[clang] [Headers][X86] Add macro descriptions to ia32intrin.h (PR #78613)

[Phoebe Wang via cfe-commits]

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@@ -411,13 +497,45 @@ __rdpmc(int __A) {
 /// \param __A
 ///    Address of where to store the 32-bit \c IA32_TSC_AUX value.
 /// \returns The 64-bit value of the time stamp counter.
+/// \see _rdtsc
 static __inline__ unsigned long long __DEFAULT_FN_ATTRS
 __rdtscp(unsigned int *__A) {
   return __builtin_ia32_rdtscp(__A);
 }
 
-#define _rdtsc() __rdtsc()
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phoebewang wrote:

I think the old code is correct. The `__rdtsc` is a built in, and we have documented in Intrinsic Guide.

https://github.com/llvm/llvm-project/pull/78613