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From: Rich Felker <dalias@libc.org>
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Subject: Re: [PATCH] x86_64/memset: use "small block" code for blocks
 up to 30 bytes long
Date: Mon, 16 Feb 2015 10:12:49 -0500
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To: Denys Vlasenko <vda.linux@googlemail.com>
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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@libc.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