https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98465
--- Comment #24 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Ok, so for GCC 11, can we just help the optimizers a little bit and at the same
time get rid of the warning?
Like:
--- libstdc++-v3/include/bits/basic_string.tcc.jj 2021-01-04
10:26:02.930960956 +0100
+++ libstdc++-v3/include/bits/basic_string.tcc 2021-02-04 17:44:10.592843195
+0100
@@ -477,7 +477,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{
if (__s + __len2 <= __p + __len1)
this->_S_move(__p, __s, __len2);
+#if _GLIBCXX_HAS_BUILTIN(__builtin_object_size) && defined(__OPTIMIZE__)
+ /* Help the optimizers rule out impossible cases and
+ get rid of false positive warnings at the same time.
+ If we know the maximum size of the __s object and
+ it is shorter than 2 * __len2 - __len1, then
+ __s >= __p + __len1 case is impossible. */
+ else if (!(__builtin_constant_p((2 * __len2 - __len1)
+ * sizeof(_CharT))
+ && __builtin_object_size(__s, 0)
+ < (2 * __len2 - __len1) * sizeof(_CharT))
+ && __s >= __p + __len1)
+#else
else if (__s >= __p + __len1)
+#endif
this->_S_copy(__p, __s + __len2 - __len1, __len2);
else
{
I don't get the warning anymore and function size on #c0 testcase shrunk with
-O2 -std=gnu++20 -Wall from 614 bytes to 568.
This code is handling the non-disjunct case, and when __s >= __p + __len1 and
__len1 < __len2, we need to copy from (__s + __len2 - __len1) __len2, i.e. have
access up to __s + 2 * __len2 - __len1 - 1. If we know that is certainly out
of bounds, most likely it isn't the non-disjunct case at all, but even if it
would, this copying wouldn't be a valid option.
