On 12/9/21 5:38 PM, Martin Sebor wrote:
On 12/9/21 4:24 PM, Jonathan Wakely via Gcc-patches wrote:
These warnings are triggered by perfectly valid code using std::string.
They're particularly bad when --enable-fully-dynamic-string is used,
because even std::string().begin() will give a warning.
Use pragmas to stop the troublesome warnings for copies done by
std::char_traits.
I'm still experimenting with some of the approaches we discussed
last week, but based on my findings so far this was going to be
my suggestion at lest for now, until or unless the problem turns
out to affect more code than just std::string.
Just minutes after I wrote this I tried following the clue
in the note printed for the test case from PR 103534 with
an enhancement I'm experimenting with:
/build/gcc-master/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/char_traits.h:426:56:
warning: ‘void* __builtin_memcpy(void*, const void*, long unsigned int)’
specified size between 18446744073709551600 and 18446744073709551615
exceeds maximum object size 9223372036854775807 [-Wstringop-overflow=]
426 | return static_cast<char_type*>(__builtin_memcpy(__s1,
__s2, __n));
|
~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~
/build/gcc-master/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/char_traits.h:426:56:
note: when
‘(<anonymous>.std::__cxx11::basic_string<char>::_M_string_length >
18446744073709551599)’
and adding an assert to string::size():
constexpr
size_type
size() const noexcept
{
if (_M_string_length >= -1LU >> 1)
__builtin_unreachable ();
return _M_string_length;
}
That gets rid of the false positive in this PR. I realize
the others happen for other reasons but this approach at least
suggests that there might be other ways to suppress them than
the #pragma. Unlike it, the alternative approaches should also
improve codegen.
That said, I noticed a typo in the patch:
libstdc++-v3/ChangeLog:
PR libstdc++/103332
PR libstdc++/102958
PR libstdc++/103483
* include/bits/char_traits.h: Suppress stringop and array-bounds
warnings.
---
libstdc++-v3/include/bits/char_traits.h | 7 +++++++
1 file changed, 7 insertions(+)
diff --git a/libstdc++-v3/include/bits/char_traits.h
b/libstdc++-v3/include/bits/char_traits.h
index da3e0ffffaa..3f7befcf8b2 100644
--- a/libstdc++-v3/include/bits/char_traits.h
+++ b/libstdc++-v3/include/bits/char_traits.h
@@ -54,6 +54,11 @@ namespace __gnu_cxx _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstringop-overflow"
+#pragma GCC diagnostic ignored "-Wstringop-overread"
+#pragma GCC diagnostic ignored "-Warray-bounds"
(Just for reference, as I mentioned in my private mail, at -O1
the same code also triggers -Wfree-nonheap-object.)
+
/**
* @brief Mapping from character type to associated types.
*
@@ -990,6 +995,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
} // namespace __detail
#endif // C++20
+#pragma GCC diagnostic push
This should be pop.
Martin
+
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace
