The mulx instruction was introduced with the BMI2 instruction set starting with the Haswell processor.
According to Intel's documentation there should be an intrinsic for mulx
unsigned __int64 umul128(unsigned __int64 a, unsigned __int64 b, unsigned __int64 * hi);
However, I find no such intrinsic from Intel's intrinsic guide online under BMI2 or in general. I do however find the addcarry intrinsics from the ADX instruction set.
According to this link the intrinsic is mulx_u64 but I don't find that one either.
MSVC added a _umul128 intrinsic in MSVC 2005 but that only produces mul and not mulx (and I have no idea how to enable BMI2 in MSVC).
I can produce the mulx instruction indirectly using __int128 in GCC with -mbmi2 (or -march=haswell) but I would prefer to do this more directly using an intrinsic.
Why do the ADX intrinsics exist but not one for mulx as defined in Intel's documentation?
The intrinsic which generates mulx instruction for 64 bit Integer multiplication is _mulx_u64(). Below is an example for the same:
#include <stdio.h>
int main()
{
unsigned __int64 a = 0x0fffffffffffffff;
unsigned __int64 b = 0xf0000000;
unsigned __int64 c, d;
d = _mulx_u64(a, b, &c);
printf_s("%#I64x * %#I64x = %#I64x%I64x\n", a, b, c, d);
}
Variable "c" will hold the higher 64 bits of the result and variable "d" will hold the lower 64 bits of the result. This intrinsic is also supported in Microsoft Visual Studio Compiler. We are working on updating the white paper (New Instructions Support Large Integer Arithmetic) with the right intrinsic. Thanks for bringing this to our attention.