PR tree-optimization/83075 - Invalid strncpy optimization

gcc/ChangeLog:

	PR tree-optimization/83075
	* tree-ssa-strlen.c (handle_builtin_stxncpy): Avoid assuming
	strncat/strncpy don't change length of source string.

gcc/testsuite/ChangeLog:

	PR tree-optimization/83075
	* gcc.dg/tree-ssa/strncat.c: New test.
	* gcc.dg/tree-ssa/strncpy-2.c: Same.

From-SVN: r255446

gcc/ChangeLog

+2017-12-06  Martin Sebor  <msebor@redhat.com>
+
+	PR tree-optimization/83075
+	* tree-ssa-strlen.c (handle_builtin_stxncpy): Avoid assuming
+	strncat/strncpy don't change length of source string.
+
 2017-12-06  Eric Botcazou  <ebotcazou@adacore.com>
 
 	Revert

gcc/testsuite/ChangeLog

+2017-12-06  Martin Sebor  <msebor@redhat.com>
+
+	PR tree-optimization/83075
+	* gcc.dg/tree-ssa/strncat.c: New test.
+	* gcc.dg/tree-ssa/strncpy-2.c: Same.
+
 2017-12-06  Bin Cheng  <bin.cheng@arm.com>
 
 	* g++.dg/graphite/pr41305.C: Refine test option.

gcc/testsuite/gcc.dg/tree-ssa/strncat.c

+/* PR tree-optimization/83075 - Invalid strncpy optimization
+   { dg-do run }
+   { dg-options "-O2 -Wno-stringop-overflow" } */
+
+int main (void)
+{
+  char a[8] = "";
+
+  __builtin_strcpy (a, "123");
+
+  unsigned n0 = __builtin_strlen (a);
+
+  __builtin_strncat (a + 3, a, n0);
+
+  unsigned n1 = __builtin_strlen (a);
+
+  if (n1 == n0)
+    __builtin_abort ();
+}

gcc/testsuite/gcc.dg/tree-ssa/strncpy-2.c

+/* PR tree-optimization/83075 - Invalid strncpy optimization
+   { dg-do run }
+   { dg-options "-O2 -Wno-stringop-overflow" } */
+
+int main (void)
+{
+  char a[8] = "";
+
+  __builtin_strcpy (a, "123");
+
+  unsigned n0 = __builtin_strlen (a);
+
+  __builtin_strncpy (a + 3, a, n0);
+
+  unsigned n1 = __builtin_strlen (a);
+
+  if (n1 == n0)
+    __builtin_abort ();
+}

gcc/tree-ssa-strlen.c

@@ -1941,10 +1941,9 @@ handle_builtin_stxncpy (built_in_function, gimple_stmt_iterator *gsi)
   int sidx = get_stridx (src);
   strinfo *sisrc = sidx > 0 ? get_strinfo (sidx) : NULL;
 
-  /* Strncpy() et al. cannot modify the source string.  Prevent the rest
-     of the pass from invalidating the strinfo data.  */
-  if (sisrc)
-    sisrc->dont_invalidate = true;
+  /* strncat() and strncpy() can modify the source string by writing
+     over the terminating nul so SISRC->DONT_INVALIDATE must be left
+     clear.  */
 
   /* Retrieve the strinfo data for the string S that LEN was computed
      from as some function F of strlen (S) (i.e., LEN need not be equal