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Re: Variations of memset()

On Fri, Aug 4, 2017 at 12:54 PM, Carlos O'Donell <> wrote:
> On 08/04/2017 03:11 PM, Carlos O'Donell wrote:
>> On 08/04/2017 03:02 PM, Matthew Wilcox wrote:
>>> Here's the sample usage from the symbios driver:
>>> -               for (i = 0 ; i < 64 ; i++)
>>> -                       tp->luntbl[i] = cpu_to_scr(vtobus(&np->badlun_sa));
>>> +               memset32(tp->luntbl, cpu_to_scr(vtobus(&np->badlun_sa)), 64);
>>> I expect a lot of users would be of this type; simply replacing the
>>> explicit for-loop equivalent with a library call.
>> Have you measured the performance of this kind of conversion when using a
>> simple application and a library implementing your various memset routines?
>> In the kernel is one thing, outside of the kernel we have dynamic linking
>> and no-inling across that shared object boundary.
> I want to  reiterate that measuring the performance of various options in
> userspace is going to be relevant (particularly when they vary from the kernel):
> * Application doing the naive loop above (-O0).
> * Application doing the naive loop above ([-O2,-O3] + <vectorize options>).
> * Application calling memset32 (-O0)
> * Application calling memset32 (-O3)
> <vectorize options>="-ftree-vectorize [-msse2,-mavx] -fopt-info-missed=missed.all"
> You need to split the memset32 into another DSO to simulate this accurately.

These functions aren't very useful for x86-64 where wmemset,
aka, memset32, is implemented with memset:

Dump of assembler code for function __wmemset_sse2_unaligned:
   0x0000000000000020 <+0>: shl    $0x2,%rdx
   0x0000000000000024 <+4>: movd   %esi,%xmm0
   0x0000000000000028 <+8>: mov    %rdi,%rax
   0x000000000000002b <+11>: pshufd $0x0,%xmm0,%xmm0
   0x0000000000000030 <+16>: jmp    0x64 <__memset_sse2_unaligned+20>
End of assembler dump.


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