Re: [PATCH v1] [RFC] Improve folding for comparisons with zero in tree-ssa-forwprop.

[Jeff Law <jeffreyalaw@gmail.com>]

On 4/25/23 01:21, Richard Biener wrote:
> On Tue, Apr 25, 2023 at 1:05 AM Jeff Law <jeffreyalaw@gmail.com> wrote
>>
>>
>>
>>
>> On 4/24/23 02:06, Richard Biener via Gcc-patches wrote:
>>> On Fri, Apr 21, 2023 at 11:01 PM Philipp Tomsich
>>> <philipp.tomsich@vrull.eu> wrote:
>>>>
>>>> Any guidance on the next steps for this patch?
>>>
>>> I think we want to perform this transform later, in particular when
>>> the test is a loop exit test we do not want to do it as it prevents
>>> coalescing of the IV on the backedge at out-of-SSA time.
>>>
>>> That means doing the transform in folding and/or before inlining
>>> (the test could become a loop exit test) would be a no-go.  In fact
>>> for SSA coalescing we'd want the reverse transform in some cases, see
>>> PRs 86270 and 70359.
>>>
>>> If we can reliably undo for the loop case I suppose we can do the
>>> canonicalization to compare against zero.  In any case please split
>>> up the patch (note
>> I've also
>>> hoped we could eventually get rid of that part of
>>> tree-ssa-forwprop.cc
>> in favor
>>> of match.pd patterns since it uses GENERIC folding :/).
>>>
>> Do we have enough information to do this at expansion time?  That would
>> avoid introducing the target dependencies to drive this in gimple.
> 
> I think so, but there isn't any convenient place to do this I think.  I suppose
> there's no hope to catch it during RTL opts?
Combine would be the most natural place in the RTL pipeline, but it'd be 
a 2->2 combination which would be rejected.

We could possibly do it as a define_insn_and_split, but the gimple->RTL 
interface seems like a better fit to me.  If TER has done its job, we 
should see a complex enough tree node to do the right thing.

jeff