[PATCH RFC 2/2 V3] Improve 64bit memset for Corei7 with avx2 instruction

[Ling Ma]

Thanks for your code, I run on haswell as below:

[root@localhost test_broadcast]# ./test
0.19    0.19
0.19    0.19
0.19    0.19
0.19    0.19
0.19    0.19
0.19    0.19
0.19    0.19
0.19    0.19
0.19    0.19
0.19    0.19



2013/7/30, Ondřej Bílka <neleai@seznam.cz>:
> On Tue, Jul 30, 2013 at 01:35:49PM +0800, Ling Ma wrote:
>> >> >> +L(less_128bytes):
>> >> >> +	xor	%esi, %esi
>> >> >> +	mov	%ecx, %esi
>> >> > And this? A C equivalent of this is
>> >> > x = 0;
>> >> > x = y;
>> >> Ling: we used mov %sil, %cl in above code, now %esi become  as
>> >> destination register(mov %ecx, %esi),  there is one false dependence
>> >> hazard, we use xor r1, r1 to ask decode stage to break the dependence,
>> >> and insight pipeline xor r1, r1  will be removed  before entering into
>> >> execution stage.
>> >>
>> > That is pointless as mov breaks false dependencies.
>> >
>> > Anyway a code you use is redudnand. You already have that computed so
>> > simple mov %xmm0, %rcx will do a job.
>>
>> Ling: Usually rename stage can help us to resolve most of WAR, WAW,
>> but we use %sil, instead of %esi, which is related with patial
>> register access.
> It does not matter. Also you have plenty of free registers available
>> i remember mov xmm0, r32/64  will cause cross-domain operation, it is
>> not good on nehalem, i may test whether it exists on haswell.
> Wrong again even on nehalem. I tested your and xmm code and you are 50%
> slowerand you are 50% slower (and I am not counting that computing
> pshufb is free in our case).
>
> A time in seconds to calculate broadcast 1000000000 times is:
> your    sse
> 0.37	0.23
> 0.36	0.23
> 0.36	0.23
> 0.36	0.24
> 0.36	0.23
> 0.36	0.23
> 0.37	0.24
> 0.36	0.23
> 0.36	0.23
> 0.36	0.23
>
> In  attached benchmark.
>
>