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From: "Jan Beulich" <JBeulich@suse.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Xen-devel <xen-devel@lists.xen.org>
Subject: Re: [PATCH] x86/current: Provide additional information to optimise get_cpu_info()
Date: Mon, 01 Sep 2014 12:24:55 +0100	[thread overview]
Message-ID: <540473A7020000780002F6C2@mail.emea.novell.com> (raw)
In-Reply-To: <1409569130-19066-1-git-send-email-andrew.cooper3@citrix.com>

>>> On 01.09.14 at 12:58, <andrew.cooper3@citrix.com> wrote:
> Exactly as with c/s d55c5eefe "x86: use compiler visible "add" instead of
> inline assembly "or" in get_cpu_info()", this is achieved by providing more
> information to the compiler.
> 
> With this modification, gcc replaces the older:
>     mov imm, %reg
>     and %rsp, %reg
> 
> with:
>     mov %rsp, %reg
>     and imm, %reg
> 
> which is one byte shorter.

I'm in no way opposed to the change, but is that really true? Afaict
it can be 1 byte shorter only when %rax gets selected as the register
here.

>  It also considers all general purpose registers
> for %reg rather than just the legacy ones (i.e. will now use %r12 etc), 
> which
> allows for better register scheduling in larger functions.

Same here - why would with the old code not all registers be
available for selection by the compiler?

Jan

  reply	other threads:[~2014-09-01 11:24 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-09-01 10:58 [PATCH] x86/current: Provide additional information to optimise get_cpu_info() Andrew Cooper
2014-09-01 11:24 ` Jan Beulich [this message]
2014-09-01 12:18   ` Andrew Cooper
2014-09-01 12:32     ` Jan Beulich
2014-09-01 15:27       ` [PATCH v2] " Andrew Cooper
2014-09-13 16:10         ` Marcin Cieslak
2014-09-15  8:16           ` Andrew Cooper

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