Add tree-ssa-strlen optimization.

From-SVN: r199006

gcc/ChangeLog

+2013-05-17  Marek Polacek  <polacek@redhat.com>
+
+	* tree-ssa-strlen.c (handle_char_store): Don't invalidate
+	cached length when doing non-zero store of storing '\0' to
+	'\0'.
+
+	* gcc.dg/strlenopt-25.c: New test.
+	* gcc.dg/strlenopt-26.c: Likewise.
+
 2013-05-17  Jakub Jelinek  <jakub@redhat.com>
 
 	* tree-vect-patterns.c (vect_recog_rotate_pattern): For

gcc/testsuite/gcc.dg/strlenopt-25.c

+/* { dg-do run } */
+/* { dg-options "-O2 -fdump-tree-strlen" } */
+
+#include "strlenopt.h"
+
+int
+main ()
+{
+  char p[] = "foobar";
+  int len, len2;
+  len = strlen (p);
+  p[0] = 'O';
+  len2 = strlen (p);
+  return len - len2;
+}
+
+/* { dg-final { scan-tree-dump-times "strlen \\(" 0 "strlen" } } */
+/* { dg-final { cleanup-tree-dump "strlen" } } */

gcc/testsuite/gcc.dg/strlenopt-26.c

+/* { dg-do run } */
+/* { dg-options "-O2 -fdump-tree-strlen" } */
+
+#include "strlenopt.h"
+
+__attribute__((noinline, noclone)) size_t
+fn1 (char *p, const char *r)
+{
+  size_t len1 = strlen (r);
+  char *q = strchr (p, '\0');
+  *q = '\0';
+  return len1 - strlen (r); // This strlen should be optimized into len1.
+}
+
+int
+main (void)
+{
+  char p[] = "foobar";
+  const char *volatile q = "xyzzy";
+  fn1 (p, q);
+  return 0;
+}
+
+/* { dg-final { scan-tree-dump-times "strlen \\(" 1 "strlen" } } */
+/* { dg-final { cleanup-tree-dump "strlen" } } */

gcc/tree-ssa-strlen.c

@@ -1694,7 +1694,8 @@ handle_char_store (gimple_stmt_iterator *gsi)
 	      else
 		{
 		  si->writable = true;
-		  si->dont_invalidate = true;
+		  gsi_next (gsi);
+		  return false;
 		}
 	    }
 	  else
@@ -1717,6 +1718,33 @@ handle_char_store (gimple_stmt_iterator *gsi)
 	    si->endptr = ssaname;
 	  si->dont_invalidate = true;
 	}
+      /* If si->length is non-zero constant, we aren't overwriting '\0',
+	 and if we aren't storing '\0', we know that the length of the
+	 string and any other zero terminated string in memory remains
+	 the same.  In that case we move to the next gimple statement and
+	 return to signal the caller that it shouldn't invalidate anything.  
+
+	 This is benefical for cases like:
+
+	 char p[20];
+	 void foo (char *q)
+	 {
+	   strcpy (p, "foobar");
+	   size_t len = strlen (p);        // This can be optimized into 6
+	   size_t len2 = strlen (q);        // This has to be computed
+	   p[0] = 'X';
+	   size_t len3 = strlen (p);        // This can be optimized into 6
+	   size_t len4 = strlen (q);        // This can be optimized into len2
+	   bar (len, len2, len3, len4);
+        }
+	*/ 
+      else if (si != NULL && si->length != NULL_TREE
+	       && TREE_CODE (si->length) == INTEGER_CST
+	       && integer_nonzerop (gimple_assign_rhs1 (stmt)))
+	{
+	  gsi_next (gsi);
+	  return false;
+	}
     }
   else if (idx == 0 && initializer_zerop (gimple_assign_rhs1 (stmt)))
     {