[PATCH] Fixup nb_iterations_upper_bound adjustment for vectorized loops

[Ilya Enkovich <enkovich dot gnu at gmail dot com>]

Hi,

Currently when loop is vectorized we adjust its nb_iterations_upper_bound
by dividing it by VF.  This is incorrect since nb_iterations_upper_bound
is upper bound for (<number of loop iterations> - 1) and therefore simple
dividing it by VF in many cases gives us bounds greater than a real one.
Correct value would be ((nb_iterations_upper_bound + 1) / VF - 1).

Also decrement due to peeling for gaps should happen before we scale it
by VF because peeling applies to a scalar loop, not vectorized one.

This patch modifies nb_iterations_upper_bound computation to resolve
these issues.

Running regression testing I got one fail due to optimized loop. Heres
is a loop:

foo (signed char s)
{
  signed char i;
  for (i = 0; i < s; i++)
    yy[i] = (signed int) i;
}

Here we vectorize for AVX512 using VF=64.  Original loop has max 127
iterations and therefore vectorized loop may be executed only once.
With the patch applied compiler detects it and transforms loop into
BB with just stores of constants vectors into yy.  Test was adjusted
to increase number of possible iterations.  A copy of test was added
to check we can optimize out the original loop.

Bootstrapped and regtested on x86_64-pc-linux-gnu.  OK for trunk?

Thanks,
Ilya
--
gcc/

2016-04-21  Ilya Enkovich  <ilya.enkovich@intel.com>

	* tree-vect-loop.c (vect_transform_loop): Fix
	nb_iterations_upper_bound computation for vectorized loop.

gcc/testsuite/

2016-04-21  Ilya Enkovich  <ilya.enkovich@intel.com>

	* gcc.target/i386/vect-unpack-2.c (avx512bw_test): Avoid
	optimization of vector loop.
	* gcc.target/i386/vect-unpack-3.c: New test.


diff --git a/gcc/testsuite/gcc.target/i386/vect-unpack-2.c b/gcc/testsuite/gcc.target/i386/vect-unpack-2.c
index 4825248..51c518e 100644
--- a/gcc/testsuite/gcc.target/i386/vect-unpack-2.c
+++ b/gcc/testsuite/gcc.target/i386/vect-unpack-2.c
@@ -6,19 +6,22 @@
 
 #define N 120
 signed int yy[10000];
+signed char zz[10000];
 
 void
-__attribute__ ((noinline)) foo (signed char s)
+__attribute__ ((noinline,noclone)) foo (int s)
 {
-   signed char i;
+   int i;
    for (i = 0; i < s; i++)
-     yy[i] = (signed int) i;
+     yy[i] = zz[i];
 }
 
 void
 avx512bw_test ()
 {
   signed char i;
+  for (i = 0; i < N; i++)
+    zz[i] = i;
   foo (N);
   for (i = 0; i < N; i++)
     if ( (signed int)i != yy [i] )
diff --git a/gcc/testsuite/gcc.target/i386/vect-unpack-3.c b/gcc/testsuite/gcc.target/i386/vect-unpack-3.c
new file mode 100644
index 0000000..eb8a93e
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/vect-unpack-3.c
@@ -0,0 +1,29 @@
+/* { dg-do run } */
+/* { dg-options "-O2 -fdump-tree-vect-details -ftree-vectorize -ffast-math -mavx512bw -save-temps" } */
+/* { dg-require-effective-target avx512bw } */
+
+#include "avx512bw-check.h"
+
+#define N 120
+signed int yy[10000];
+
+void
+__attribute__ ((noinline)) foo (signed char s)
+{
+   signed char i;
+   for (i = 0; i < s; i++)
+     yy[i] = (signed int) i;
+}
+
+void
+avx512bw_test ()
+{
+  signed char i;
+  foo (N);
+  for (i = 0; i < N; i++)
+    if ( (signed int)i != yy [i] )
+      abort ();
+}
+
+/* { dg-final { scan-tree-dump "LOOP VECTORIZED" "vect" } } */
+/* { dg-final { scan-assembler-not "vpmovsxbw\[ \\t\]+\[^\n\]*%zmm" } } */
diff --git a/gcc/tree-vect-loop.c b/gcc/tree-vect-loop.c
index d813b86..da98211 100644
--- a/gcc/tree-vect-loop.c
+++ b/gcc/tree-vect-loop.c
@@ -6921,11 +6921,13 @@ vect_transform_loop (loop_vec_info loop_vinfo)
   /* Reduce loop iterations by the vectorization factor.  */
   scale_loop_profile (loop, GCOV_COMPUTE_SCALE (1, vectorization_factor),
 		      expected_iterations / vectorization_factor);
-  loop->nb_iterations_upper_bound
-    = wi::udiv_floor (loop->nb_iterations_upper_bound, vectorization_factor);
   if (LOOP_VINFO_PEELING_FOR_GAPS (loop_vinfo)
       && loop->nb_iterations_upper_bound != 0)
     loop->nb_iterations_upper_bound = loop->nb_iterations_upper_bound - 1;
+  loop->nb_iterations_upper_bound
+    = wi::udiv_floor (loop->nb_iterations_upper_bound + 1,
+		      vectorization_factor) - 1;
+
   if (loop->any_estimate)
     {
       loop->nb_iterations_estimate