Re: [RFC] Improve memset

[Andy Lutomirski]

On Mon, Sep 16, 2019 at 2:30 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Mon, Sep 16, 2019 at 10:41 AM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> >
> > After some experimentation, I think y'all are just doing it wrong.
> > GCC is very clever about this as long as it's given the chance.  This
> > test, for example, generates excellent code:
> >
> > #include <string.h>
> >
> > __THROW __nonnull ((1)) __attribute__((always_inline)) void
> > *memset(void *s, int c, size_t n)
> > {
> >     asm volatile ("nop");
> >     return s;
> > }
> >
> > /* generates 'nop' */
> > void zero(void *dest, size_t size)
> > {
> >     __builtin_memset(dest, 0, size);
> > }
>
> I think the point was that we'd like to get the default memset (for
> when __builtin_memset() doesn't generate inline code) also inlined
> into just "rep stosb", instead of that tail-call "jmp memset".

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.

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

#include <stddef.h>

inline void *memset(void *dest, int c, size_t n)
{
    /* Boris' implementation */
    void *ret, *dummy;

    asm volatile("push %%rdi\n\t"
                     "mov %%rax, %%rsi\n\t"
                     "mov %%rcx, %%rdx\n\t"
                     "andl $7,%%edx\n\t"
                     "shrq $3,%%rcx\n\t"
                     "movzbl %%sil,%%esi\n\t"
                     "movabs $0x0101010101010101,%%rax\n\t"
                     "imulq %%rsi,%%rax\n\t"
                     "rep stosq\n\t"
                     "movl %%edx,%%ecx\n\t"
                     "rep stosb\n\t"
                     "pop %%rax\n\t"
                     : "=&D" (ret), "=c" (dummy)
                     : "0" (dest), "a" (c), "c" (n)
                     : "rsi", "rdx", "memory");

    return ret;
}

int one_word(void)
{
    int x;
    memset(&x, 0, sizeof(x));
    return x;
}

So maybe Boris' patch is good after all.