PHIOPT: Improve replace_phi_edge_with_variable for diamond shapped bb

While looking at differences between what minmax_replacement
and match_simplify_replacement does. I noticed that they sometimes
chose different edges to remove. I decided we should be able to do
better and be able to remove both empty basic blocks in the
case of match_simplify_replacement as that moves the statements.

This also updates the testcases as now match_simplify_replacement
will remove the unused MIN/MAX_EXPR and they were checking for
those.

OK? Bootstrapped and tested on x86_64-linux-gnu with no regressions.

gcc/ChangeLog:

	* tree-ssa-phiopt.cc (replace_phi_edge_with_variable): Handle
	diamond form bb with forwarder only empty blocks better.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/minmax-15.c: Update test.
	* gcc.dg/tree-ssa/minmax-16.c: Update test.
	* gcc.dg/tree-ssa/minmax-3.c: Update test.
	* gcc.dg/tree-ssa/minmax-4.c: Update test.
	* gcc.dg/tree-ssa/minmax-5.c: Update test.
	* gcc.dg/tree-ssa/minmax-8.c: Update test.

1 files changed, 34 insertions, 1 deletions

diff --git a/gcc/tree-ssa-phiopt.cc b/gcc/tree-ssa-phiopt.cc
index 154a5ab..2f70463 100644
--- a/gcc/tree-ssa-phiopt.cc
+++ b/gcc/tree-ssa-phiopt.cc
@@ -94,6 +94,7 @@ replace_phi_edge_with_variable (basic_block cond_block,
   basic_block bb = gimple_bb (phi);
   gimple_stmt_iterator gsi;
   tree phi_result = PHI_RESULT (phi);
+  bool deleteboth = false;
 
   /* Duplicate range info if they are the only things setting the target PHI.
      This is needed as later on, the new_tree will be replacing
@@ -137,7 +138,14 @@ replace_phi_edge_with_variable (basic_block cond_block,
       keep_edge = EDGE_SUCC (cond_block, 1);
     }
   else if ((keep_edge = find_edge (cond_block, e->src)))
-    ;
+    {
+      basic_block bb1 = EDGE_SUCC (cond_block, 0)->dest;
+      basic_block bb2 = EDGE_SUCC (cond_block, 1)->dest;
+      if (single_pred_p (bb1) && single_pred_p (bb2)
+	  && single_succ_p (bb1) && single_succ_p (bb2)
+	  && empty_block_p (bb1) && empty_block_p (bb2))
+	deleteboth = true;
+    }
   else
     gcc_unreachable ();
 
@@ -152,6 +160,31 @@ replace_phi_edge_with_variable (basic_block cond_block,
       gsi = gsi_last_bb (cond_block);
       gsi_remove (&gsi, true);
     }
+  else if (deleteboth)
+    {
+      basic_block bb1 = EDGE_SUCC (cond_block, 0)->dest;
+      basic_block bb2 = EDGE_SUCC (cond_block, 1)->dest;
+
+      edge newedge = redirect_edge_and_branch (keep_edge, bb);
+
+      /* The new edge should be the same. */
+      gcc_assert (newedge == keep_edge);
+
+      keep_edge->flags |= EDGE_FALLTHRU;
+      keep_edge->flags &= ~(EDGE_TRUE_VALUE | EDGE_FALSE_VALUE);
+      keep_edge->probability = profile_probability::always ();
+
+      /* Copy the edge's phi entry from the old one. */
+      copy_phi_arg_into_existing_phi (e, keep_edge);
+
+      /* Delete the old 2 empty basic blocks */
+      delete_basic_block (bb1);
+      delete_basic_block (bb2);
+
+      /* Eliminate the COND_EXPR at the end of COND_BLOCK.  */
+      gsi = gsi_last_bb (cond_block);
+      gsi_remove (&gsi, true);
+    }
   else
     {
       /* If there are other edges into the middle block make