Re: [PATCH] x86_64: use -mskip-rax-setup when available

Borislav Petkov
2015-04-20T15:07:42Z

On Mon, Apr 20, 2015 at 02:28:16PM +0200, Rasmus Villemoes wrote:
> gcc 5 supports the option -mskip-rax-setup to avoid emitting the
> two-byte instruction xor %eax,%eax before a vararg function
> call. Clearing %eax is redundant in the kernel, since no callee
> expects any floating point arguments, and hence never use the incoming
> value of %eax for anything.
>
> For a defconfig kernel, the .text saving is around 26 kB:
>
> $ size /tmp/vmlinux.{old,new}
> text data bss dec hex filename
> 12221850 1735352 1077248 15034450 e56852 /tmp/vmlinux.old
> 12195955 1735352 1077248 15008555 e5032b /tmp/vmlinux.new
>
> Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> ---
>
> Nothing seems to have happened on the kernel side since H.J. Lu
> implemented this in December. Since gcc 5 is officially released

I see the same patch from him from Apr. 16th:

https://lkml.kernel.org/r/CAMe9rOrfDo3MGaQc_s5crw3TrYhS8NtqeyRcUFiA0yAV8e9Nig@mail.gmail.com

Why are you sending it too?

--
Regards/Gruss,
Boris.

ECO tip #101: Trim your mails when you reply.
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