gcc intrinsic for extended division/multiplication

Question

Modern CPU\'s can perform extended multiplication between two native-size words and store the low and high result in separate registers. Similarly, when performing division,

Answer 1

For those wondering about the other half of the question (division), gcc does not provide an intrinsic for that because the processor division instructions don't conform to the standard.

This is true both with 128-bit dividends on 64-bit x86 targets and 64-bit dividends on 32-bit x86 targets. The problem is that DIV will cause divide overflow exceptions in cases where the standard says the result should be truncated. For example (unsigned long long) (((unsigned _int128) 1 << 64) / 1) should evaluate to 0, but would cause divide overflow exception if evaluated with DIV.

(Thanks to @ross-ridge for this info)

Answer 2

For gcc since version 4.6 you can use __int128. This works on most 64 bit hardware. For instance

To get the 128 bit result of a 64x64 bit multiplication just use

void extmul(size_t a, size_t b, size_t *lo, size_t *hi) {
    __int128 result = (__int128)a * (__int128)b;
    *lo = (size_t)result;
    *hi = result >> 64;
}

On x86_64 gcc is smart enough to compile this to

   0:   48 89 f8                mov    %rdi,%rax
   3:   49 89 d0                mov    %rdx,%r8
   6:   48 f7 e6                mul    %rsi
   9:   49 89 00                mov    %rax,(%r8)
   c:   48 89 11                mov    %rdx,(%rcx)
   f:   c3                      retq   

No native 128 bit support or similar required, and after inlining only the mul instruction remains.

Edit: On a 32 bit arch this works in a similar way, you need to replace __int128_t by uint64_t and the shift width by 32. The optimization will work on even older gccs.