Re: [PATCH]middle-end: don't create LC-SSA PHI variables for PHI nodes who dominate loop

[Richard Biener <rguenther@suse.de>]

On Thu, 19 Oct 2023, Tamar Christina wrote:

> Hi All,
> 
> As the testcase shows, when a PHI node dominates the loop there is no new
> definition inside the loop.  As such there would be no PHI nodes to update.
> 
> When we maintain LCSSA form we create an intermediate node in between the two
> loops to thread alongt the value.  However later on when we update the second
> loop we don't have any PHI nodes to update and so adjust_phi_and_debug_stmts
> does nothing.   This leaves us with an incorrect phi node.  Normally this does
> nothing and just gets ignored.  But in the case of the vUSE chain we end up
> corrupting the chain.
> 
> As such whenever a PHI node's argument dominates the loop, we should remove
> the newly created PHI node after edge redirection.
> 
> The one exception to this is when the loop has been versioned.  In such cases
> the versioned loop may not use the value but the second loop can.
> 
> When this happens and we add the loop guard unless the join block has the PHI
> it can't find the original value for use inside the guard block.
> 
> The next refactoring in the series moves the formation of the guard block
> inside peeling itself.  Here we have all the information and wouldn't
> need to re-create it later.
> 
> Bootstrapped Regtested on aarch64-none-linux-gnu, x86_64-linux-gnu
> and no issues and issues in libgomp fixed.
> 
> Ok for master?

OK.

Thanks,
Richard.

> Thanks,
> Tamar
> 
> gcc/ChangeLog:
> 
> 	PR tree-optimization/111860
> 	* tree-vect-loop-manip.cc (slpeel_tree_duplicate_loop_to_edge_cfg):
> 	Remove PHI nodes that dominate loop.
> 
> gcc/testsuite/ChangeLog:
> 
> 	PR tree-optimization/111860
> 	* gcc.dg/vect/pr111860.c: New test.
> 
> --- inline copy of patch -- 
> diff --git a/gcc/testsuite/gcc.dg/vect/pr111860.c b/gcc/testsuite/gcc.dg/vect/pr111860.c
> new file mode 100644
> index 0000000000000000000000000000000000000000..36f0774601040418bc6b7f27c9425b2bf93b18cb
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/vect/pr111860.c
> @@ -0,0 +1,16 @@
> +/* { dg-do compile } */
> +
> +int optimize_path_n, optimize_path_d;
> +int *optimize_path_d_0;
> +extern void path_threeOpt( long);
> +void optimize_path() {
> +  int i;
> +  long length;
> +  i = 0;
> +  for (; i <= optimize_path_n; i++)
> +    optimize_path_d = 0;
> +  i = 0;
> +  for (; i < optimize_path_n; i++)
> +    length += optimize_path_d_0[i];
> +  path_threeOpt(length);
> +}
> diff --git a/gcc/tree-vect-loop-manip.cc b/gcc/tree-vect-loop-manip.cc
> index 1f7779b9834c3aef3c6a993fab916224fab03147..db1d4f867ead5c6079cda3ff0d0870234d11e39d 100644
> --- a/gcc/tree-vect-loop-manip.cc
> +++ b/gcc/tree-vect-loop-manip.cc
> @@ -1633,6 +1633,21 @@ slpeel_tree_duplicate_loop_to_edge_cfg (class loop *loop, edge loop_exit,
>  	{
>  	  tree new_arg = gimple_phi_arg (phi, 0)->def;
>  	  new_phi_args.put (new_arg, gimple_phi_result (phi));
> +
> +	  if (TREE_CODE (new_arg) != SSA_NAME)
> +	    continue;
> +	  /* If the PHI node dominates the loop then we shouldn't create
> +	      a new LC-SSSA PHI for it in the intermediate block.  Unless the
> +	      the loop has been versioned.  If it has then we need the PHI
> +	      node such that later when the loop guard is added the original
> +	      dominating PHI can be found.  */
> +	  basic_block def_bb = gimple_bb (SSA_NAME_DEF_STMT (new_arg));
> +	  if (loop == scalar_loop
> +	      && (!def_bb || !flow_bb_inside_loop_p (loop, def_bb)))
> +	    {
> +	      auto gsi = gsi_for_stmt (phi);
> +	      remove_phi_node (&gsi, true);
> +	    }
>  	}
>  
>        /* Copy the current loop LC PHI nodes between the original loop exit
> 
> 
> 
> 
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
GF: Ivo Totev, Andrew McDonald, Werner Knoblich; (HRB 36809, AG Nuernberg)