Re: [RFC] Improve memset

[Linus Torvalds]

On Mon, Sep 16, 2019 at 4:14 PM Andy Lutomirski <luto@xxxxxxxxxx> 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