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

[Jeff Law <jeffreyalaw@gmail.com>]

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.

Jeff