[PATCH] D132141: [X86] Emulate _rdrand64_step with two rdrand32 if it is 32bit

[Bing Yu via Phabricator via cfe-commits]

yubing added inline comments.


================
Comment at: clang/lib/Headers/immintrin.h:301
+  unsigned long long tmp;
+  if (__builtin_ia32_rdrand32_step((unsigned int *)&tmp) &
+      __builtin_ia32_rdrand32_step(((unsigned int *)&tmp) + 1)) {
----------------
RKSimon wrote:
> RKSimon wrote:
> > craig.topper wrote:
> > > craig.topper wrote:
> > > > Should `&` be `&&`?
> > > Can we avoid the pointer cast here? Use two unsigned ints and manually concatenate them to a 64-bit value.
> > +1
> > ```
> > unsigned int lo, hi;
> > if (__builtin_ia32_rdrand32_step(&lo) &&
> >     __builtin_ia32_rdrand32_step(&hi)) {
> >   *p = ((unsigned long)hi << 32) | lo;
> >   return 1;
> > }
> > ```
> Are there any sideeffects that we might encounter by not always performing both __builtin_ia32_rdrand32_step calls?
> ```
>   unsigned int __lo, __hi;
>   int __res_lo = __builtin_ia32_rdrand32_step(&__lo);
>   int __res_hi = __builtin_ia32_rdrand32_step(&__hi);
>   if (__res_lo && __res_hi) {
>     *__p = ((unsigned long long)__hi << 32) | (unsigned long long)__lo;
>     return 1;
>   } else {
>     *__p = 0;
>     return 0;
>   }
> ```
however, if the first rdrand32 failed, then we don't need to execute the second one.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D132141/new/

https://reviews.llvm.org/D132141