this still. Using standard semantics in general I think is preferable, since nothing prevents another compiler implementation to still do something similar.

[rnk]

This feels to me like we are still working around some incompatibilities between the MSVC intrin.h / intrin0.h model. I would prefer it if we could always use static inline consistently in our intrinsic headers so we don't have to worry about ODR violation problems. However, we're using always_inline so there's not much to worry about anyway.

Anyway, if it comes up again, we should try to come up with a better fix. +@hans

clang/lib/Headers/adxintrin.h

 #endif
 
 #ifndef __ADXINTRIN_H
 #define __ADXINTRIN_H
 
 /* Define the default attributes for the functions in this file. */
 #define __DEFAULT_FN_ATTRS __attribute__((__always_inline__, __nodebug__))
 
+/* Use C++ inline semantics in C++, GNU inline for C mode. */
+#if defined(__cplusplus)
+#define __INLINE __inline
+#else
+#define __INLINE static __inline
+#endif
+
+#if defined(__cplusplus)
+extern "C" {
+#endif
+
 /* Intrinsics that are available only if __ADX__ defined */
-static __inline unsigned char __attribute__((__always_inline__, __nodebug__, __target__("adx")))
+__INLINE unsigned char
+    __attribute__((__always_inline__, __nodebug__, __target__("adx")))
     _addcarryx_u32(unsigned char __cf, unsigned int __x, unsigned int __y,
-               unsigned int *__p)
-{
+                   unsigned int *__p) {
   return __builtin_ia32_addcarryx_u32(__cf, __x, __y, __p);
 }
 
 #ifdef __x86_64__
-static __inline unsigned char __attribute__((__always_inline__, __nodebug__, __target__("adx")))
+__INLINE unsigned char
+    __attribute__((__always_inline__, __nodebug__, __target__("adx")))
     _addcarryx_u64(unsigned char __cf, unsigned long long __x,
-               unsigned long long __y, unsigned long long  *__p)
-{
+                   unsigned long long __y, unsigned long long *__p) {
   return __builtin_ia32_addcarryx_u64(__cf, __x, __y, __p);
 }
 #endif
 
 /* Intrinsics that are also available if __ADX__ undefined */
-static __inline unsigned char __DEFAULT_FN_ATTRS
-_addcarry_u32(unsigned char __cf, unsigned int __x, unsigned int __y,
-              unsigned int *__p)
-{
+__INLINE unsigned char __DEFAULT_FN_ATTRS _addcarry_u32(unsigned char __cf,
+                                                        unsigned int __x,
+                                                        unsigned int __y,
+                                                        unsigned int *__p) {
   return __builtin_ia32_addcarryx_u32(__cf, __x, __y, __p);
 }
 
 #ifdef __x86_64__
-static __inline unsigned char __DEFAULT_FN_ATTRS
+__INLINE unsigned char __DEFAULT_FN_ATTRS
 _addcarry_u64(unsigned char __cf, unsigned long long __x,
-              unsigned long long __y, unsigned long long  *__p)
-{
+              unsigned long long __y, unsigned long long *__p) {
   return __builtin_ia32_addcarryx_u64(__cf, __x, __y, __p);
 }
 #endif
 
-static __inline unsigned char __DEFAULT_FN_ATTRS
-_subborrow_u32(unsigned char __cf, unsigned int __x, unsigned int __y,
-              unsigned int *__p)
-{
+__INLINE unsigned char __DEFAULT_FN_ATTRS _subborrow_u32(unsigned char __cf,
+                                                         unsigned int __x,
+                                                         unsigned int __y,
+                                                         unsigned int *__p) {
   return __builtin_ia32_subborrow_u32(__cf, __x, __y, __p);
 }
 
 #ifdef __x86_64__
-static __inline unsigned char __DEFAULT_FN_ATTRS
+__INLINE unsigned char __DEFAULT_FN_ATTRS
 _subborrow_u64(unsigned char __cf, unsigned long long __x,
-               unsigned long long __y, unsigned long long  *__p)
-{
+               unsigned long long __y, unsigned long long *__p) {
   return __builtin_ia32_subborrow_u64(__cf, __x, __y, __p);
 }
 #endif
 
+#if defined(__cplusplus)
+}
+#endif
+
 #undef __DEFAULT_FN_ATTRS
 
 #endif /* __ADXINTRIN_H */