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

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Date:	Thu, 10 Oct 2013 10:31:07 +0200
From:	Jakub Jelinek <jakub@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
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

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

> --- a/include/linux/compiler-gcc4.h
> +++ b/include/linux/compiler-gcc4.h
> @@ -65,6 +65,19 @@
>  #define __visible __attribute__((externally_visible))
>  #endif
>  
> +/*
> + * GCC 'asm goto' miscompiles certain code sequences:
> + *
> + *   http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58670
> + *
> + * Work it around via quirk suggested by Jakub Jelinek.
> + * Not yet fixed, so use the quirk on all compiler versions:
> + */
> +#if GCC_VERSION <= 99999
> +# define asm_goto(x...) do { asm goto(x); asm (""); } while (0)
> +#else
> +# define asm_goto(x...) do { asm goto(x); } while (0)
> +#endif
>  
>  #ifdef CONFIG_ARCH_USE_BUILTIN_BSWAP
>  #if GCC_VERSION >= 40400

	Jakub
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