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Re: [PATCH] x86-64: Optimize strcat/strncat, strcpy/strncpy and stpcpy/stpncpy with AVX2
On 31/10/2018 17:23, Leonardo Sandoval wrote:
> On Wed, 2018-10-31 at 15:36 -0300, Adhemerval Zanella wrote:
>>
>>
>>> diff --git a/sysdeps/x86_64/multiarch/strcat-avx2.S
>>> b/sysdeps/x86_64/multiarch/strcat-avx2.S
>>> new file mode 100644
>>> index 00000000000..b0623564276
>>> --- /dev/null
>>> +++ b/sysdeps/x86_64/multiarch/strcat-avx2.S
>>> @@ -0,0 +1,275 @@
>>> +/* strcat with AVX2
>>
>> Is this really a gain on real work usage comparing to generic strcat
>> (
>> (strcpy (dest + strlen (dest), src)) assuming optimized strcpy /
>> strlen?
>
> The speedup is briefly summarized on the commit description but the
> comparison was done between SSE2 Unaligned and AVX2 (current code).
> Either SSE2 and AVX2 are much faster the generic strcat (I can provide
> numbers).
>
As Rich has put, this strcat optimization seems to use the very
strategy of strcpy/strlen and basically brings a constant gain with
downside of i-cache overhead plus maintainability cost (since a possible
bug in strcpy/strlen implementation will need to be checked against
strcat as well).
I also tend to agree with Rich that strcat is an antipattern, and I
would prefer we use simpler generic optimization for such cases.
>
>> Wouldn't be simple and more i-cache friendly to use a custom generic
>> implementation that calls AVX2 strcpy/strlen (such as powerpc64
>> does)?
>
> I believe this is what we have right now through IFUNC_SELECTOR.
>
What I meant was use something like:
char *strcat_avx2 (char *dest, const char *src)
{
__strcpy_avx2 (dest + __strlen_avx2 (dest), src);
return dest;
}