Re: [PATCH], PR target/68404, Fix PowerPC fusion error

[David Edelsohn <dje dot gcc at gmail dot com>]

On Tue, Feb 9, 2016 at 9:49 AM, Michael Meissner
<meissner@linux.vnet.ibm.com> wrote:
> This bug fixes PR 68404, which created an insn for the fusion operation when
> accessing an array with a large constant offset that the downstream passes
> (regrenam in particular don't like).  Because fusion in general adds so little
> to the performance of power8, I just eliminated the compiler from generating
> this case for GCC 6.  In the GCC 7 timeframe, I likely will revist fusion for
> power9 support.  I ran a spec 2006 benchmark suite comparing the current
> behavior and the fix for PR 68404, and it was in the noise level (mcf was 1%
> slower, others ranged from 0.3% slower to 0.4% faster).
>
> I did a bootstrap build, including a bootstrap profiled build with LTO (which
> is how the problem was found) and it was found.  I rewrote 2 of the 3 fusion
> tests so that it uses fusion from a medium code toc entry instead of accessing
> an array element with a constant index over 65536 bytes.
>
> Is this patch ok to apply?  If you would prefer, I can eliminate the code
> inside of the fusion_gpr_addis predicate instead of using #if 0.
>
> [gcc]
> 2016-02-08  Michael Meissner  <meissner@linux.vnet.ibm.com>
>
>         PR target/68404
>         * config/rs6000/predicates.md (fusion_gpr_addis): Prevent fusing
>         an ADDIS that adds a pointer to a large constant that sets the
>         upper16 bits with a load operation.
>
> [gcc/testsuite]
> 2016-02-08  Michael Meissner  <meissner@linux.vnet.ibm.com>
>
>         PR target/68404
>         * gcc.target/powerpc/fusion.c: Rewrite test to use TOC fusion
>         instead accessing a really large arrray.
>         * gcc.target/powerpc/fusion3.c: Likewise.

Please remove the code entirely, not #if 0.

Okay with that change.

Thanks, David