linux-kernel - Re: [RFC] Improve memset

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Message-ID: <20190919125542.GB18148@nazgul.tnic>
Date:   Thu, 19 Sep 2019 14:55:42 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>, x86-ml <x86@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] Improve memset

On Tue, Sep 17, 2019 at 01:45:20PM -0700, Linus Torvalds wrote:
> That sounds better, but I'm a bit nervous about the whole thing
> because who knows when the alternatives code itself internally uses
> memset() and then we have a nasty little chicken-and-egg problem.

You mean memcpy()...?

> Also, for it to make sense to inline rep stosb, I think we also need
> to just make the calling conventions for the alternative calls be that
> they _don't_ clobber other registers than the usual rep ones
> (cx/di/si). Otherwise one big code generation advantage of inlining
> the thing just goes away.

Yah, that is tricky and I have no smart idea how. The ABI puts the
operands in rdi,rsi,rdx, ... while REP; STOSB wants them in rax,rcx,rdi.
And if it were only that, then we could probably accept the 2 movs and
a push but then the old functions clobber three more: "rdx", "r8", "r9".

I could try to rewrite the old functions to see if I can save some regs...

> On the whole I get the feeling that this is all painful complexity and
> we shouldn't do it. At least not without some hard performance numbers
> for some huge improvement, which I don't think we've seen.

Yap, it is starting to become hairy.

> Because I find the thing fascinating conceptually, but am not at all
> convinced I want to deal with the pain in practice ;)

I hear ya.

Thx.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
--