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From: Jonathan Wakely <redi@gcc.gnu.org> To: gcc-cvs@gcc.gnu.org, libstdc++-cvs@gcc.gnu.org Subject: [gcc r12-2437] libstdc++: Make __gnu_cxx::sequence_buffer move-aware [PR101542] Date: Wed, 21 Jul 2021 16:20:51 +0000 (GMT) [thread overview] Message-ID: <20210721162051.2B3C9386FC29@sourceware.org> (raw) https://gcc.gnu.org/g:8edb61420502c62fa2cccdd98876a9aa039b72a6 commit r12-2437-g8edb61420502c62fa2cccdd98876a9aa039b72a6 Author: Jonathan Wakely <jwakely@redhat.com> Date: Wed Jul 21 15:29:19 2021 +0100 libstdc++: Make __gnu_cxx::sequence_buffer move-aware [PR101542] 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. Diff: --- libstdc++-v3/include/ext/rope | 9 ++++++++- libstdc++-v3/testsuite/ext/rope/101542.cc | 27 +++++++++++++++++++++++++++ 2 files changed, 35 insertions(+), 1 deletion(-) diff --git a/libstdc++-v3/include/ext/rope b/libstdc++-v3/include/ext/rope index 81e4f23708f..9681dbc6225 100644 --- a/libstdc++-v3/include/ext/rope +++ b/libstdc++-v3/include/ext/rope @@ -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) { diff --git a/libstdc++-v3/testsuite/ext/rope/101542.cc b/libstdc++-v3/testsuite/ext/rope/101542.cc new file mode 100644 index 00000000000..e89f23d3d48 --- /dev/null +++ b/libstdc++-v3/testsuite/ext/rope/101542.cc @@ -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" ); +}
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