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

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Message-ID: <AANLkTinr8wFmXibOy8SgAsbH=u8G7QVp4UUcjTRjOOqs@mail.gmail.com>
Date:	Mon, 15 Nov 2010 10:56:14 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jim Bos <jim876@...all.nl>
Cc:	Jakub Jelinek <jakub@...hat.com>, Andi Kleen <andi@...stfloor.org>,
	James Cloos <cloos@...loos.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andreas Schwab <schwab@...hat.com>,
	Michael Matz <matz@...e.de>,
	Dave Korn <dave.korn.cygwin@...il.com>,
	Richard Guenther <richard.guenther@...il.com>, gcc@....gnu.org
Subject: Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?

On Mon, Nov 15, 2010 at 10:30 AM, Jim Bos <jim876@...all.nl> wrote:
>
> Attached version with plain 2.6.36 source and version with the committed
> patch, i.e with the '"+m" (*regs)'

Looks 100% identical in i8k_smm() itself, and I'm not seeing anything
bad. The asm has certainly not been optimized away as implied in the
archlinux thread.

There are differences, but they are with code generation *elsewhere*.

To me it is starting to look like the real problem is that gcc has
decided that the "i8k_smm()" function is "__attribute__((const))".

Which is clearly totally bogus. If a function has an inline asm that
has a memory clobber, it is clearly *not* 'const'. But that does
explain the bug, and does explain why "+m" makes a difference and why
"noinline" does not.

So what I _think_ happens is that

 - gcc logic for the automatic 'const' attribute for functions is
broken, so it marks that function 'const'.

 - since the rule for a const function is that it only _looks_ at its
attributes and has no side effects, now the callers will decide that
'i8k_smm()' cannot change the passed-in structure, so they'll happily
optimize away all the accesses to it.

                       Linus
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