Re: More aggressive threading causing loop-interchange-9.c regression

[Jeff Law <jeffreyalaw@gmail.com>]

On 9/10/2021 7:53 AM, Aldy Hernandez via Gcc-patches wrote:
>
>
> On 9/10/21 3:16 PM, Michael Matz wrote:
>> Hi,
>>
>> On Fri, 10 Sep 2021, Aldy Hernandez via Gcc-patches wrote:
>>
>>>       }
>>> +
>>> +  /* Threading through a non-empty latch would cause code to be added
>>
>> "through an *empty* latch".  The test in code is correct, though.
>
> Whoops.
>
>>
>> And for the before/after loops flag you added: we have a
>> cfun->curr_properties field which can be used.  We even already have a
>> PROP_loops flag but that is set throughout compilation from CFG
>> construction until the RTL loop optimizers, so can't be re-used for what
>> is needed here.  But you still could invent another PROP_ value 
>> instead of
>> adding a new field in struct function.
>
> Oooo, even better.  No inline functions.
>
> Like this?
> Aldy
>
> 0001-Disable-threading-through-latches-until-after-loop-o.patch
>
>  From ff25faa8dd8721da9bb4715706c662fc09fd4e8c Mon Sep 17 00:00:00 2001
> From: Aldy Hernandez <aldyh@redhat.com>
> Date: Thu, 9 Sep 2021 20:30:28 +0200
> Subject: [PATCH] Disable threading through latches until after loop
>   optimizations.
>
> The motivation for this patch was enabling the use of global ranges in
> the path solver, but this caused certain properties of loops being
> destroyed which made subsequent loop optimizations to fail.
> Consequently, this patch's mail goal is to disable jump threading
> involving the latch until after loop optimizations have run.
>
> As can be seen in the test adjustments, we mostly shift the threading
> from the early threaders (ethread, thread[12] to the late threaders
> thread[34]).  I have nuked some of the early notes in the testcases
> that came as part of the jump threader rewrite.  They're mostly noise
> now.
>
> Note that we could probably relax some other restrictions in
> profitable_path_p when loop optimizations have completed, but it would
> require more testing, and I'm hesitant to touch more things than needed
> at this point.  I have added a reminder to the function to keep this
> in mind.
>
> Finally, perhaps as a follow-up, we should apply the same restrictions to
> the forward threader.  At some point I'd like to combine the cost models.
>
> Tested on x86-64 Linux.
>
> p.s. There is a thorough discussion involving the limitations of jump
> threading involving loops here:
>
> 	https://gcc.gnu.org/pipermail/gcc/2021-September/237247.html
>
> gcc/ChangeLog:
>
> 	* tree-pass.h (PROP_loop_opts_done): New.
> 	* gimple-range-path.cc (path_range_query::internal_range_of_expr):
> 	Intersect with global range.
> 	* tree-ssa-loop.c (tree_ssa_loop_done): Set PROP_loop_opts_done.
> 	* tree-ssa-threadbackward.c
> 	(back_threader_profitability::profitable_path_p): Disable
> 	threading through latches until after loop optimizations have run.
>
> gcc/testsuite/ChangeLog:
>
> 	* gcc.dg/tree-ssa/ssa-dom-thread-2b.c: Adjust for disabling of
> 	threading through latches.
> 	* gcc.dg/tree-ssa/ssa-dom-thread-6.c: Same.
> 	* gcc.dg/tree-ssa/ssa-dom-thread-7.c: Same.
OK
jeff