Re: Make ifcvt clean up dead comparisons

[Richard Sandiford <richard dot sandiford at arm dot com>]

Richard Biener <richard.guenther@gmail.com> writes:
> On Fri, Jul 12, 2019 at 12:28 PM Richard Sandiford
> <richard.sandiford@arm.com> wrote:
>>
>> Richard Biener <richard.guenther@gmail.com> writes:
>> > On Fri, Jul 12, 2019 at 10:00 AM Richard Sandiford
>> > <richard.sandiford@arm.com> wrote:
>> >>
>> >> This change is needed to avoid a regression in gcc.dg/ifcvt-3.c
>> >> for a later patch.  Without it, we enter CSE with a dead comparison left
>> >> by if-conversion and then eliminate the second (live) comparison in
>> >> favour of the dead one.  That's functionally correct in itself, but it
>> >> meant that we'd combine the subtraction and comparison into a SUBS
>> >> before we have a chance to fold away the subtraction.
>> >>
>> >> Tested on aarch64-linux-gnu, aarch64_be-elf and x86_64-linux-gnu.
>> >> OK to install?
>> >
>> > cfg_cleanup if it does, runs fast-dce after cleaning up the CFG so does
>> > it make sense to do this in the caller instead?  (and after removing the
>> > live problem just in case dce tries to keep that updated?)
>>
>> I think that only happens with CLEANUP_CROSSJUMP, which we don't use
>> here as things stand.  But that could easily change in future, and yeah,
>> doing it after removing the live problem would be better.
>>
>> So something like the attached?  (Only lightly tested, but it does
>> still fix the original problem.)
>
> Yeah, that works, too (I was originally thinking to simply put it after
> the cleanup_cfg (0) call ...).
>
> Any reason you use 0x100 instead of 256?

Just trying to set a precedent for future flags. :-)  Beyond 65536 I have
to look it up.

> If nobody objects this is OK.

Thanks.

Richard