Re: gcc 4.3.0 i386 default question

[Joel Sherrill <joel dot sherrill at oarcorp dot com>]

Richard Guenther wrote:
> On Wed, Mar 12, 2008 at 4:23 PM, Joel Sherrill
> <joel.sherrill@oarcorp.com> wrote:
>   
> > Hi,
> >
> >  Did the default i386 CPU model that gcc generates
> >  code for change between 4.2.x and 4.3.0?  I didn't
> >  see anything in the release notes that jumps out at
> >  me about this.
> >
> >  Using i386-rtems4.9 as the target, I was running
> >  code compiled by gcc 4.3.0 on a  vanilla i386 and
> >  was getting illegal instruction  exceptions.  I looked
> >  at the assembly and noticed instructions which were
> >  not on the i386.  Obviosuly, when I added -mtune=i386
> >  and the issue went away.
> >
> >  This probably impacts no one on a GNU/Linux computer
> >  since very few of those are base i386's anymore.  Just
> >  venerable embedded systems. :)
> >     
>
> -mtune doesn't change the instruction set, -march/-mcpu does.
> So this is probably a bug in the instruction predicates.  What
> is the illegal instruction gcc emits?
>   
I saw a series of instructions with something like
mm0 as a register.  I can't duplicate it now but
also realized that a background automated run
of tests may have been replacing the boot image
behind my back. <sigh>  The danger of multitasking.

I am sorry to have raised this.  I think this points out
what is happening in the failures I see when running
the gcc testsuite though:

http://gcc.gnu.org/ml/gcc-testresults/2008-02/msg00944.html

Lots of tests with "vect" or "sse" in the name fail when run
on the same vanilla i386 target.  For example,
gcc.dg/vect/costmodel/i386/costmodel-vect-31.c fails with
a similar illegal instruction exception.  Looking at that particular
case, I see that it is compiled with -msse2 and fails with an
illegal instruction exception when it tries this one:

 10022a:       f2 0f 10 c0             movsd  %xmm0,%xmm0

Is there any way to skip these tests for particular HW features
that are not present?  There are similar failures on the PowerPC
target I use for reporting because it doesn't have Altivec.

Thanks.  Sorry for the bogus question.

> Richard.
>   


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