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Re: Conditional negation elimination in tree-ssa-phiopt.c
- From: Segher Boessenkool <segher at kernel dot crashing dot org>
- To: Richard Earnshaw <rearnsha at arm dot com>
- Cc: Kyrill Tkachov <kyrylo dot tkachov at arm dot com>, Jeff Law <law at redhat dot com>, Richard Biener <richard dot guenther at gmail dot com>, "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>
- Date: Wed, 13 Aug 2014 12:32:26 -0500
- Subject: Re: Conditional negation elimination in tree-ssa-phiopt.c
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On Wed, Aug 13, 2014 at 03:57:31PM +0100, Richard Earnshaw wrote:
> The problem with the frankenmonster patterns is that they tend to
> proliferate into the machine description, and before you know where you
> are the back-end is full of them.
> Furthermore, they are very sensitive to the greedy first-match nature of
> combine: a better, later, combination is missed because a less good,
> earlier, optimization matched. If the first insn in the sequence is
> merged into an earlier instruction then you can end up with a junk
> sequence that completely fails to simplify. That ends up with
> super-frankenmonster patterns to deal with all the subcases and the
> problems grow exponentially from there.
Right. Of course, combine should be fixed, yadda yadda.
> I really do think that the best solution would be to try and catch this
> during expand if possible and generate the right pattern from the start;
> then you don't risk combine failing to come to the rescue after several
> intermediate transformations have taken place.
I think ssa-phiopt should simply not do this obfuscation at all. Without
it, RTL ifcvt picks it up just fine on targets with conditional assignment
instructions. I agree on targets without expand should do a better job
(also for more generic conditional assignment).
Instruction selection belongs in RTL land.