[patch] Tame if-combining when loop unswitching is enabled

[Eric Botcazou <botcazou@adacore.com>]

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Hi,

in order to make it possible to vectorize loops running over arrays in Ada, 
which generally contain index checks, hence control-flow instructions, we rely 
on loop unswitching to generate two copies of the loop, one guarded with a 
global condition (no index check fails in the loop) and vectorizable and one 
with the original index checks and non-vectorizable.  This is achieved by the 
simple trick of prepending the global_condition to the condition of the index 
checks and letting the loop unswitching pass do its magic.

But there is an enemy, namely if-combining, which can turn a simple boolean 
conjunction into something else that loop unswitching cannot deal with, and a 
testcase is attached with 3 slightly different versions of the same issue.

Therefore the attached patch attempts to tame if-combining by reasoning on the 
loop invariant status (really loop depths) of the conditions.

Bootstrapped/regtested on x86-64/Linux, OK for the mainline?


2021-10-15  Eric Botcazou  <ebotcazou@adacore.com>

	* tree-ssa-ifcombine.c: Include cfgloop.h.
	(operand_loop_depth): New function.
	(ifcombine_ifandif): When loop unswitching is enabled, do not merge
	conditions whose loop invariant status is different.


2021-10-15  Eric Botcazou  <ebotcazou@adacore.com>

	* gnat.dg/vect19.ads, gnat.dg/vect19.adb: New test.

-- 
Eric Botcazou

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diff --git a/gcc/tree-ssa-ifcombine.c b/gcc/tree-ssa-ifcombine.c
index f93e04aa4df..986084049da 100644
--- a/gcc/tree-ssa-ifcombine.c
+++ b/gcc/tree-ssa-ifcombine.c
@@ -35,6 +35,7 @@ along with GCC; see the file COPYING3.  If not see
    BRANCH_COST.  */
 #include "fold-const.h"
 #include "cfganal.h"
+#include "cfgloop.h"
 #include "gimple-fold.h"
 #include "gimple-iterator.h"
 #include "gimplify-me.h"
@@ -378,6 +379,19 @@ update_profile_after_ifcombine (basic_block inner_cond_bb,
   outer2->probability = profile_probability::never ();
 }
 
+/* Return the loop depth of GIMPLE operand OP.  */
+
+static int
+operand_loop_depth (tree op)
+{
+  basic_block bb;
+
+  if (TREE_CODE (op) == SSA_NAME && (bb = gimple_bb (SSA_NAME_DEF_STMT (op))))
+    return bb_loop_depth (bb);
+
+  return 0;
+}
+
 /* If-convert on a and pattern with a common else block.  The inner
    if is specified by its INNER_COND_BB, the outer by OUTER_COND_BB.
    inner_inv, outer_inv and result_inv indicate whether the conditions
@@ -554,6 +568,22 @@ ifcombine_ifandif (basic_block inner_cond_bb, bool inner_inv,
 	  HONOR_NANS (gimple_cond_lhs (outer_cond)));
       if (outer_cond_code == ERROR_MARK)
 	return false;
+      /* Do not merge if the loop invariant status of the conditions is not
+	 the same and we'll be unswitching loops downstream.  */
+      if (flag_unswitch_loops)
+	{
+	  const int current_depth
+	    = MIN (bb_loop_depth (inner_cond_bb),
+		   bb_loop_depth (outer_cond_bb));
+	  const int inner_depth
+	    = MAX (operand_loop_depth (gimple_cond_lhs (inner_cond)),
+		   operand_loop_depth (gimple_cond_rhs (inner_cond)));
+	  const int outer_depth
+	    = MAX (operand_loop_depth (gimple_cond_lhs (outer_cond)),
+		   operand_loop_depth (gimple_cond_rhs (outer_cond)));
+	  if ((inner_depth < current_depth) != (outer_depth < current_depth))
+	    return false;
+	}
       /* Don't return false so fast, try maybe_fold_or_comparisons?  */
 
       if (!(t = maybe_fold_and_comparisons (boolean_type_node, inner_cond_code,

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-- { dg-do compile { target i?86-*-* x86_64-*-* } }
-- { dg-options "-O3 -msse2 -fno-vect-cost-model -fdump-tree-vect-details" }
-- { dg-additional-options "-gnatX" }

package body Vect19 is

   function "+" (X, Y : Varray) return Varray is
      R : Varray (X'Range);
   begin
      for I in X'Range loop
         R(I) := X(I) + Y(I);
      end loop;
      return R;
   end;

   procedure Add (X, Y : Varray; R : out Varray) is
   begin
      for I in X'Range loop
         R(I) := X(I) + Y(I);
      end loop;
   end;

   procedure Add (X, Y : not null access Varray; R : not null access Varray) is
   begin
      for I in X'Range loop
         R(I) := X(I) + Y(I);
      end loop;
   end;

end Vect19;

-- { dg-final { scan-tree-dump-times "vectorized 1 loops" 3 "vect"  } }

[-- Attachment #4: vect19.ads --]
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package Vect19 is

   type Varray is array (Natural range 1 .. <>) of Long_Float;
   for Varray'Alignment use 16;

   function "+" (X, Y : Varray) return Varray;
   procedure Add (X, Y : Varray; R : out Varray);
   procedure Add (X, Y : not null access Varray; R : not null access Varray);

end Vect19;