On Mon, Mar 7, 2016 at 5:56 AM, David Laight <david.lai...@aculab.com> wrote:
> From: Alexander Duyck
> ...
>> Actually probably the easiest way to go on x86 is to just replace the
>> use of len with (len >> 6) and use decl or incl instead of addl or
>> subl, and lea instead of addq for the buff address. None of those
>> instructions effect the carry flag as this is how such loops were
>> intended to be implemented.
>>
>> I've been doing a bit of testing and that seems to work without
>> needing the adcq until after you exit the loop, but doesn't give that
>> much of a gain in speed for dropping the instruction from the
>> hot-path. I suspect we are probably memory bottle-necked already in
>> the loop so dropping an instruction or two doesn't gain you much.
>
> Right, any superscalar architecture gives you some instructions
> 'for free' if they can execute at the same time as those on the
> critical path (in this case the memory reads and the adc).
> This is why loop unrolling can be pointless.
>
> So the loop:
> 10: addc %rax,(%rdx,%rcx,8)
> inc %rcx
> jnz 10b
> could easily be as fast as anything that doesn't use the 'new'
> instructions that use the overflow flag.
> That loop might be measurable faster for aligned buffers.
Tested by replacing the unrolled loop in my patch with just:
if (len >= 8) {
asm("clc\n\t"
"0: adcq (%[src],%%rcx,8),%[res]\n\t"
"decl %%ecx\n\t"
"jge 0b\n\t"
"adcq $0, %[res]\n\t"
: [res] "=r" (result)
: [src] "r" (buff), "[res]" (result), "c"
((len >> 3) - 1));
}
This seems to be significantly slower:
1400 bytes: 797 nsecs vs. 202 nsecs
40 bytes: 6.5 nsecs vs. 26.8 nsecs
Tom
>
> David
>
