Re: [PATCH] Allow loop header copying when first iteration condition is known.

[Richard Biener <richard.guenther@gmail.com>]

On Thu, Nov 11, 2021 at 11:33 AM Aldy Hernandez <aldyh@redhat.com> wrote:
>
> On Thu, Nov 11, 2021 at 8:30 AM Richard Biener
> <richard.guenther@gmail.com> wrote:
> >
> > On Wed, Nov 10, 2021 at 9:42 PM Jeff Law <jeffreyalaw@gmail.com> wrote:
> > >
> > >
> > >
> > > On 11/10/2021 11:20 AM, Aldy Hernandez via Gcc-patches wrote:
> > > > As discussed in the PR, the loop header copying pass avoids doing so
> > > > when optimizing for size.  However, sometimes we can determine the
> > > > loop entry conditional statically for the first iteration of the loop.
> > > >
> > > > This patch uses the path solver to determine the outgoing edge
> > > > out of preheader->header->xx.  If so, it allows header copying.  Doing
> > > > this in the loop optimizer saves us from doing gymnastics in the
> > > > threader which doesn't have the context to determine if a loop
> > > > transformation is profitable.
> > > >
> > > > I am only returning true in entry_loop_condition_is_static for
> > > > a true conditional.  Technically a false conditional is also
> > > > provably static, but allowing any boolean value causes a regression
> > > > in gfortran.dg/vector_subscript_1.f90.
> > > >
> > > > I would have preferred not passing around the query object, but the
> > > > layout of pass_ch and should_duplicate_loop_header_p make it a bit
> > > > awkward to get it right without an outright refactor to the
> > > > pass.
> > > >
> > > > Tested on x86-64 Linux.
> > > >
> > > > OK?
> > > >
> > > > gcc/ChangeLog:
> > > >
> > > >       PR tree-optimization/102906
> > > >       * tree-ssa-loop-ch.c (entry_loop_condition_is_static): New.
> > > >       (should_duplicate_loop_header_p): Call entry_loop_condition_is_static.
> > > >       (class ch_base): Add m_ranger and m_query.
> > > >       (ch_base::copy_headers): Pass m_query to
> > > >       entry_loop_condition_is_static.
> > > >       (pass_ch::execute): Allocate and deallocate m_ranger and
> > > >       m_query.
> > > >       (pass_ch_vect::execute): Same.
> > > >
> > > > gcc/testsuite/ChangeLog:
> > > >
> > > >       * gcc.dg/tree-ssa/pr102906.c: New test.
> > > OK.  It also makes a nice little example of how to use a Ranger within
> > > an existing pass.
> >
> > Note if you just test for the condition to be true it will only catch 50%
> > of the desired cases since we have no idea whether the 'true' edge
> > is the edge existing the loop or the edge remaining in the loop.
> > For loop header copying we like to resolve statically to the edge
> > remaining in the loop, so you want
>
> Ahh, I figured there was some block shuffling needed.
>
> I was cautious not to touch much because of the
> gfortran.dg/vector_subscript_1.f90 regression, but now I see that the
> test fails for all optimization levels except -Os.  With this fix we
> properly fail for all levels.  I assume this is expected ;-).
>
> >
> > extract_true_false_edges_from_block (gimple_bb (last), &true_e, &false_e);
> >
> > /* If neither edge is the exit edge this is not a case we'd like to
> >    special-case.  */
> > if (!loop_exit_edge_p (l, true_e) && !loop_exit_edge_p (l, false_e))
> >  return false;
> >
> > tree desired_static_value;
> > if (loop_exit_edge_p (l, true_e))
> >  desired_static_value = boolean_false_node;
> > else
> >   desired_static_value = boolean_true_node;
> >
> > and test for desired_static_value.
>
> Thanks for the code!
>
> OK pending tests?

OK, thanks!
Richard.