Messages in this thread |  | | Subject | Re: [RFC] Improve memset | From | Rasmus Villemoes <> | Date | Fri, 13 Sep 2019 11:18:00 +0200 |
| |
On 13/09/2019 11.00, Linus Torvalds wrote: > On Fri, Sep 13, 2019 at 8:22 AM Borislav Petkov <bp@alien8.de> wrote: >> >> since the merge window is closing in and y'all are on a conference, I >> thought I should take another stab at it. It being something which Ingo, >> Linus and Peter have suggested in the past at least once. >> >> 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: > > That's probably fine for when the memset *is* a call, but: > >> The result is this: >> >> static __always_inline void *memset(void *dest, int c, size_t n) >> { >> void *ret, *dummy; >> >> asm volatile(ALTERNATIVE_2_REVERSE("rep; stosb", > > Forcing this code means that if you do > > struct { long hi, low; } a; > memset(&a, 0, sizeof(a)); > > you force that "rep stosb". Which is HORRID. > > The compiler should turn it into just one single 8-byte store. But > because you took over all of memset(), now that doesn't happen.
OK, that answers my question.
> So we do need to have gcc do the __builtin_memset() for the simple cases..
Something like
if (__builtin_constant_p(c) && __builtin_constant_p(n) && n <= 32) return __builtin_memset(dest, c, n);
might be enough? Of course it would be sad if 32 was so high that this turned into a memset() call, but there's -mmemset-strategy= if one wants complete control. Though that's of course build-time, so can't consider differences between cpu models.
Rasmus
|  |