linux-kernel - Re: [RFC] Improve memset

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Message-ID: <CAHk-=wiw6q1hk7QHP+8M6H9zRDj-yJ8p-KyhQS9gTJA8NopYkA@mail.gmail.com>
Date:   Mon, 16 Sep 2019 16:26:20 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        Borislav Petkov <bp@...en8.de>,
        Rasmus Villemoes <mail@...musvillemoes.dk>,
        x86-ml <x86@...nel.org>, Josh Poimboeuf <jpoimboe@...hat.com>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] Improve memset

On Mon, Sep 16, 2019 at 4:14 PM Andy Lutomirski <luto@...nel.org> wrote:
>
> Well, when I wrote this email, I *thought* it was inlining the
> 'memset' function, but maybe I just can't read gcc's output today.

Not having your compiler, it's also possible that it works for you,
but just doesn't work for me.

> It seems like gcc is maybe smart enough to occasionally optimize
> memset just because it's called 'memset'.  This generates good code:

Yup, that does the rigth thing for me and ignores the definition of
memset() in favor of the built-in one.

However, at least part of this discussion started because of the
reverse problem (turning a couple of assignments into memset), and the
suggestion that we might be able to use -ffreestanding together with

  #define memset __builtin_memset

and then your nice code generation goes away, because the magical
treatment of memset goes away. I get

        one_word:
                xorl    %eax, %eax
                ret

        not_one_word:
                movq    %rsi, %rdx
                xorl    %esi, %esi
                jmp     memset

despite having that "inline void *memset()" definition.

             Linus