Re: [PATCH] Optimize certain end of loop conditions into min/max operation

[Richard Biener <richard dot guenther at gmail dot com>]

On Fri, Jul 31, 2015 at 8:27 PM, Jeff Law <law@redhat.com> wrote:
> On 07/31/2015 12:18 PM, Michael Collison wrote:
>>
>> Hi Jeff,
>>
>> Yes I will create a test case. I'm not quite sure what to check for even
>> in the machine dependent test case. It's quite possible for the
>> instructions that are generated to change over time.
>
> I think we're going to want to look at the gimple IR and search for the
> MIN/MAX expressions rather than the instructions.  Given we don't know where
> the transformation is going to land (yet), you can probably start with
> -fdump-tree-optimized and scanning the .optimized dump.

Yeah.  FWIW I'm ok with the specialized pattern in match.pd.  Indeed phiopt
isn't a good fit here (though on some targets you may end up seeing a PHI node,
dependent on LOGICAL_OP_NON_SHORT_CIRCUIT).  For longer chains
I'd say reassoc is the appropriate place to handle this as it has
already code to
combine &&s and ||s of conditions.  Maybe you can give it a short try to only
handle it there.

I'm also somewhat worried about code-generation on random targets.  So can
you have a quick look (just for your testcase) what the code-gen difference is
on primary platforms (just build some stage1 cc1s)?

Thanks,
Richard.

> We can still do that and have the test be target specific.
>
> jeff
>