This is the mail archive of the
gcc@gcc.gnu.org
mailing list for the GCC project.
Re: [RFC] Combine related fail of gcc.target/powerpc/ti_math1.c
- From: Richard Henderson <rth at redhat dot com>
- To: Segher Boessenkool <segher at kernel dot crashing dot org>
- Cc: gcc at gcc dot gnu dot org, Alan Modra <amodra at gmail dot com>
- Date: Thu, 21 May 2015 11:59:21 -0700
- Subject: Re: [RFC] Combine related fail of gcc.target/powerpc/ti_math1.c
- Authentication-results: sourceware.org; auth=none
- References: <20150521103604 dot GK6140 at bubble dot grove dot modra dot org> <20150521123916 dot GA26096 at gate dot crashing dot org> <555E2526 dot 5070301 at redhat dot com> <20150521184431 dot GC10871 at gate dot crashing dot org>
On 05/21/2015 11:44 AM, Segher Boessenkool wrote:
> On Thu, May 21, 2015 at 11:34:14AM -0700, Richard Henderson wrote:
>> Actually, I believe that the way CA is modeled at the moment is dangerous.
>> It's not a 64-bit value, but a 1-bit value.
>
> It's a fixed register and it is only ever set to 0 or 1. There are
> more targets that do such things, and it is safe.
Old Cygnus proverb: Lie to the compiler and it will always bite you in the end.
> I've tried with BImode before, with two effects: 1) the patterns become
> much more unmanageable; and 2) the optimisers do a lousy job on it.
> BImode isn't so well supported.
Really? Zero-extending from BImode should be no different than from SImode,
and we handle that all the time.
> Let's wait for Alan's patch that makes combine not reorder things
> unnecessarily, that should take care of it all as far as I see.
I remain skeptical, but I'm also willing to let someone else worry about it. ;-)
r~