Re: [PATCH v3 1/3] reassoc: Do not bias loop-carried PHIs early

[Richard Biener <rguenther@suse.de>]

On Sun, 26 Sep 2021, Ilya Leoshkevich wrote:

> Biasing loop-carried PHIs during the 1st reassociation pass interferes
> with reduction chains and does not bring measurable benefits, so do it
> only during the 2nd reassociation pass.

OK.

Thanks,
Richard.

> gcc/ChangeLog:
> 
>             * passes.def (pass_reassoc): Rename parameter to early_p.
>             * tree-ssa-reassoc.c (reassoc_bias_loop_carried_phi_ranks_p):
>             New variable.
>             (phi_rank): Don't bias loop-carried phi ranks
>             before vectorization pass.
>             (execute_reassoc): Add bias_loop_carried_phi_ranks_p parameter.
>             (pass_reassoc::pass_reassoc): Add bias_loop_carried_phi_ranks_p
>             initializer.
>             (pass_reassoc::set_param): Set bias_loop_carried_phi_ranks_p
>             value.
>             (pass_reassoc::execute): Pass bias_loop_carried_phi_ranks_p to
>             execute_reassoc.
>             (pass_reassoc::bias_loop_carried_phi_ranks_p): New member.
> ---
>  gcc/passes.def         |  4 ++--
>  gcc/tree-ssa-reassoc.c | 16 ++++++++++++++--
>  2 files changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/gcc/passes.def b/gcc/passes.def
> index d7a1f8c97a6..c5f915d04c6 100644
> --- a/gcc/passes.def
> +++ b/gcc/passes.def
> @@ -242,7 +242,7 @@ along with GCC; see the file COPYING3.  If not see
>        /* Identify paths that should never be executed in a conforming
>  	 program and isolate those paths.  */
>        NEXT_PASS (pass_isolate_erroneous_paths);
> -      NEXT_PASS (pass_reassoc, true /* insert_powi_p */);
> +      NEXT_PASS (pass_reassoc, true /* early_p */);
>        NEXT_PASS (pass_dce);
>        NEXT_PASS (pass_forwprop);
>        NEXT_PASS (pass_phiopt, false /* early_p */);
> @@ -325,7 +325,7 @@ along with GCC; see the file COPYING3.  If not see
>        NEXT_PASS (pass_lower_vector_ssa);
>        NEXT_PASS (pass_lower_switch);
>        NEXT_PASS (pass_cse_reciprocals);
> -      NEXT_PASS (pass_reassoc, false /* insert_powi_p */);
> +      NEXT_PASS (pass_reassoc, false /* early_p */);
>        NEXT_PASS (pass_strength_reduction);
>        NEXT_PASS (pass_split_paths);
>        NEXT_PASS (pass_tracer);
> diff --git a/gcc/tree-ssa-reassoc.c b/gcc/tree-ssa-reassoc.c
> index 8498cfc7aa8..420c14e8cf5 100644
> --- a/gcc/tree-ssa-reassoc.c
> +++ b/gcc/tree-ssa-reassoc.c
> @@ -180,6 +180,10 @@ along with GCC; see the file COPYING3.  If not see
>     point 3a in the pass header comment.  */
>  static bool reassoc_insert_powi_p;
>  
> +/* Enable biasing ranks of loop accumulators.  We don't want this before
> +   vectorization, since it interferes with reduction chains.  */
> +static bool reassoc_bias_loop_carried_phi_ranks_p;
> +
>  /* Statistics */
>  static struct
>  {
> @@ -269,6 +273,9 @@ phi_rank (gimple *stmt)
>    use_operand_p use;
>    gimple *use_stmt;
>  
> +  if (!reassoc_bias_loop_carried_phi_ranks_p)
> +    return bb_rank[bb->index];
> +
>    /* We only care about real loops (those with a latch).  */
>    if (!father->latch)
>      return bb_rank[bb->index];
> @@ -6940,9 +6947,10 @@ fini_reassoc (void)
>     optimization of a gimple conditional.  Otherwise returns zero.  */
>  
>  static unsigned int
> -execute_reassoc (bool insert_powi_p)
> +execute_reassoc (bool insert_powi_p, bool bias_loop_carried_phi_ranks_p)
>  {
>    reassoc_insert_powi_p = insert_powi_p;
> +  reassoc_bias_loop_carried_phi_ranks_p = bias_loop_carried_phi_ranks_p;
>  
>    init_reassoc ();
>  
> @@ -6983,15 +6991,19 @@ public:
>      {
>        gcc_assert (n == 0);
>        insert_powi_p = param;
> +      bias_loop_carried_phi_ranks_p = !param;
>      }
>    virtual bool gate (function *) { return flag_tree_reassoc != 0; }
>    virtual unsigned int execute (function *)
> -    { return execute_reassoc (insert_powi_p); }
> +  {
> +    return execute_reassoc (insert_powi_p, bias_loop_carried_phi_ranks_p);
> +  }
>  
>   private:
>    /* Enable insertion of __builtin_powi calls during execute_reassoc.  See
>       point 3a in the pass header comment.  */
>    bool insert_powi_p;
> +  bool bias_loop_carried_phi_ranks_p;
>  }; // class pass_reassoc
>  
>  } // anon namespace
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)