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Re: [PATCH] Remove some restrictions on loop shape in tree-if-conv.c


Alan Lawrence wrote:
Tree if-conversion currently bails out for loops that (a) contain nested loops; (b) have more than one exit; (c) where the exit block (source of the exit edge) does not dominate the loop latch; (d) where the exit block is the loop header, or there are statements after the exit.

This patch removes restrictions (c) and (d). The intuition is that, for (c), "if (P) {... if (Q) break;}" is equivalent to "if (P) {...}; if (P&&Q) break;" and this is mostly handled by existing code for propagating conditions. For (d), "if (P) break; stmts" is equivalent to "if (!P) stmts; if (P) break;" - this requires inserting the predicated stmts before the branch rather than after.

Mostly thus this patch is just removing assumptions about when we do/don't need to store predicates. One 'gotcha' was in some test cases the latch block passed into if-conversion is non-empty; in such cases, if-conversion will now restore "good form" by moving the statement into the exit block (predicated with !exit-condition).

The condition on dominance in add_to_predicate_list, I haven't quite managed to convince myself is right; we _do_ want to store a predicate for the latch block to handle the above case, but I'm not totally sure of the postdominance condition - I think it may store conditions in cases where we don't really need to (e.g. "for (;;) { ... if (P) { for (;;) ; } }" which might look nested but isn't, and has no route to the function exit). However, storing conditions when we don't need to, is OK, unlike failing to store when we do need to ;).

A simple example of the patch at work:

int
foo ()
{
   for (int i = 0; i < N ; i++)
   {
     int m = (a[i] & i) ? 5 : 4;
     b[i] = a[i] * m;
   }
}

compiled at -O3, -fdump-tree-ivcanon shows this immediately before tree-if-conversion:

...function entry, variables, etc...
   <bb 2>:
   _10 = a[0];
   goto <bb 6>;

   <bb 3>:
   _5 = a[i_9];
   _6 = _5 & i_9;
   if (_6 != 0)
     goto <bb 5>;
   else
     goto <bb 4>;

   <bb 4>:

   <bb 5>:
   # m_14 = PHI <5(3), 4(4)>

   <bb 6>:
   # m_2 = PHI <m_14(5), 4(2)>
   # _15 = PHI <_5(5), _10(2)>
   # i_16 = PHI <i_9(5), 0(2)>
   # ivtmp_13 = PHI <ivtmp_3(5), 32(2)>
   _7 = m_2 * _15;
   b[i_16] = _7;
   i_9 = i_16 + 1;
   ivtmp_3 = ivtmp_13 - 1;
   if (ivtmp_3 != 0)
     goto <bb 3>;
   else
     goto <bb 7>;

which previously was not if-converted. With this patch:

   <bb 2>:
   _10 = a[0];
   goto <bb 4>;

   <bb 3>:

   <bb 4>:
   # m_2 = PHI <m_14(3), 4(2)>
   # _15 = PHI <_5(3), _10(2)>
   # i_16 = PHI <i_9(3), 0(2)>
   # ivtmp_13 = PHI <ivtmp_3(3), 32(2)>
   _7 = m_2 * _15;
   b[i_16] = _7;
   i_9 = i_16 + 1;
   ivtmp_3 = ivtmp_13 - 1;
   _5 = a[i_9];
   _6 = _5 & i_9;
   m_14 = _6 != 0 ? 5 : 4;
   if (ivtmp_3 != 0)
     goto <bb 3>;
   else
     goto <bb 5>;

   <bb 5>:
   return;

(Unfortunately the vectorizer still doesn't handle this loop either, but that's another issue/patch...)

Bootstrapped + check-gcc on x86_64-unknown-linux-gnu and aarch64-none-linux-gnu.
Cross-tested check-gcc on aarch64-none-elf.
I'm investigating impact on benchmarks - on AArch64 Spec2k6, this touches a number of object files, leading to an overall slight decrease in the number of instructions, but no change that looks significant (specifically, no more or less vectorization).

Is this OK for trunk?

Cheers, Alan

Attached!

