strncmp: Add a testcase for page boundary [BZ #25933]

Add a strncmp testcase to cover cases where one of strings ends on the
page boundary with the maximum string length less than the number bytes
of each AVX2 loop iteration and different offsets from page boundary.

The updated string/test-strncmp fails on Intel Core i7-8559U without

ommit 1c6432316bc434a72108d7b0c7cfbfdde64c3124
Author: Sunil K Pandey <skpgkp1@gmail.com>
Date:   Fri Jun 12 08:57:16 2020 -0700

    Fix avx2 strncmp offset compare condition check [BZ #25933]

1 files changed, 33 insertions, 0 deletions

diff --git a/string/test-strncmp.c b/string/test-strncmp.c
index d961ac4493..962679b384 100644
--- a/string/test-strncmp.c
+++ b/string/test-strncmp.c
@@ -403,6 +403,38 @@ check2 (void)
   free (s2);
 }
 
+static void
+check3 (void)
+{
+  /* To trigger bug 25933, we need a size that is equal to the vector
+     length times 4. In the case of AVX2 for Intel, we need 32 * 4.  We
+     make this test generic and run it for all architectures as additional
+     boundary testing for such related algorithms.  */
+  size_t size = 32 * 4;
+  CHAR *s1 = (CHAR *) (buf1 + (BUF1PAGES - 1) * page_size);
+  CHAR *s2 = (CHAR *) (buf2 + (BUF1PAGES - 1) * page_size);
+  int exp_result;
+
+  memset (s1, 'a', page_size);
+  memset (s2, 'a', page_size);
+  s1[(page_size / CHARBYTES) - 1] = (CHAR) 0;
+
+  /* Iterate over a size that is just below where we expect the bug to
+     trigger up to the size we expect will trigger the bug e.g. [99-128].
+     Likewise iterate the start of two strings between 30 and 31 bytes
+     away from the boundary to simulate alignment changes.  */
+  for (size_t s = 99; s <= size; s++)
+    for (size_t s1a = 30; s1a < 32; s1a++)
+      for (size_t s2a = 30; s2a < 32; s2a++)
+	{
+	  CHAR *s1p = s1 + (page_size / CHARBYTES - s) - s1a;
+	  CHAR *s2p = s2 + (page_size / CHARBYTES - s) - s2a;
+	  exp_result = SIMPLE_STRNCMP (s1p, s2p, s);
+	  FOR_EACH_IMPL (impl, 0)
+	    check_result (impl, s1p, s2p, s, exp_result);
+	}
+}
+
 int
 test_main (void)
 {
@@ -412,6 +444,7 @@ test_main (void)
 
   check1 ();
   check2 ();
+  check3 ();
 
   printf ("%23s", "");
   FOR_EACH_IMPL (impl, 0)