[Headers] Add some intrinsic function descriptions to immintrin.h

[probinson]

FTR, I'll be working my way through a bunch of intrinsics over the next month or so, trying not to do too many at once. Mostly AVX2 but also some others.

[pengfei]

LGTM, thanks for working on it!

clang/lib/Headers/immintrin.h

294

memory location

294

Nit: I saw others prefer to A pointer

309

ditto.

324

ditto.

414

Nit: I saw others prefer to A 32-bit integer value.

clang/lib/Headers/immintrin.h

 static __inline__ unsigned int __attribute__((__always_inline__, __nodebug__, __target__("rdpid")))
 _rdpid_u32(void) {
   return __builtin_ia32_rdpid();
 }
 #endif // __RDPID__
 
 #if !(defined(_MSC_VER) || defined(__SCE__)) || __has_feature(modules) ||      \
     defined(__RDRND__)
+/// Returns a 16-bit hardware-generated random value.
+///
+/// \headerfile <immintrin.h>
+///
+/// This intrinsic corresponds to the <c> RDRAND </c> instruction.
+///
+/// \param __p
+///    A pointer to a 16-bit memory location to place the random value.
+/// \returns 1 if the value was successfully generated, 0 otherwise.
 static __inline__ int __attribute__((__always_inline__, __nodebug__, __target__("rdrnd")))
 _rdrand16_step(unsigned short *__p)
 {
   return (int)__builtin_ia32_rdrand16_step(__p);
 }
 
+/// Returns a 32-bit hardware-generated random value.
+///
+/// \headerfile <immintrin.h>
+///
+/// This intrinsic corresponds to the <c> RDRAND </c> instruction.
+///
+/// \param __p
+///    A pointer to a 32-bit memory location to place the random value.
+/// \returns 1 if the value was successfully generated, 0 otherwise.
 static __inline__ int __attribute__((__always_inline__, __nodebug__, __target__("rdrnd")))
 _rdrand32_step(unsigned int *__p)
 {
   return (int)__builtin_ia32_rdrand32_step(__p);
 }
 
+/// Returns a 64-bit hardware-generated random value.
+///
+/// \headerfile <immintrin.h>
+///
+/// This intrinsic corresponds to the <c> RDRAND </c> instruction.
+///
+/// \param __p
+///    A pointer to a 64-bit memory location to place the random value.
+/// \returns 1 if the value was successfully generated, 0 otherwise.
 #ifdef __x86_64__
 static __inline__ int __attribute__((__always_inline__, __nodebug__, __target__("rdrnd")))
 _rdrand64_step(unsigned long long *__p)
 {
   return (int)__builtin_ia32_rdrand64_step(__p);
 }
 #else
 // We need to emulate the functionality of 64-bit rdrand with 2 32-bit
_rdrand64_step(unsigned long long *__p)
   }
 }
 #endif
 #endif /* __RDRND__ */
 
 #if !(defined(_MSC_VER) || defined(__SCE__)) || __has_feature(modules) ||      \
     defined(__FSGSBASE__)
 #ifdef __x86_64__
+/// Reads the FS base register.
+///
+/// \headerfile <immintrin.h>
+///
+/// This intrinsic corresponds to the <c> RDFSBASE </c> instruction.
+///
+/// \returns The lower 32 bits of the FS base register.
 static __inline__ unsigned int __attribute__((__always_inline__, __nodebug__, __target__("fsgsbase")))
 _readfsbase_u32(void)
 {
   return __builtin_ia32_rdfsbase32();
 }
 
+/// Reads the FS base register.
+///
+/// \headerfile <immintrin.h>
+///
+/// This intrinsic corresponds to the <c> RDFSBASE </c> instruction.
+///
+/// \returns The contents of the FS base register.
 static __inline__ unsigned long long __attribute__((__always_inline__, __nodebug__, __target__("fsgsbase")))
 _readfsbase_u64(void)
 {
   return __builtin_ia32_rdfsbase64();
 }
 
+/// Reads the GS base register.
+///
+/// \headerfile <immintrin.h>
+///
+/// This intrinsic corresponds to the <c> RDGSBASE </c> instruction.
+///
+/// \returns The lower 32 bits of the GS base register.
 static __inline__ unsigned int __attribute__((__always_inline__, __nodebug__, __target__("fsgsbase")))
 _readgsbase_u32(void)
 {
   return __builtin_ia32_rdgsbase32();
 }
 
+/// Reads the GS base register.
+///
+/// \headerfile <immintrin.h>
+///
+/// This intrinsic corresponds to the <c> RDGSBASE </c> instruction.
+///
+/// \returns The contents of the GS base register.
 static __inline__ unsigned long long __attribute__((__always_inline__, __nodebug__, __target__("fsgsbase")))
 _readgsbase_u64(void)
 {
   return __builtin_ia32_rdgsbase64();
 }
 
+/// Modifies the FS base register.
+///
+/// \headerfile <immintrin.h>
+///
+/// This intrinsic corresponds to the <c> WRFSBASE </c> instruction.
+///
+/// \param __V
+///    Value to use for the lower 32 bits of the FS base register.
 static __inline__ void __attribute__((__always_inline__, __nodebug__, __target__("fsgsbase")))
 _writefsbase_u32(unsigned int __V)
 {
   __builtin_ia32_wrfsbase32(__V);
 }
 
+/// Modifies the FS base register.
+///
+/// \headerfile <immintrin.h>
+///
+/// This intrinsic corresponds to the <c> WRFSBASE </c> instruction.
+///
+/// \param __V
+///    Value to use for the FS base register.
 static __inline__ void __attribute__((__always_inline__, __nodebug__, __target__("fsgsbase")))
 _writefsbase_u64(unsigned long long __V)
 {
   __builtin_ia32_wrfsbase64(__V);
 }
 
+/// Modifies the GS base register.
+///
+/// \headerfile <immintrin.h>
+///
+/// This intrinsic corresponds to the <c> WRGSBASE </c> instruction.
+///
+/// \param __V
+///    Value to use for the lower 32 bits of the GS base register.
 static __inline__ void __attribute__((__always_inline__, __nodebug__, __target__("fsgsbase")))
 _writegsbase_u32(unsigned int __V)
 {
   __builtin_ia32_wrgsbase32(__V);
 }
 
+/// Modifies the GS base register.
+///
+/// \headerfile <immintrin.h>
+///
+/// This intrinsic corresponds to the <c> WRFSBASE </c> instruction.
+///
+/// \param __V
+///    Value to use for GS base register.
 static __inline__ void __attribute__((__always_inline__, __nodebug__, __target__("fsgsbase")))
 _writegsbase_u64(unsigned long long __V)
 {
   __builtin_ia32_wrgsbase64(__V);
 }
 
 #endif
 #endif /* __FSGSBASE__ */