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* [gcc r12-4383] libstdc++: Fix regression in memory use when constructing paths
@ 2021-10-13 22:48 Jonathan Wakely
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From: Jonathan Wakely @ 2021-10-13 22:48 UTC (permalink / raw)
To: gcc-cvs, libstdc++-cvs
https://gcc.gnu.org/g:f874a13ca3870a56036a90758b0d41c8c217f4f7
commit r12-4383-gf874a13ca3870a56036a90758b0d41c8c217f4f7
Author: Jonathan Wakely <jwakely@redhat.com>
Date: Wed Oct 13 21:32:14 2021 +0100
libstdc++: Fix regression in memory use when constructing paths
The changes in r12-4381 were intended to reduce memory usage, but
replacing the __contiguous constant in __string_from_range with the new
__is_contiguous variable template caused a regression. The old code
checked is_pointer_v<decltype(std::__niter_base(__first))> but he new
code just checks is_pointer_v<_InputIterator>. This means that we no
longer recognise basic_string::iterator and vector::iterator as
contiguous, and so return a temporary basic_string instead of a
basic_string_view. This only affects C++17 mode, because the
std::contiguous_iterator concept is used in C++20 which gives the right
answer for __normal_iterator (and more types as well).
The fix is to specialize the new __is_contiguous variable template so it
is true for __normal_iterator<T*, C> specializations. The new partial
specializations are defined for C++20 too, because it should be cheaper
to match the partial specialization than to check whether the
std::contiguous_iterator concept is satisfied.
libstdc++-v3/ChangeLog:
* include/bits/fs_path.h (__detail::__is_contiguous): Add
partial specializations for pointers and __normal_iterator.
Diff:
---
libstdc++-v3/include/bits/fs_path.h | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/libstdc++-v3/include/bits/fs_path.h b/libstdc++-v3/include/bits/fs_path.h
index 05db792fbae..c51bfa3095a 100644
--- a/libstdc++-v3/include/bits/fs_path.h
+++ b/libstdc++-v3/include/bits/fs_path.h
@@ -158,9 +158,16 @@ namespace __detail
constexpr bool __is_contiguous = std::contiguous_iterator<_Iter>;
#else
template<typename _Iter>
- constexpr bool __is_contiguous = is_pointer_v<_Iter>;
+ constexpr bool __is_contiguous = false;
#endif
+ template<typename _Tp>
+ constexpr bool __is_contiguous<_Tp*> = true;
+
+ template<typename _Tp, typename _Seq>
+ constexpr bool
+ __is_contiguous<__gnu_cxx::__normal_iterator<_Tp*, _Seq>> = true;
+
#if !defined _GLIBCXX_FILESYSTEM_IS_WINDOWS && defined _GLIBCXX_USE_CHAR8_T
// For POSIX treat char8_t sequences as char without encoding conversions.
template<typename _EcharT>
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