Re: kernel + gcc 4.1 = several problems

From: "Albert Cahalan" <acahalan@gmail.com>
To: mikpe@it.uu.se, s0348365@sms.ed.ac.uk, torvalds@osdl.org,
	linux-kernel@vger.kernel.org, akpm@osdl.org, bunk@stusta.de
Subject: Re: kernel + gcc 4.1 = several problems
Date: Thu, 4 Jan 2007 02:11:42 -0500	[thread overview]
Message-ID: <787b0d920701032311l2c37c248s3a97daf111fe88f3@mail.gmail.com> (raw)

Linus Torvalds writes:
> [probably Mikael Pettersson] writes:

>> The suggestions I've had so far which I have not yet tried:
>>
>> - Select a different x86 CPU in the config.
>>   - Unfortunately the C3-2 flags seem to simply tell GCC to
>>     schedule for ppro (like i686) and enabled MMX and SSE
>>   - Probably useless
>
> Actually, try this one. Try using something that doesn't like "cmov".
> Maybe the C3-2 simply has some internal cmov bugginess.

Of course that changes register usage, register spilling,
and thus ultimately even the stack layout. :-(

Adjusting gcc flags to eliminate optimizations is another way to go.
Adding -fwrapv would be an excellent start. Lack of this flag breaks
most code which checks for integer wrap-around. The compiler "knows"
that signed integers don't ever wrap, and thus eliminates any code
which checks for values going negative after a wrap-around. I could
imagine this affecting a switch() or other jump table.