linux-kernel - Re: [PATCH] gcc4: Add 'asm goto' miscompilation quirk

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Date:	Thu, 10 Oct 2013 10:45:35 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Jakub Jelinek <jakub@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Richard Henderson <rth@...ddle.net>
Subject: Re: [PATCH] gcc4: Add 'asm goto' miscompilation quirk


* Jakub Jelinek <jakub@...hat.com> wrote:

> On Thu, Oct 10, 2013 at 10:24:30AM +0200, Ingo Molnar wrote:
> > Something like the patch below? (Totally untested and all that.)
> > 
> > Notes:
> > 
> > - If the bug is fixed in 4.8.3 then the version check can be sharpened
> >   from 99999 to 40803.
> 
> The bug is likely going to be fixed already for 4.8.2 (to be released
> next week or so).
> 
> > - I'd really prefer this quirk versus having to add the extra barrier to 
> >   the label, as it makes the actual usage sites a lot less painful.
> 
> Please check how much it bloats the generated code.

on Linus's latest tree plain 'asm goto' is only rarely used, so the size 
impact of the patch, on x86-64 defconfig, is zero:

 vmlinux:
     text           data     bss      dec            hex filename
 11166823        1448024 1187840 13802687         d29cbf vmlinux.before
 11166823        1448024 1187840 13802687         d29cbf vmlinux.after
 1f7c3c1fd71b4b1327965bcfbc7a77fb  vmlinux.before
 3b246add801bc170e47096b712942395  vmlinux.after

Using GCC 4.8.1.

> Also, for the bitops patch, you probably want an asm_volatile_goto variant.

Also, for mutex_64.h - good point, will fix that.

Thanks,

	Ingo
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