linux-kernel - Re: [RFC] Improve memset

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Message-ID: <20190914092915.GA17409@avx2>
Date:   Sat, 14 Sep 2019 12:29:15 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     bp@...en8.de
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org, x86@...nel.org,
        linux@...musvillemoes.dk, torvalds@...ux-foundation.org
Subject: Re: [RFC] Improve memset

> Instead of calling memset:
> 
> ffffffff8100cd8d:       e8 0e 15 7a 00          callq  ffffffff817ae2a0 <__memset>
> 
> and having a JMP inside it depending on the feature supported, let's simply
> have the REP; STOSB directly in the code:
> 
> ...
> ffffffff81000442:       4c 89 d7                mov    %r10,%rdi
> ffffffff81000445:       b9 00 10 00 00          mov    $0x1000,%ecx
> 
> <---- new memset
> ffffffff8100044a:       f3 aa                   rep stos %al,%es:(%rdi)
> ffffffff8100044c:       90                      nop
> ffffffff8100044d:       90                      nop
> ffffffff8100044e:       90                      nop

You can fit entire "xor eax, eax; rep stosb" inside call instruction.

> /* clobbers used by memset_orig() and memset_rep_good() */
>                 : "rsi", "rdx", "r8", "r9", "memory");

eh... I'd just drop it. These registers screw up everything.

Time to rebase memset0().