linux-kernel - Re: [RFC][PATCH 3/2] x86/jump labels: Count and display the short jumps used

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Message-ID: <5202ABEF.10107@akamai.com>
Date:	Wed, 07 Aug 2013 16:19:59 -0400
From:	Jason Baron <jbaron@...mai.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC][PATCH 3/2] x86/jump labels: Count and display the short
 jumps used

On 08/07/2013 03:22 PM, Linus Torvalds wrote:
> On Wed, Aug 7, 2013 at 10:54 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
>> On another box, using a distro config, I had even better results:
>>
>> [    2.352448] short jumps: 193
>> [    2.355407]  long jumps: 219
> .. well, another way of looking at this is to say that all of this
> effort saves just 579 bytes.
>
> Yes, maybe some of those bytes are in really hot paths, but the other
> side of *that* coin is that the 2-vs-5 byte jump doesn't much matter
> if it's already cached.
>
> So I'd vote for not doing this. If we had some simple way to do the
> short jumps, I think it would be lovely. Or if we had to parse the ELF
> files and do instruction rewriting for various other reasons, and the
> jump rewriting was just one small detail.
>
> But using 576 new lines (the diffstat for your patch 1/2 that adds the
> infrastructure to do the rewriting) in order to same just about
> exactly that many bytes in the binary - the effort just doesn't work
> out, imnsho.
>
>               Linus

The whole point of the thread started with wanting to move the default
'disabled' branch further out-of-line. We could get there with better
compiler support for the 'cold' label attribute. Thus, in theory the
whole 2-byte jmp is just an intermediate step. (Yeah, I know that
support doesn't seem to be happening anytime soon...)

Thanks,

-Jason


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