[PATCH 1/4][PR tree-optimization/78496] Don't simplify conditionals too early in VRP

Jeff Law law@redhat.com
Wed May 3 16:39:00 GMT 2017


[ With the patch attached... ]

On 05/03/2017 10:31 AM, Jeff Law wrote:
> This is the first of 3-5 patches to address pr78496.
> 
> The goal of these patches is to catch jump threads earlier in the 
> pipeline to avoid undesirable behavior in PRE and more generally be able 
> to exploit the secondary opportunities exposed by jump threading.
> 
> One of the more serious issues I found while investigating 78496 was VRP 
> failing to find what should have been obvious jump threads.  The 
> fundamental issue is VRP will simplify conditionals which are fed by a 
> typecast prior to jump threading.   So something like this:
> 
> x = (typecast) y;
> if (x == 42)
> 
> Can often be transformed into:
> 
> if (y == 42)
> 
> 
> The problem is any ASSERT_EXPRS after the conditional will reference "x" 
> rather than "y".  That in turn makes it impossible for VRP to use those 
> ASSERT_EXPRs to thread later jumps that use x == <whatever>
> 
> 
> More concretely consider this gimple code:
> 
> 
> ;;   basic block 5, loop depth 0, count 0, freq 10000, maybe hot
> ;;    prev block 4, next block 12, flags: (NEW, REACHABLE, VISITED)
> ;;    pred:       3 [50.0%]  (TRUE_VALUE,EXECUTABLE)
> ;;                4 [100.0%]  (FALLTHRU,EXECUTABLE)
>    # iftmp.0_2 = PHI <1(3), 0(4)>
>    in_loop_7 = (unsigned char) iftmp.0_2;
>    if (in_loop_7 != 0)
>      goto <bb 6>; [33.00%]
>    else
>      goto <bb 12>; [67.00%]
> 
> ;;    succ:       6 [33.0%]  (TRUE_VALUE,EXECUTABLE)
> ;;                12 [67.0%]  (FALSE_VALUE,EXECUTABLE)
> 
> ;;   basic block 12, loop depth 0, count 0, freq 6700, maybe hot
> ;;    prev block 5, next block 6, flags: (NEW)
> ;;    pred:       5 [67.0%]  (FALSE_VALUE,EXECUTABLE)
>    in_loop_15 = ASSERT_EXPR <in_loop_7, in_loop_7 == 0>;
>    goto <bb 7>; [100.00%]
> ;;    succ:       7 [100.0%]  (FALLTHRU)
> 
> ;;   basic block 6, loop depth 0, count 0, freq 3300, maybe hot
> ;;    prev block 12, next block 7, flags: (NEW, REACHABLE, VISITED)
> ;;    pred:       5 [33.0%]  (TRUE_VALUE,EXECUTABLE)
>    in_loop_14 = ASSERT_EXPR <in_loop_7, in_loop_7 != 0>;
>    simple_iv ();
> ;;    succ:       7 [100.0%]  (FALLTHRU,EXECUTABLE)
> 
> And later we have:
> 
> ;;   basic block 9, loop depth 0, count 0, freq 8476, maybe hot
> ;;    prev block 8, next block 10, flags: (NEW, REACHABLE, VISITED)
> ;;    pred:       7 [84.8%]  (FALSE_VALUE,EXECUTABLE)
>    if (in_loop_7 == 0)
>      goto <bb 10>; [36.64%]
>    else
>      goto <bb 11>; [63.36%]
> 
> VRP knows it can replace the uses of in_loop_7 in the conditionals in 
> blocks 5 and 9 with iftmp.0_2 and happily does so *before* jump 
> threading (but well after ASSERT_EXPR insertion).
> 
> As a result VRP is unable to utilize the ASSERT_EXPRs in blocks 12 and 6 
> (which reference in_loop_7) to thread the jump at bb9 (which now 
> references iftmp.0_2).
> 
> 
> The cases in pr78496 are slightly more complex, but boil down to the 
> same core issue -- simplifying the conditional too early.
> 
> Thankfully this is easy to fix.  We just split the conditional 
> simplification into two steps so that the transformation noted above 
> occurs after jump threading (the other simplifications we want to occur 
> before jump threading).
> 
> This allows VRP1 to pick up 27 missed jump threads in the testcase from 
> 78496.  It could well be enough to address 78496, but since we don't 
> have a solid description of the desired end result I won't consider 
> 78496 fixed quite yet as there's significant further improvements we can 
> make.
> 
> Bootstrapped and regression tested on x86_64-linux-gnu.  Installing on 
> the trunk.
> 
> Jeff
> 

-------------- next part --------------
diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 92a4e395ba8..32cc81978df 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,14 @@
+2017-05-03  Jeff Law  <law@redhat.com>
+
+	PR tree-optimization/78496
+	* tree-vrp.c (simplify_cond_using_ranges_1): Renamed
+	from simplify_cond_using_ranges.  Split off code to walk
+	backwards through casts into ...
+	(simplify_cond_using_ranges_2): New function.
+	(simplify_stmt_using_ranges): Call simplify_cond_using_ranges_1.
+	(execute_vrp): After identifying jump threads, call
+	simplify_cond_using_ranges_2.
+
 2017-05-03  Jeff Downs  <heydowns@somuchpressure.net>
 	    Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>
 
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 596468d33e9..55a44e4635a 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2017-05-03  Jeff Law  <law@redhat.com>
+
+	PR tree-optimization/78496
+	* gcc.dg/tree-ssa/ssa-thread-15.c: New test.
+
 2017-05-03  Uros Bizjak  <ubizjak@gmail.com>
 
