Re: [committed] More gimple const/copy propagation opportunities

[Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>]

On Fri, 30 Sep 2022 17:32:34 -0600
Jeff Law <jlaw@ventanamicro.com> wrote:

> +  /* This looks good from a CFG standpoint.  Now look at the guts
> +     of PRED.  Basically we want to verify there are no PHI nodes
> +     and no real statements.  */
> +  if (! gimple_seq_empty_p (phi_nodes (pred)))
> +    return false;

So, given the below, neither DEBUG nor labels do count towards an
empty seq [coming in from any PHI that is, otherwise it's a different
thing], which is a bit surprising but well, ok. It looks at PHI IL, so
probably yes. Allegedly that's what it is. Neat if that's true.

> +
> +  gimple_stmt_iterator gsi;
> +  for (gsi = gsi_last_bb (pred); !gsi_end_p (gsi); gsi_prev (&gsi))
> +    {
> +      gimple *stmt = gsi_stmt (gsi);
> +
> +      switch (gimple_code (stmt))
> +	{
> +	  case GIMPLE_LABEL:
> +	    if (DECL_NONLOCAL (gimple_label_label (as_a <glabel *> (stmt))))
> +	      return false;
> +	    break;
> +
> +	  case GIMPLE_DEBUG:
> +	    break;
> +
> +	  default:
> +	    return false;

don't like, sounds odd. Are we sure there's no other garbage that can
manifest here? int meow=42;, and meow unused won't survive?, pragmas
neither or stuff ?

> +	}
> +    }
> +
> +  return true;
> +}
> +
>  /* We have finished optimizing BB, record any information implied by
>     taking a specific outgoing edge from BB.  */
>  

> @@ -583,6 +656,62 @@ record_edge_info (basic_block bb)
>                if (can_infer_simple_equiv && TREE_CODE (inverted) == EQ_EXPR)
>  		edge_info->record_simple_equiv (op0, op1);
>              }
> +
> +	  /* If this block is a single block loop, then we may be able to
> +	     record some equivalences on the loop's exit edge.  */
> +	  if (single_block_loop_p (bb))
> +	    {
> +	      /* We know it's a single block loop.  Now look at the loop
> +		 exit condition.  What we're looking for is whether or not
> +		 the exit condition is loop invariant which we can detect
> +		 by checking if all the SSA_NAMEs referenced are defined
> +		 outside the loop.  */
> +	      if ((TREE_CODE (op0) != SSA_NAME
> +		   || gimple_bb (SSA_NAME_DEF_STMT (op0)) != bb)
> +		  && (TREE_CODE (op1) != SSA_NAME
> +		      || gimple_bb (SSA_NAME_DEF_STMT (op1)) != bb))
> +		{
> +		  /* At this point we know the exit condition is loop
> +		     invariant.  The only way to get out of the loop is
> +		     if never traverses the backedge to begin with.  This

s/if /if it /

> +		     implies that any PHI nodes create equivalances we can

that any threw me off asking for "that if any". Would have been nicer,
i think?

> +		     attach to the loop exit edge.  */

attach it to

> +		  int alternative

bool

> +		    = (EDGE_PRED (bb, 0)->flags & EDGE_DFS_BACK) ? 1 : 0;
> +
> +		  gphi_iterator gsi;
> +		  for (gsi = gsi_start_phis (bb);
> +		       !gsi_end_p (gsi);
> +		       gsi_next (&gsi))
> +		    {
> +		      /* If the other alternative is the same as the result,
> +			 then this is a degenerate and can be ignored.  */
> +		      if (dst == PHI_ARG_DEF (phi, !alternative))
> +			continue;
> +
> +		      /* Now get the EDGE_INFO class so we can append
> +			 it to our list.  We want the successor edge
> +			 where the destination is not the source of
> +			 an incoming edge.  */
> +		      gphi *phi = gsi.phi ();
> +		      tree src = PHI_ARG_DEF (phi, alternative);
> +		      tree dst = PHI_RESULT (phi);
> +
> +		      if (EDGE_SUCC (bb, 0)->dest
> +			  != EDGE_PRED (bb, !alternative)->src)

by now, alternative would be easier to grok if it would have been spelled
from_backedge_p or something. IMHO.
thanks,

> +			edge_info = (class edge_info *)EDGE_SUCC (bb, 0)->aux;
> +		      else
> +			edge_info = (class edge_info *)EDGE_SUCC (bb, 1)->aux;
> +
> +		      /* Note that since this processing is done independently
> +			 of other edge equivalency processing, we may not
> +			 have an EDGE_INFO structure set up yet.  */
> +		      if (edge_info == NULL)
> +			edge_info = new class edge_info (false_edge);
> +		      edge_info->record_simple_equiv (dst, src);
> +		    }
> +		}
> +	    }
>          }
>      }
>  }