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[PATCH v2] Fix bogus strncpy source length warning on source bound by constant


Avoid issuing a bogus warning when the source of strncpy is bound by a
constant known to be less than the minimum size of the destination.

Changes from v1:

- Use range-info instead of the MIN_EXPR hack
- Get the minimum size of dst and check for NULL_TREE return

The patch bootstraps successfully and introduces no new regressions in
the testsuite.

gcc/

	* tree-ssa-strlen.c (handle_builtin_stxncpy): Check bounds of
	source length if available.

gcc/testsuite/

	* gcc.dg/builtin-stringop-chk-10.c: New test case.
---
 gcc/testsuite/gcc.dg/builtin-stringop-chk-10.c | 17 +++++++++++++++++
 gcc/tree-ssa-strlen.c                          | 15 +++++++++++++++
 2 files changed, 32 insertions(+)
 create mode 100644 gcc/testsuite/gcc.dg/builtin-stringop-chk-10.c

diff --git a/gcc/testsuite/gcc.dg/builtin-stringop-chk-10.c b/gcc/testsuite/gcc.dg/builtin-stringop-chk-10.c
new file mode 100644
index 00000000000..13e4bd2f049
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/builtin-stringop-chk-10.c
@@ -0,0 +1,17 @@
+/* Bogus -Wstringop-overflow on strncpy when size is based on strlen but is
+   bound by a constant.
+   { dg-do compile }
+   { dg-options "-O2 -Wstringop-overflow" } */
+
+char dst[1024];
+
+void
+f1 (const char *src)
+{
+  unsigned long limit = 512;
+  unsigned long len = __builtin_strlen (src);	/* { dg-bogus "length computed here" } */
+  if (len > limit)
+    len = limit;
+
+  __builtin_strncpy (dst, src, len);	/* { dg-bogus "specified bound depends on the length of the source argument" } */
+}
diff --git a/gcc/tree-ssa-strlen.c b/gcc/tree-ssa-strlen.c
index 72f6a17cd32..265f351ea85 100644
--- a/gcc/tree-ssa-strlen.c
+++ b/gcc/tree-ssa-strlen.c
@@ -2125,6 +2125,21 @@ handle_builtin_stxncpy (built_in_function, gimple_stmt_iterator *gsi)
       return;
     }
 
+  /* Don't bother about the strlen (SRC) in the LEN computation if the range of
+     values for LEN ends up within dstsize.  */
+  if (TREE_CODE (len) == SSA_NAME)
+    {
+      wide_int min, max;
+      enum value_range_type vr = get_range_info (len, &min, &max);
+      tree dstsize = compute_objsize (dst, 3);
+      if (vr == VR_RANGE && dstsize)
+	{
+	  tree len_max = wide_int_to_tree (TREE_TYPE (dstsize), max);
+	  if (tree_int_cst_lt (len_max, dstsize))
+	    return;
+	}
+    }
+
   /* 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
      to strlen(S)).  */
-- 
2.14.3


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