GCC modified for the FreeChainXenon project
10f26de915
Avoid creating arbitrarily large objects on the stack when emplacing trivially copyable objects into a variant. Currently we provide the strong exception-safety guarantee for all trivially copyable types, by constructing a second variant and then doing a non-throwing move assignment from the temporary. This patch restricts that behaviour to trivially copyable types that are no larger than 256 bytes. For larger types the object will be emplaced directly into the variant, and if its initialization throws then the variant becomes valueless. Also implement Antony Polukhin's suggestion to whitelist specific types that are not trivially copyable but can be efficiently move-assigned. Emplacing those types will never cause a variant to become valueless. The whitelisted types are: std::shared_ptr, std::weak_ptr, std::unique_ptr, std::function, and std::any. Additionally, std::basic_string, std::vector, and __gnu_debug::vector are whitelisted if their allocator traits give them a non-throwing move assignment operator. Specifically, this means std::string is whitelisted, but std::pmr::string is not. As part of this patch, additional if-constexpr branches are added for the cases where the initialization is known to be non-throwing (so the overhead of the try-catch block can be avoided) and where a scalar is being produced by a potentially-throwing conversion operator (so that the overhead of constructing and move-assigning a variant is avoided). These changes should have no semantic effect, just better codegen. PR libstdc++/87431 (again) * include/bits/basic_string.h (__variant::_Never_valueless_alt): Define partial specialization for basic_string. * include/bits/shared_ptr.h (_Never_valueless_alt): Likewise for shared_ptr and weak_ptr. * include/bits/std_function.h (_Never_valueless_alt): Likewise for function. * include/bits/stl_vector.h (_Never_valueless_alt): Likewise for vector. * include/bits/unique_ptr.h (_Never_valueless_alt): Likewise for unique_ptr. * include/debug/vector (_Never_valueless_alt): Likewise for debug vector. * include/std/any (_Never_valueless_alt): Define explicit specialization for any. * include/std/variant (_Never_valueless_alt): Define primary template. (__never_valueless): Use _Never_valueless_alt instead of is_trivially_copyable. (variant::emplace<N>(Args&&...)): Add special case for non-throwing initializations to avoid try-catch overhead. Add special case for scalars produced by potentially-throwing conversions. Use _Never_valueless_alt instead of is_trivially_copyable for the remaining strong exception-safety cases. (variant::emplace<N>(initializer_list<U>, Args&&...)): Likewise. * testsuite/20_util/variant/87431.cc: Run both test functions. * testsuite/20_util/variant/exception_safety.cc: New test. * testsuite/20_util/variant/run.cc: Use pmr::string instead of string, so the variant becomes valueless. From-SVN: r270170 |
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| config | ||
| contrib | ||
| fixincludes | ||
| gcc | ||
| gnattools | ||
| gotools | ||
| include | ||
| INSTALL | ||
| intl | ||
| libada | ||
| libatomic | ||
| libbacktrace | ||
| libcc1 | ||
| libcpp | ||
| libdecnumber | ||
| libffi | ||
| libgcc | ||
| libgfortran | ||
| libgo | ||
| libgomp | ||
| libhsail-rt | ||
| libiberty | ||
| libitm | ||
| libobjc | ||
| liboffloadmic | ||
| libphobos | ||
| libquadmath | ||
| libsanitizer | ||
| libssp | ||
| libstdc++-v3 | ||
| libvtv | ||
| lto-plugin | ||
| maintainer-scripts | ||
| zlib | ||
| .dir-locals.el | ||
| .gitattributes | ||
| .gitignore | ||
| ABOUT-NLS | ||
| ar-lib | ||
| ChangeLog | ||
| ChangeLog.jit | ||
| ChangeLog.tree-ssa | ||
| compile | ||
| config-ml.in | ||
| config.guess | ||
| config.rpath | ||
| config.sub | ||
| configure | ||
| configure.ac | ||
| COPYING | ||
| COPYING.LIB | ||
| COPYING.RUNTIME | ||
| COPYING3 | ||
| COPYING3.LIB | ||
| depcomp | ||
| install-sh | ||
| libtool-ldflags | ||
| libtool.m4 | ||
| ltgcc.m4 | ||
| ltmain.sh | ||
| ltoptions.m4 | ||
| ltsugar.m4 | ||
| ltversion.m4 | ||
| lt~obsolete.m4 | ||
| MAINTAINERS | ||
| Makefile.def | ||
| Makefile.in | ||
| Makefile.tpl | ||
| missing | ||
| mkdep | ||
| mkinstalldirs | ||
| move-if-change | ||
| multilib.am | ||
| README | ||
| symlink-tree | ||
| test-driver | ||
| ylwrap | ||
This directory contains the GNU Compiler Collection (GCC). The GNU Compiler Collection is free software. See the files whose names start with COPYING for copying permission. The manuals, and some of the runtime libraries, are under different terms; see the individual source files for details. The directory INSTALL contains copies of the installation information as HTML and plain text. The source of this information is gcc/doc/install.texi. The installation information includes details of what is included in the GCC sources and what files GCC installs. See the file gcc/doc/gcc.texi (together with other files that it includes) for usage and porting information. An online readable version of the manual is in the files gcc/doc/gcc.info*. See http://gcc.gnu.org/bugs/ for how to report bugs usefully. Copyright years on GCC source files may be listed using range notation, e.g., 1987-2012, indicating that every year in the range, inclusive, is a copyrightable year that could otherwise be listed individually.