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

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

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

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.

Richard.

> Jeff
>