linux-kernel - Re: Using __int128 on 64-bit architectures

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Date:	Sun, 21 Apr 2013 05:29:28 +0100 (BST)
From:	"Maciej W. Rozycki" <macro@...ux-mips.org>
To:	"H. Peter Anvin" <hpa@...or.com>
cc:	Linux Arch Mailing List <linux-arch@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Using __int128 on 64-bit architectures

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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