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

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Date:	Wed, 07 Aug 2013 12:27:51 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Jason Baron <jbaron@...mai.com>
Subject: Re: [RFC][PATCH 3/2] x86/jump labels: Count and display the short jumps used

Well we do... both to extract relocations and to sort the exception table.  Perhaps we need to merge those kinds of postprocessing tools?

Linus Torvalds <torvalds@...ux-foundation.org> 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

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