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Date: Mon, 12 Oct 2020 14:18:54 +0200
From: Denys Vlasenko <dvlasenk@...hat.com>
To: Rich Felker <dalias@...c.org>, Denys Vlasenko <vda.linux@...glemail.com>
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH] x86/memset: avoid performing final store twice
On 10/11/20 2:25 AM, Rich Felker wrote:
> On Sun, Oct 04, 2020 at 12:32:09AM +0200, Denys Vlasenko wrote:
>> From: Denys Vlasenko <dvlasenk@...hat.com>
>>
>> For not very short NBYTES case:
>>
>> To handle the tail alignment, the code performs a potentially
>> misaligned word store to fill the final 8 bytes of the buffer.
>> This is done even if the buffer's end is aligned.
>>
>> Eventually code fills the rest of the buffer, which is a multiple
>> of 8 bytes now, with NBYTES / 8 aligned word stores.
>>
>> However, this means that if NBYTES *was* divisible by 8,
>> we store last word too, again.
>>
>> This patch decrements byte count before dividing it by 8,
>> making one less store in "NBYTES is divisible by 8" case,
>> and not changing anything in all other cases.
>>
...
>> --- a/src/string/x86_64/memset.s
>> +++ b/src/string/x86_64/memset.s
>> @@ -53,7 +53,7 @@ memset:
>> 2: test $15,%edi
>> mov %rdi,%r8
>> mov %rax,-8(%rdi,%rdx)
>> - mov %rdx,%rcx
>> + lea -1(%rdx),%rcx
>> jnz 2f
>>
>> 1: shr $3,%rcx
>> --
>> 2.25.0
>
> Does this have measurably better performance on a system you've tested
> it on?
I did not test performance, I predict it will hardly be detectable.
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