Re: [PATCH] C undefined behavior fix

From: Alexandre Oliva (
Date: Sun Jan 06 2002 - 20:40:28 EST

On Jan 6, 2002, Tom Rini <> wrote:

> On Sun, Jan 06, 2002 at 11:19:40PM +0100, Jakub Jelinek wrote:
>> On Mon, Jan 07, 2002 at 08:59:47AM +1100, Paul Mackerras wrote:

>> > asm("" : "=r" (x) : "0" (y));

>> Even if gcc learned to analyze asm statements (and use it in something other
>> than scheduling), I'm sure this wouldn't be optimized away exactly because
>> this construct is used by various projects exactly for this purpose (make
>> gcc think it can have any value allowed for the type in question).

> Yes, but there's no gaurentee of that. It'd probably break a few things
> if they did, but there's nothing stopping them from doing it.

If we documented this side effect of such asm statements, one would
have to come up with a very strong case to change it.

Alexandre Oliva   Enjoy Guarana', see
Red Hat GCC Developer                  aoliva@{,}
CS PhD student at IC-Unicamp        oliva@{,}
Free Software Evangelist    *Please* write to mailing lists, not to me
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