Messages in this thread | | | Date | Thu, 19 Sep 2019 14:55:42 +0200 | From | Borislav Petkov <> | 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. --
| |