On Sun, 17 Mar 2013, H. Peter Anvin wrote:
> How desirable/portable is it to use __int128 on non-x86 64-bit
> architectures to get a 64*64 -> 128 bit multiply? On x86-64 this works
> extremely well, but I'm worried about that needlessly breaking on other
> architectures.
Hmm, nobody has replied, so just FYI such widening multiplication is
available in all 64-bit MIPS hardware and GCC has supported it since 4.4
or mid 2008 (older versions used a libgcc __multi3 helper, not quite so
efficient as you can imagine).
$ cat mulditi3.c
typedef int int128_t __attribute__((mode(TI)));
int128_t mulditi3(long x, long y)
{
int128_t _x = x, _y = y;
return _x * _y;
}
$ mips64-linux-gcc -O2 -c mulditi3.c
$ mips64-linux-objdump -d mulditi3.o
mulditi3.o: file format elf64-tradbigmips
Disassembly of section .text:
0000000000000000 <mulditi3>:
0: 0085001c dmult a0,a1
4: 00001812 mflo v1
8: 03e00008 jr ra
c: 00001010 mfhi v0
$
(MFLO and MFHI are register moves from the MD accumulator to GPRs).
There's an unsigned instruction variant as well.
HTH,
Maciej
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