gcc 4.3.0 i386 default question

[Joel Sherrill]

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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Joel Sherrill, Ph.D.             Director of Research & Development
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