linux-kernel - Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?

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Message-ID: <4CD843DC.10106@gmail.com>
Date:	Mon, 08 Nov 2010 18:39:24 +0000
From:	Dave Korn <dave.korn.cygwin@...il.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Richard Guenther <richard.guenther@...il.com>,
	Andreas Schwab <schwab@...ux-m68k.org>, Jim <jim876@...all.nl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	gcc@....gnu.org
Subject: Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?

On 08/11/2010 11:20, Andi Kleen wrote:

> An asm with live inputs and outputs should never be optimized
> way. If 4.5.1 started doing that it's seriously broken.

  I don't see that.  Consider:

void foo (void)
{
  int x, y, z;
  x = 23;
  y = x + 1;
  z = y + 1;
}

  So far, you'd agree the compiler may optimise the entire function away?  So
why not this:

void foo (void)
{
  int x, y, z;
  x = 23;
  asm ("do something" : "=r" (y) : "r" (x) );
  z = y + 1;
}

  ?

    cheers,
      DaveK
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