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libstdc++: Make __gnu_cxx::sequence_buffer move-aware [PR101542]

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The PR explains that Clang trunk now selects a different constructor
when a non-const sequence_buffer is returned in a context where it
qualifies as an implicitly-movable entity. Because lookup is first
performed using an rvalue, the sequence_buffer(const sequence_buffer&)
constructor gets chosen, which makes a copy instead of a "pseudo-move"
via the sequence_buffer(sequence_buffer&) constructor. The problem isn't
seen with GCC because as noted in the r11-2412 commit log, GCC actually
implements a slightly modified rule that avoids breaking exactly this
type of code.

This patch adds a move constructor to sequence_buffer, so that implicit
or explicit moves will have the same effect, calling the
sequence_buffer(sequence_buffer&) constructor. A move assignment
operator is also added to make move assignment work similarly.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	PR libstdc++/101542
	* include/ext/rope (sequence_buffer): Add move constructor and
	move assignment operator.
	* testsuite/ext/rope/101542.cc: New test.
This commit is contained in:
Jonathan Wakely 2021-07-21 15:29:19 +01:00
parent e4d306cf70
commit 8edb614205
2 changed files with 35 additions and 1 deletions
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9
libstdc++-v3/include/ext/rope
View file

@ -203,6 +203,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
std::copy(__x._M_buffer, __x._M_buffer + __x._M_buf_count, _M_buffer);
}
// Non-const "copy" modifies the parameter - yuck
sequence_buffer(sequence_buffer& __x)
{
__x.flush();
@ -213,6 +214,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
sequence_buffer(_Sequence& __s)
: _M_prefix(&__s), _M_buf_count(0) { }
// Non-const "copy" modifies the parameter - yuck
sequence_buffer&
operator=(sequence_buffer& __x)
{
@ -230,7 +232,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
std::copy(__x._M_buffer, __x._M_buffer + __x._M_buf_count, _M_buffer);
return *this;
}
#if __cplusplus >= 201103L
sequence_buffer(sequence_buffer&& __x) : sequence_buffer(__x) { }
sequence_buffer& operator=(sequence_buffer&& __x) { return *this = __x; }
#endif
void
push_back(value_type __x)
{

27
libstdc++-v3/testsuite/ext/rope/101542.cc Normal file
View file

@ -0,0 +1,27 @@
// { dg-do run { target c++11 } }
// PR libstdc++/101542
#include <ext/rope>
#include <testsuite_hooks.h>
template<typename T> T f(T x) { return x; }
template<typename T> T g(T x) { return std::move(x); }
int main()
{
std::string s;
{
__gnu_cxx::sequence_buffer<std::string> a(s);
{
__gnu_cxx::sequence_buffer<std::string> b = std::move(a);
b.push_back('h');
b.push_back('e');
b.push_back('l');
b.push_back('l');
b.push_back('o');
__gnu_cxx::sequence_buffer<std::string> c;
c = f(g((std::move(b))));
}
}
VERIFY( s == "hello" );
}