[PATCH GCC 7/9]Skip loops iterating only 1 time in predictive commoning

[Bin Cheng]

Hi,
For loops which are bounded to iterate only 1 time (thus loop's latch doesn't 
roll), there is nothing to predictive common, this patch detects/skips these 
cases.  A test is also added in 
gcc/testsuite/gfortran.dg/vect/fast-math-mgrid-resid.f for this.

Thanks,
bin

2016-09-01  Bin Cheng  <bin.ch...@arm.com>

        * tree-predcom.c (tree_predictive_commoning_loop): Skip loop that only
        iterates 1 time.

gcc/testsuite/ChangeLog
2016-09-01  Bin Cheng  <bin.ch...@arm.com>

        * gfortran.dg/vect/fast-math-mgrid-resid.f: New test string.
diff --git a/gcc/testsuite/gfortran.dg/vect/fast-math-mgrid-resid.f 
b/gcc/testsuite/gfortran.dg/vect/fast-math-mgrid-resid.f
index fcf475d..88238f9 100644
--- a/gcc/testsuite/gfortran.dg/vect/fast-math-mgrid-resid.f
+++ b/gcc/testsuite/gfortran.dg/vect/fast-math-mgrid-resid.f
@@ -42,3 +42,4 @@ C
 ! { dg-final { scan-tree-dump-times "Executing predictive commoning without 
unrolling" 1 "pcom" { target lp64 } } }
 ! { dg-final { scan-tree-dump-times "Executing predictive commoning without 
unrolling" 2 "pcom" { target ia32 } } }
 ! { dg-final { scan-tree-dump-times "Predictive commoning failed: no suitable 
chains" 0 "pcom" } }
+! { dg-final { scan-tree-dump-times "Loop iterates only 1 time, nothing to do" 
1 "pcom" } }
diff --git a/gcc/tree-predcom.c b/gcc/tree-predcom.c
index 096365a..14d53c2 100644
--- a/gcc/tree-predcom.c
+++ b/gcc/tree-predcom.c
@@ -2465,6 +2465,15 @@ tree_predictive_commoning_loop (struct loop *loop)
   if (dump_file && (dump_flags & TDF_DETAILS))
     fprintf (dump_file, "Processing loop %d\n",  loop->num);
 
+  /* Nothing for predicitive commoning if loop only iterates 1 time.  */
+  if (get_max_loop_iterations_int (loop) == 0)
+    {
+      if (dump_file && (dump_flags & TDF_DETAILS))
+       fprintf (dump_file, "Loop iterates only 1 time, nothing to do.\n");
+
+      return false;
+    }
+
   /* Find the data references and split them into components according to their
      dependence relations.  */
   auto_vec<loop_p, 3> loop_nest;