tree-optimization/102880 - make PHI-OPT recognize more CFGs

This allows extra edges into the middle BB for the PHI-OPT
transforms using replace_phi_edge_with_variable that do not
end up moving stmts from that middle BB.  This avoids regressing
gcc.dg/tree-ssa/ssa-hoist-4.c with the actual fix for PR102880
where CFG cleanup has the choice to remove two forwarders and
picks "the wrong" leading to

   if (a > b) /
       /\    /
      /  <BB>
     /    |
  # PHI <a, b>

rather than

   if (a > b)  |
       /\      |
    <BB> \     |
     /    \    |
  # PHI <a, b, b>

but it's relatively straight-forward to support extra edges
into the middle-BB in paths ending in replace_phi_edge_with_variable
and that do not require moving stmts.  That's because we really
only want to remove the edge from the condition to the middle BB.
Of course actually doing that means updating dominators in non-trival
ways which is why I kept the original code for the single edge
case and simply defer to CFG cleanup by adjusting the condition for
the complicated case.

The testcase needs to be a GIMPLE one since it's quite unreliable
to produce the desired CFG.

2021-11-15  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/102880
	* tree-ssa-phiopt.c (tree_ssa_phiopt_worker): Push
	single_pred (bb1) condition to places that really need it.
	(match_simplify_replacement): Likewise.
	(value_replacement): Likewise.
	(replace_phi_edge_with_variable): Deal with extra edges
	into the middle BB.

	* gcc.dg/tree-ssa/phi-opt-26.c: New testcase.

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/phi-opt-26.c b/gcc/testsuite/gcc.dg/tree-ssa/phi-opt-26.c
new file mode 100644 (file)
index 0000000..21aa66e
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/phi-opt-26.c
@@ -0,0 +1,31 @@
+/* { dg-do compile } */
+/* { dg-options "-O -fgimple -fdump-tree-phiopt1" } */
+
+int __GIMPLE (ssa,startwith("phiopt"))
+foo (int a, int b, int flag)
+{
+  int res;
+
+  __BB(2):
+  if (flag_2(D) != 0)
+    goto __BB6;
+  else
+    goto __BB4;
+
+  __BB(4):
+  if (a_3(D) > b_4(D))
+    goto __BB7;
+  else
+    goto __BB6;
+
+  __BB(6):
+  goto __BB7;
+
+  __BB(7):
+  res_1 = __PHI (__BB4: a_3(D), __BB6: b_4(D));
+  return res_1;
+}
+
+/* We should be able to detect MAX despite the extra edge into
+   the middle BB.  */
+/* { dg-final { scan-tree-dump "MAX" "phiopt1" } } */

diff --git a/gcc/tree-ssa-phiopt.c b/gcc/tree-ssa-phiopt.c
index 173ac835ca62ba92931e4970b51bcfc5ee35d296..6b22f6bedd47f2fee92818d16d1b2f2538afd63a 100644 (file)
--- a/gcc/tree-ssa-phiopt.c
+++ b/gcc/tree-ssa-phiopt.c
@@ -220,7 +220,6 @@ tree_ssa_phiopt_worker (bool do_store_elim, bool do_hoist_loads, bool early_p)
 
       /* If either bb1's succ or bb2 or bb2's succ is non NULL.  */
       if (EDGE_COUNT (bb1->succs) == 0
-          || bb2 == NULL
          || EDGE_COUNT (bb2->succs) == 0)
         continue;
 
@@ -276,14 +275,14 @@ tree_ssa_phiopt_worker (bool do_store_elim, bool do_hoist_loads, bool early_p)
          || (e1->flags & EDGE_FALLTHRU) == 0)
         continue;
 
-      /* Also make sure that bb1 only have one predecessor and that it
-        is bb.  */
-      if (!single_pred_p (bb1)
-          || single_pred (bb1) != bb)
-       continue;
-
       if (do_store_elim)
        {
+         /* Also make sure that bb1 only have one predecessor and that it
+            is bb.  */
+         if (!single_pred_p (bb1)
+             || single_pred (bb1) != bb)
+           continue;
+
          /* bb1 is the middle block, bb2 the join block, bb the split block,
             e1 the fallthrough edge from bb1 to bb2.  We can't do the
             optimization if the join block has more than two predecessors.  */
@@ -328,10 +327,11 @@ tree_ssa_phiopt_worker (bool do_store_elim, bool do_hoist_loads, bool early_p)
             node.  */
          gcc_assert (arg0 != NULL_TREE && arg1 != NULL_TREE);
 