--Alan

diff --git a/gcc/tree-if-conv.c b/gcc/tree-if-conv.c
index 400ee01a5122c0c74b21a9b100f36b2951e45a46..6bd642e54848ed2faf72aa99987ab9f838634ca6 100644
--- a/gcc/tree-if-conv.c
+++ b/gcc/tree-if-conv.c
@@ -455,8 +455,8 @@ add_to_predicate_list (struct loop *loop, basic_block bb, tree nc)
     return;
 
   /* If dominance tells us this basic block is always executed,
-     don't record any predicates for it.  */
-  if (dominated_by_p (CDI_DOMINATORS, loop->latch, bb))
+     i.e. it postdominates the header, don't record any predicates for it.  */
+  if (dominated_by_p (CDI_POST_DOMINATORS, loop->header, bb))
     return;
 
   dom_bb = get_immediate_dominator (CDI_DOMINATORS, bb);
@@ -1025,11 +1025,10 @@ has_pred_critical_p (basic_block bb)
 
    Last restriction is valid if aggressive_if_conv is false.
 
-   EXIT_BB is the basic block containing the exit of the LOOP.  BB is
-   inside LOOP.  */
+   BB is inside LOOP.  */
 
 static bool
-if_convertible_bb_p (struct loop *loop, basic_block bb, basic_block exit_bb)
+if_convertible_bb_p (struct loop *loop, basic_block bb)
 {
   edge e;
   edge_iterator ei;
@@ -1044,30 +1043,6 @@ if_convertible_bb_p (struct loop *loop, basic_block bb, basic_block exit_bb)
       && !aggressive_if_conv)
     return false;
 
-  if (exit_bb)
-    {
-      if (bb != loop->latch)
-	{
-	  if (dump_file && (dump_flags & TDF_DETAILS))
-	    fprintf (dump_file, "basic block after exit bb but before latch\n");
-	  return false;
-	}
-      else if (!empty_block_p (bb))
-	{
-	  if (dump_file && (dump_flags & TDF_DETAILS))
-	    fprintf (dump_file, "non empty basic block after exit bb\n");
-	  return false;
-	}
-      else if (bb == loop->latch
-	       && bb != exit_bb
-	       && !dominated_by_p (CDI_DOMINATORS, bb, exit_bb))
-	  {
-	    if (dump_file && (dump_flags & TDF_DETAILS))
-	      fprintf (dump_file, "latch is not dominated by exit_block\n");
-	    return false;
-	  }
-    }
-
   /* Be less adventurous and handle only normal edges.  */
   FOR_EACH_EDGE (e, ei, bb->succs)
     if (e->flags & (EDGE_EH | EDGE_ABNORMAL | EDGE_IRREDUCIBLE_LOOP))
@@ -1199,15 +1174,6 @@ predicate_bbs (loop_p loop)
       tree cond;
       gimple stmt;
 
-      /* The loop latch and loop exit block are always executed and
-	 have no extra conditions to be processed: skip them.  */
-      if (bb == loop->latch
-	  || bb_with_exit_edge_p (loop, bb))
-	{
-	  reset_bb_predicate (bb);
-	  continue;
-	}
-
       cond = bb_predicate (bb);
       stmt = last_stmt (bb);
       if (stmt && gimple_code (stmt) == GIMPLE_COND)
@@ -1271,7 +1237,6 @@ if_convertible_loop_p_1 (struct loop *loop,
 {
   bool res;
   unsigned int i;
-  basic_block exit_bb = NULL;
 
   /* Don't if-convert the loop when the data dependences cannot be
      computed: the loop won't be vectorized in that case.  */
@@ -1295,11 +1260,8 @@ if_convertible_loop_p_1 (struct loop *loop,
     {
       basic_block bb = ifc_bbs[i];
 
-      if (!if_convertible_bb_p (loop, bb, exit_bb))
+      if (!if_convertible_bb_p (loop, bb))
 	return false;
-
-      if (bb_with_exit_edge_p (loop, bb))
-	exit_bb = bb;
     }
 
   for (i = 0; i < loop->num_nodes; i++)
@@ -1375,14 +1337,11 @@ if_convertible_loop_p_1 (struct loop *loop,
    - it is innermost,
    - it has two or more basic blocks,
    - it has only one exit,
-   - loop header is not the exit edge,
    - if its basic blocks and phi nodes are if convertible.  */
 
 static bool
 if_convertible_loop_p (struct loop *loop, bool *any_mask_load_store)
 {
-  edge e;
-  edge_iterator ei;
   bool res = false;
   vec<data_reference_p> refs;
   vec<ddr_p> ddrs;
@@ -1411,12 +1370,6 @@ if_convertible_loop_p (struct loop *loop, bool *any_mask_load_store)
       return false;
     }
 
