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

[Christophe Lyon <christophe.lyon.oss@gmail.com>]

On Fri, Sep 10, 2021 at 6:32 PM Jeff Law via Gcc-patches <
gcc-patches@gcc.gnu.org> wrote:

>
>
> 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.
>

Hi,

This last test now fails on aarch64:
FAIL:  gcc.dg/tree-ssa/ssa-dom-thread-7.c scan-tree-dump thread3 "Jumps
threaded: 8"

Can you check?

Thanks,

Christophe

OK
> jeff
>
>