Re: [PATCH] Fix PR46728 (move pow/powi folds to tree phases)

[Richard Guenther <richard.guenther@gmail.com>]

On Fri, May 13, 2011 at 5:01 PM, Nathan Froyd <froydnj@codesourcery.com> wrote:
> On 05/13/2011 10:52 AM, William J. Schmidt wrote:
>> This patch addresses PR46728, which notes that pow and powi need to be
>> lowered in tree phases to restore lost FMA opportunities and expose
>> vectorization opportunities.
>>
>> +struct gimple_opt_pass pass_lower_pow =
>> +{
>> + {
>> +  GIMPLE_PASS,
>> +  "lower_pow",                               /* name */
>> +  NULL,                                      /* gate */
>
> Please make this controlled by an option; this pass doesn't need to be run all
> the time.
>
> IMHO, the pass shouldn't run at anything less than -O3, but that's for other
> people to decide.

It was run unconditionally before, so unless we preserve the code at
expansion time we have to do it here.

I will have a closer look at the patch early next week.  Btw, I thought
of adding a POW_EXPR tree code that can take mixed-mode operands
to make foldings (eventually) simpler, but I'm not sure it's worth the
trouble.

The position of the pass is odd - why did you place it there?  I would
have placed it alongside pass_cse_sincos and pass_optimize_bswap.
The foldings should probably be done via fold-stmt only (where they
should already apply), and you won't catch things like pow(sqrt(...))
there because you only see the outer call.  That said, I'd be happier
if the patch just did the powi expansion and left the rest to somebody
else.

Richard.