-  /* If one of the loop header's edge is an exit edge then do not
-     apply if-conversion.  */
-  FOR_EACH_EDGE (e, ei, loop->header->succs)
-    if (loop_exit_edge_p (loop, e))
-      return false;
-
   refs.create (5);
   ddrs.create (25);
   auto_vec<loop_p, 3> loop_nest;
@@ -2249,7 +2202,6 @@ remove_conditions_and_labels (loop_p loop)
 static void
 combine_blocks (struct loop *loop, bool any_mask_load_store)
 {
-  basic_block bb, exit_bb, merge_target_bb;
   unsigned int orig_loop_num_nodes = loop->num_nodes;
   unsigned int i;
   edge e;
@@ -2265,10 +2217,10 @@ combine_blocks (struct loop *loop, bool any_mask_load_store)
 
   /* Merge basic blocks: first remove all the edges in the loop,
      except for those from the exit block.  */
-  exit_bb = NULL;
+  basic_block exit_bb = NULL;
   for (i = 0; i < orig_loop_num_nodes; i++)
     {
-      bb = ifc_bbs[i];
+      basic_block bb = ifc_bbs[i];
       free_bb_predicate (bb);
       if (bb_with_exit_edge_p (loop, bb))
 	{
@@ -2280,7 +2232,7 @@ combine_blocks (struct loop *loop, bool any_mask_load_store)
 
   for (i = 1; i < orig_loop_num_nodes; i++)
     {
-      bb = ifc_bbs[i];
+      basic_block bb = ifc_bbs[i];
 
       for (ei = ei_start (bb->preds); (e = ei_safe_edge (ei));)
 	{
@@ -2315,37 +2267,53 @@ combine_blocks (struct loop *loop, bool any_mask_load_store)
       set_immediate_dominator (CDI_DOMINATORS, loop->latch, loop->header);
     }
 
-  merge_target_bb = loop->header;
+  basic_block merge_target_bb = loop->header;
+  /* Indicates that any further statements must be inserted
+     _before_ the final jump.  */
+  bool seen_exit = (loop->header == exit_bb);
   for (i = 1; i < orig_loop_num_nodes; i++)
     {
-      gimple_stmt_iterator gsi;
-      gimple_stmt_iterator last;
-
-      bb = ifc_bbs[i];
+      basic_block bb = ifc_bbs[i];
 
-      if (bb == exit_bb || bb == loop->latch)
-	continue;
+      if (bb == exit_bb)
+	{
+	  /* If possible, merge loop header to the block with the exit edge.
+	     This reduces the number of basic blocks to two, to please the
+	     vectorizer that handles only loops with two nodes.  */
+	  if (can_merge_blocks_p (loop->header, bb))
+	    merge_blocks (loop->header, bb);
+	  else
+	    {
+	      /* Must leave stmts in this BB; move following stmts here too.  */
+	      merge_target_bb = bb;
+	    }
+	  seen_exit = true;
+	  continue;
+	}
 
       /* Make stmts member of loop->header.  */
-      for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
+      for (gimple_stmt_iterator gsi = gsi_start_bb (bb);
+	   !gsi_end_p (gsi);
+	   gsi_next (&gsi))
 	gimple_set_bb (gsi_stmt (gsi), merge_target_bb);
 
       /* Update stmt list.  */
-      last = gsi_last_bb (merge_target_bb);
-      gsi_insert_seq_after (&last, bb_seq (bb), GSI_NEW_STMT);
+      gimple_stmt_iterator last = gsi_last_bb (merge_target_bb);
+
+      if (seen_exit)
+	{
+	  gcc_assert (is_ctrl_stmt (gsi_stmt (last)));
+	  gsi_insert_seq_before (&last, bb_seq(bb), GSI_SAME_STMT);
+	}
+      else
+	  gsi_insert_seq_after (&last, bb_seq (bb), GSI_NEW_STMT);
+
       set_bb_seq (bb, NULL);
 
-      delete_basic_block (bb);
+      if (bb != loop->latch)
+	delete_basic_block (bb);
     }
 
-  /* If possible, merge loop header to the block with the exit edge.
-     This reduces the number of basic blocks to two, to please the
-     vectorizer that handles only loops with two nodes.  */
-  if (exit_bb
-      && exit_bb != loop->header
-      && can_merge_blocks_p (loop->header, exit_bb))
-    merge_blocks (loop->header, exit_bb);
-
   free (ifc_bbs);
   ifc_bbs = NULL;
 }

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