-         gphi *newphi = factor_out_conditional_conversion (e1, e2, phi,
-                                                           arg0, arg1,
-                                                           cond_stmt);
-         if (newphi != NULL)
+         gphi *newphi;
+         if (single_pred_p (bb1)
+             && (newphi = factor_out_conditional_conversion (e1, e2, phi,
+                                                             arg0, arg1,
+                                                             cond_stmt)))
            {
              phi = newphi;
              /* factor_out_conditional_conversion may create a new PHI in
@@ -350,12 +350,14 @@ tree_ssa_phiopt_worker (bool do_store_elim, bool do_hoist_loads, bool early_p)
                                               early_p))
            cfgchanged = true;
          else if (!early_p
+                  && single_pred_p (bb1)
                   && cond_removal_in_builtin_zero_pattern (bb, bb1, e1, e2,
                                                            phi, arg0, arg1))
            cfgchanged = true;
          else if (minmax_replacement (bb, bb1, e1, e2, phi, arg0, arg1))
            cfgchanged = true;
-         else if (spaceship_replacement (bb, bb1, e1, e2, phi, arg0, arg1))
+         else if (single_pred_p (bb1)
+                  && spaceship_replacement (bb, bb1, e1, e2, phi, arg0, arg1))
            cfgchanged = true;
        }
     }
@@ -386,7 +388,6 @@ replace_phi_edge_with_variable (basic_block cond_block,
                                edge e, gphi *phi, tree new_tree)
 {
   basic_block bb = gimple_bb (phi);
-  basic_block block_to_remove;
   gimple_stmt_iterator gsi;
   tree phi_result = PHI_RESULT (phi);
 
@@ -422,28 +423,33 @@ replace_phi_edge_with_variable (basic_block cond_block,
   SET_USE (PHI_ARG_DEF_PTR (phi, e->dest_idx), new_tree);
 
   /* Remove the empty basic block.  */
+  edge edge_to_remove;
   if (EDGE_SUCC (cond_block, 0)->dest == bb)
+    edge_to_remove = EDGE_SUCC (cond_block, 1);
+  else
+    edge_to_remove = EDGE_SUCC (cond_block, 0);
+  if (EDGE_COUNT (edge_to_remove->dest->preds) == 1)
     {
-      EDGE_SUCC (cond_block, 0)->flags |= EDGE_FALLTHRU;
-      EDGE_SUCC (cond_block, 0)->flags &= ~(EDGE_TRUE_VALUE | EDGE_FALSE_VALUE);
-      EDGE_SUCC (cond_block, 0)->probability = profile_probability::always ();
+      e->flags |= EDGE_FALLTHRU;
+      e->flags &= ~(EDGE_TRUE_VALUE | EDGE_FALSE_VALUE);
+      e->probability = profile_probability::always ();
+      delete_basic_block (edge_to_remove->dest);
 
-      block_to_remove = EDGE_SUCC (cond_block, 1)->dest;
+      /* Eliminate the COND_EXPR at the end of COND_BLOCK.  */
+      gsi = gsi_last_bb (cond_block);
+      gsi_remove (&gsi, true);
     }
   else
     {
-      EDGE_SUCC (cond_block, 1)->flags |= EDGE_FALLTHRU;
-      EDGE_SUCC (cond_block, 1)->flags
-       &= ~(EDGE_TRUE_VALUE | EDGE_FALSE_VALUE);
-      EDGE_SUCC (cond_block, 1)->probability = profile_probability::always ();
-
-      block_to_remove = EDGE_SUCC (cond_block, 0)->dest;
+      /* If there are other edges into the middle block make
+        CFG cleanup deal with the edge removal to avoid
+        updating dominators here in a non-trivial way.  */
+      gcond *cond = as_a <gcond *> (last_stmt (cond_block));
+      if (edge_to_remove->flags & EDGE_TRUE_VALUE)
+       gimple_cond_make_false (cond);
+      else
+       gimple_cond_make_true (cond);
     }
-  delete_basic_block (block_to_remove);
-
-  /* Eliminate the COND_EXPR at the end of COND_BLOCK.  */
-  gsi = gsi_last_bb (cond_block);
-  gsi_remove (&gsi, true);
 
   statistics_counter_event (cfun, "Replace PHI with variable", 1);
 
@@ -959,6 +965,9 @@ match_simplify_replacement (basic_block cond_bb, basic_block middle_bb,
      allow it and move it once the transformation is done. */
   if (!empty_block_p (middle_bb))
     {
+      if (!single_pred_p (middle_bb))
+       return false;
+
       stmt_to_move = last_and_only_stmt (middle_bb);
       if (!stmt_to_move)
        return false;
@@ -1351,7 +1360,10 @@ value_replacement (basic_block cond_bb, basic_block middle_bb,
        }
       else
        {
-         statistics_counter_event (cfun, "Replace PHI with variable/value_replacement", 1);
+         if (!single_pred_p (middle_bb))
+           return 0;
+         statistics_counter_event (cfun, "Replace PHI with "
+                                   "variable/value_replacement", 1);
 
          /* Replace the PHI arguments with arg. */
          SET_PHI_ARG_DEF (phi, e0->dest_idx, arg);
@@ -1367,7 +1379,6 @@ value_replacement (basic_block cond_bb, basic_block middle_bb,
             }
           return 1;
        }
-
     }
 
   /* Now optimize (x != 0) ? x + y : y to just x + y.  */