Intrin: Add _umul128

[majnemer]

Summary

Implement _umul128; it provides the high and low halves of a 128-bit
multiply. We can simply use our __int128 arithmetic to implement this,
we generate great code for it:

movq    %rdx, %rax
mulq    %rcx
movq    %rdx, (%r8)
retq

[hans]

lgtm with comment

lib/Headers/Intrin.h

419

While you're here, is shrx_u64 missing an underscore?

423

use an underscore for FullProduct to be safe?

[rnk]

lgtm

Nice!

lib/Headers/Intrin.h

423

Why not unsigned __int128? I believe this affects the shift right below, but all the bits that would be 1 are truncated out.

[majnemer]

Closed by commit rL223175 (authored by @majnemer).

lib/Headers/Intrin.h

 __int64 _mul128(__int64 _Multiplier, __int64 _Multiplicand,
                 __int64 *_HighProduct);
 unsigned __int64 _rorx_u64(unsigned __int64, const unsigned int);
 __int64 _sarx_i64(__int64, unsigned int);
 #if __STDC_HOSTED__
 int __cdecl _setjmpex(jmp_buf);
 #endif
 unsigned __int64 _shlx_u64(unsigned __int64, unsigned int);
 unsigned __int64 shrx_u64(unsigned __int64, unsigned int);
-unsigned __int64 _umul128(unsigned __int64 _Multiplier,
-                          unsigned __int64 _Multiplicand,
-                          unsigned __int64 *_HighProduct);
+static __inline__ unsigned __int64 __attribute__((__always_inline__, __nodebug__))
+_umul128(unsigned __int64 _Multiplier, unsigned __int64 _Multiplicand,
+         unsigned __int64 *_HighProduct) {
+  __int128 FullProduct =
+      (unsigned __int128)_Multiplier * (unsigned __int128)_Multiplicand;
+  *_HighProduct = FullProduct >> 64;
+  return FullProduct;
+}
 void __cdecl _xrstor64(void const *, unsigned __int64);
 void __cdecl _xsave64(void *, unsigned __int64);
 void __cdecl _xsaveopt64(void *, unsigned __int64);
 
 #endif /* __x86_64__ */
 
 /*----------------------------------------------------------------------------*\
 |* Bit Twiddling