Re: [PATCH] Abstract PHI and forwarder block checks in jump threader.

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

On Fri, Sep 3, 2021 at 3:59 PM Aldy Hernandez via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> This patch abstracts out a couple common idioms in the forward
> threader that I found useful while navigating the code base.
>
> Tested on x86-64 Linux.
>
> OK?
>
> gcc/ChangeLog:
>
>         * tree-ssa-threadedge.c (has_phis_p): New.
>         (forwarder_block_p): New.
>         (potentially_threadable_block): Call forwarder_block_p.
>         (jump_threader::thread_around_empty_blocks): Call has_phis_p.
>         (jump_threader::thread_through_normal_block): Call
>         forwarder_block_p.
> ---
>  gcc/tree-ssa-threadedge.c | 25 +++++++++++++++++++------
>  1 file changed, 19 insertions(+), 6 deletions(-)
>
> diff --git a/gcc/tree-ssa-threadedge.c b/gcc/tree-ssa-threadedge.c
> index e57f6d3e39c..3db54a199fd 100644
> --- a/gcc/tree-ssa-threadedge.c
> +++ b/gcc/tree-ssa-threadedge.c
> @@ -95,6 +95,21 @@ jump_threader::thread_through_all_blocks (bool may_peel_loop_headers)
>    return m_registry->thread_through_all_blocks (may_peel_loop_headers);
>  }
>
> +static inline bool
> +has_phis_p (basic_block bb)
> +{
> +  return !gsi_end_p (gsi_start_phis (bb));

gimple_seq_empty_p (phi_nodes (bb)) shoud be cheaper.  Do virtual PHIs
count as PHIs for you?

> +}
> +
> +/* Return TRUE for a forwarder block which is defined as having PHIs
> +   but no instructions.  */
> +
> +static bool
> +forwarder_block_p (basic_block bb)

There exists a function with exactly the same signature in cfgrtl.h, likewise
several similar implementations might exist elsewhere.

Your definition is also quite odd, not matching what one would expect
(the PHI requirement).  The tree-cfgcleanup.c variant has
tree_forwarder_block_p which is explicit about this.

Btw, gsi_start_nondebug_bb does not ignore labels.

> +{
> +  return gsi_end_p (gsi_start_nondebug_bb (bb)) && has_phis_p (bb);
> +}
> +
>  /* Return TRUE if we may be able to thread an incoming edge into
>     BB to an outgoing edge from BB.  Return FALSE otherwise.  */
>
> @@ -107,9 +122,8 @@ potentially_threadable_block (basic_block bb)
>       not optimized away because they forward from outside a loop
>       to the loop header.   We want to thread through them as we can
>       sometimes thread to the loop exit, which is obviously profitable.
> -     the interesting case here is when the block has PHIs.  */
> -  if (gsi_end_p (gsi_start_nondebug_bb (bb))
> -      && !gsi_end_p (gsi_start_phis (bb)))
> +     The interesting case here is when the block has PHIs.  */
> +  if (forwarder_block_p (bb))
>      return true;
>
>    /* If BB has a single successor or a single predecessor, then
> @@ -854,7 +868,7 @@ jump_threader::thread_around_empty_blocks (vec<jump_thread_edge *> *path,
>    /* The key property of these blocks is that they need not be duplicated
>       when threading.  Thus they cannot have visible side effects such
>       as PHI nodes.  */
> -  if (!gsi_end_p (gsi_start_phis (bb)))
> +  if (has_phis_p (bb))
>      return false;
>
>    /* Skip over DEBUG statements at the start of the block.  */
> @@ -994,8 +1008,7 @@ jump_threader::thread_through_normal_block (vec<jump_thread_edge *> *path,
>      {
>        /* First case.  The statement simply doesn't have any instructions, but
>          does have PHIs.  */
> -      if (gsi_end_p (gsi_start_nondebug_bb (e->dest))
> -         && !gsi_end_p (gsi_start_phis (e->dest)))
> +      if (forwarder_block_p (e->dest))
>         return 0;
>
>        /* Second case.  */
> --
> 2.31.1
>