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* [gcc/devel/ranger] libstdc++: Fix comment on __memcpyable
@ 2020-06-17 19:25 Aldy Hernandez
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From: Aldy Hernandez @ 2020-06-17 19:25 UTC (permalink / raw)
To: gcc-cvs, libstdc++-cvs
https://gcc.gnu.org/g:94f7d7ec6ebef49a50da777fd71db3d03ee03aa0
commit 94f7d7ec6ebef49a50da777fd71db3d03ee03aa0
Author: Jonathan Wakely <jwakely@redhat.com>
Date: Wed Mar 4 15:34:05 2020 +0000
libstdc++: Fix comment on __memcpyable
The discussion of iterator_traits<volatile T*>::value_type and the
example with three tempalte arguments related to an earlier version of
the patch, not the one committed.
Also improve the comment on __memcmpable.
* include/bits/cpp_type_traits.h (__memcpyable): Fix comment.
Diff:
---
libstdc++-v3/ChangeLog | 4 ++++
libstdc++-v3/include/bits/cpp_type_traits.h | 11 +++++++----
2 files changed, 11 insertions(+), 4 deletions(-)
diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index ff8ae64d477..df3b040abca 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,7 @@
+2020-03-04 Jonathan Wakely <jwakely@redhat.com>
+
+ * include/bits/cpp_type_traits.h (__memcpyable): Fix comment.
+
2020-03-04 Patrick Palka <ppalka@redhat.com>
PR libstdc++/94017
diff --git a/libstdc++-v3/include/bits/cpp_type_traits.h b/libstdc++-v3/include/bits/cpp_type_traits.h
index fac6e4bbea2..979ad9c2c69 100644
--- a/libstdc++-v3/include/bits/cpp_type_traits.h
+++ b/libstdc++-v3/include/bits/cpp_type_traits.h
@@ -429,10 +429,9 @@ __INT_N(__GLIBCXX_TYPE_INT_N_3)
enum { __value = __is_trivially_copyable(_Tp) };
};
- // Cannot use memcpy/memmove/memcmp on volatile types, but before C++20
- // iterator_traits<volatile T*>::value_type is volatile T and so the
- // partial specializations below match for volatile-qualified pointers
- // e.g. __memcpyable<volatile int*, volatile int*, volatile int>.
+ // Cannot use memcpy/memmove/memcmp on volatile types even if they are
+ // trivially copyable, so ensure __memcpyable<volatile int*, volatile int*>
+ // and similar will be false.
template<typename _Tp>
struct __is_nonvolatile_trivially_copyable<volatile _Tp>
{
@@ -457,6 +456,10 @@ __INT_N(__GLIBCXX_TYPE_INT_N_3)
{ };
// Whether two iterator types can be used with memcmp.
+ // This trait only says it's well-formed to use memcmp, not that it
+ // gives the right answer for a given algorithm. So for example, std::equal
+ // needs to add additional checks that the types are integers or pointers,
+ // because other trivially copyable types can overload operator==.
template<typename _Iter1, typename _Iter2>
struct __memcmpable
{
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