 	* g++.dg/lto/pr79671_0.C (foo): Fix asm constraints.
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-15.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-15.c
new file mode 100644
index 00000000000..f73268cacbb
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-15.c
@@ -0,0 +1,51 @@
+/* { dg-do compile } */ 
+/* { dg-options "-O2 -fdump-tree-vrp1" } */
+
+/* We should thread the if (!in_loop) completely leaving
+   just two conditionals.  */
+/* { dg-final { scan-tree-dump-times "if \\(" 2 "vrp1" } } */
+
+
+union tree_node;
+typedef union tree_node *tree;
+
+enum size_type_kind
+{
+  SIZETYPE,
+  SSIZETYPE,
+  BITSIZETYPE,
+  SBITSIZETYPE,
+  TYPE_KIND_LAST
+};
+extern tree size_int_kind (long, enum size_type_kind);
+
+
+
+typedef struct
+{
+
+  tree base, step;
+
+} affine_iv;
+
+struct loop
+{
+
+  int num;
+};
+extern unsigned char simple_iv ();
+
+unsigned char
+dr_analyze_innermost (struct loop *loop, tree poffset)
+{
+  affine_iv offset_iv;
+  unsigned char in_loop = (loop && loop->num);
+
+
+  if (in_loop)
+    simple_iv ();
+
+  if (!in_loop)
+    offset_iv.step = size_int_kind (0, SSIZETYPE);
+
+}
diff --git a/gcc/tree-vrp.c b/gcc/tree-vrp.c
index df5e4267491..6a4035dc924 100644
--- a/gcc/tree-vrp.c
+++ b/gcc/tree-vrp.c
@@ -9977,7 +9977,7 @@ range_fits_type_p (value_range *vr, unsigned dest_precision, signop dest_sgn)
    the original conditional.  */
 
 static bool
-simplify_cond_using_ranges (gcond *stmt)
+simplify_cond_using_ranges_1 (gcond *stmt)
 {
   tree op0 = gimple_cond_lhs (stmt);
   tree op1 = gimple_cond_rhs (stmt);
@@ -10080,6 +10080,22 @@ simplify_cond_using_ranges (gcond *stmt)
 	    }
 	}
     }
+  return false;
+}
+
+/* STMT is a conditional at the end of a basic block.
+
+   If the conditional is of the form SSA_NAME op constant and the SSA_NAME
+   was set via a type conversion, try to replace the SSA_NAME with the RHS
+   of the type conversion.  Doing so makes the conversion dead which helps
+   subsequent passes.  */
+
+static void
+simplify_cond_using_ranges_2 (gcond *stmt)
+{
+  tree op0 = gimple_cond_lhs (stmt);
+  tree op1 = gimple_cond_rhs (stmt);
+  enum tree_code cond_code = gimple_cond_code (stmt);
 
   /* If we have a comparison of an SSA_NAME (OP0) against a constant,
      see if OP0 was set by a type conversion where the source of
@@ -10097,7 +10113,7 @@ simplify_cond_using_ranges (gcond *stmt)
 
       if (!is_gimple_assign (def_stmt)
 	  || !CONVERT_EXPR_CODE_P (gimple_assign_rhs_code (def_stmt)))
-	return false;
+	return;
 
       innerop = gimple_assign_rhs1 (def_stmt);
 
@@ -10142,12 +10158,16 @@ simplify_cond_using_ranges (gcond *stmt)
 	      tree newconst = fold_convert (TREE_TYPE (innerop), op1);
 	      gimple_cond_set_lhs (stmt, innerop);
 	      gimple_cond_set_rhs (stmt, newconst);
-	      return true;
+	      update_stmt (stmt);
+	      if (dump_file && (dump_flags & TDF_DETAILS))
+		{
+		  fprintf (dump_file, "Folded into: ");
+		  print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
+		  fprintf (dump_file, "\n");
+		}
 	    }
 	}
     }
-
-  return false;
 }
 
 /* Simplify a switch statement using the value range of the switch
@@ -10746,7 +10766,7 @@ simplify_stmt_using_ranges (gimple_stmt_iterator *gsi)
 	}
     }
   else if (gimple_code (stmt) == GIMPLE_COND)
-    return simplify_cond_using_ranges (as_a <gcond *> (stmt));
+    return simplify_cond_using_ranges_1 (as_a <gcond *> (stmt));
   else if (gimple_code (stmt) == GIMPLE_SWITCH)
     return simplify_switch_using_ranges (as_a <gswitch *> (stmt));
   else if (is_gimple_call (stmt)
@@ -11759,6 +11779,21 @@ execute_vrp (bool warn_array_bounds_p)
      the datastructures built by VRP.  */
   identify_jump_threads ();
 
+  /* A comparison of an SSA_NAME against a constant where the SSA_NAME
+     was set by a type conversion can often be rewritten to use the
+     RHS of the type conversion.
+
+     However, doing so inhibits jump threading through the comparison.
+     So that transformation is not performed until after jump threading
+     is complete.  */
+  basic_block bb;
+  FOR_EACH_BB_FN (bb, cfun)
+    {
+      gimple *last = last_stmt (bb);
+      if (last && gimple_code (last) == GIMPLE_COND)
+	simplify_cond_using_ranges_2 (as_a <gcond *> (last));
+    }
+
   vrp_free_lattice ();
 
   free_numbers_of_iterations_estimates (cfun);


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