Date: Mon, 16 Feb 2015 10:12:49 -0500 From: Rich Felker <dalias@...c.org> To: Denys Vlasenko <vda.linux@...glemail.com> Cc: musl <musl@...ts.openwall.com> Subject: Re: [PATCH] x86_64/memset: use "small block" code for blocks up to 30 bytes long On Mon, Feb 16, 2015 at 11:09:25AM +0100, Denys Vlasenko wrote: > On Sun, Feb 15, 2015 at 11:55 PM, Rich Felker <dalias@...c.org> wrote: > >> "sub $8,%rcx" can be folded into lea. > > > > No, it can't, for the above reason. > > Why can't? > > lea -9(%rdx), %rcx > > will also underflow for %rdx == 0, just like "lea -1". Then n==1..8 would also fall into the rep stosq path and either fail horribly or need to use slow fixup code there. Rich
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