https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108846
--- Comment #32 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jonathan Wakely <r...@gcc.gnu.org>: https://gcc.gnu.org/g:3abe751ea86e3472fa2c97bf2014f9f93f569019 commit r15-4473-g3abe751ea86e3472fa2c97bf2014f9f93f569019 Author: Jonathan Wakely <jwak...@redhat.com> Date: Wed Oct 9 12:55:54 2024 +0100 libstdc++: Refactor std::uninitialized_{copy,fill,fill_n} algos [PR68350] This refactors the std::uninitialized_copy, std::uninitialized_fill and std::uninitialized_fill_n algorithms to directly perform memcpy/memset optimizations instead of dispatching to std::copy/std::fill/std::fill_n. The reasons for this are: - Use 'if constexpr' to simplify and optimize compilation throughput, so dispatching to specialized class templates is only needed for C++98 mode. - Use memcpy instead of memmove, because the conditions on non-overlapping ranges are stronger for std::uninitialized_copy than for std::copy. Using memcpy might be a minor optimization. - No special case for creating a range of one element, which std::copy needs to deal with (see PR libstdc++/108846). The uninitialized algos create new objects, which reuses storage and is allowed to clobber tail padding. - Relax the conditions for using memcpy/memset, because the C++20 rules on implicit-lifetime types mean that we can rely on memcpy to begin lifetimes of trivially copyable types. We don't need to require trivially default constructible, so don't need to limit the optimization to trivial types. See PR 68350 for more details. - Remove the dependency on std::copy and std::fill. This should mean that stl_uninitialized.h no longer needs to include all of stl_algobase.h. This isn't quite true yet, because we still use std::fill in __uninitialized_default and still use std::fill_n in __uninitialized_default_n. That will be fixed later. Several tests need changes to the diagnostics matched by dg-error because we no longer use the __constructible() function that had a static assert in. Now we just get straightforward errors for attempting to use a deleted constructor. Two tests needed more signficant changes to the actual expected results of executing the tests, because they were checking for old behaviour which was incorrect according to the standard. 20_util/specialized_algorithms/uninitialized_copy/64476.cc was expecting std::copy to be used for a call to std::uninitialized_copy involving two trivially copyable types. That was incorrect behaviour, because a non-trivial constructor should have been used, but using std::copy used trivial default initialization followed by assignment. 20_util/specialized_algorithms/uninitialized_fill_n/sizes.cc was testing the behaviour with a non-integral Size passed to uninitialized_fill_n, but I wrote the test looking at the requirements of uninitialized_copy_n which are not the same as uninitialized_fill_n. The former uses --n and tests n > 0, but the latter just tests n-- (which will never be false for a floating-point value with a fractional part). libstdc++-v3/ChangeLog: PR libstdc++/68350 PR libstdc++/93059 * include/bits/stl_uninitialized.h (__check_constructible) (_GLIBCXX_USE_ASSIGN_FOR_INIT): Remove. [C++98] (__unwrappable_niter): New trait. (__uninitialized_copy<true>): Replace use of std::copy. (uninitialized_copy): Fix Doxygen comments. Open-code memcpy optimization for C++11 and later. (__uninitialized_fill<true>): Replace use of std::fill. (uninitialized_fill): Fix Doxygen comments. Open-code memset optimization for C++11 and later. (__uninitialized_fill_n<true>): Replace use of std::fill_n. (uninitialized_fill_n): Fix Doxygen comments. Open-code memset optimization for C++11 and later. * testsuite/20_util/specialized_algorithms/uninitialized_copy/64476.cc: Adjust expected behaviour to match what the standard specifies. * testsuite/20_util/specialized_algorithms/uninitialized_fill_n/sizes.cc: Likewise. * testsuite/20_util/specialized_algorithms/uninitialized_copy/1.cc: Adjust dg-error directives. * testsuite/20_util/specialized_algorithms/uninitialized_copy/89164.cc: Likewise. * testsuite/20_util/specialized_algorithms/uninitialized_copy_n/89164.cc: Likewise. * testsuite/20_util/specialized_algorithms/uninitialized_fill/89164.cc: Likewise. * testsuite/20_util/specialized_algorithms/uninitialized_fill_n/89164.cc: Likewise. * testsuite/23_containers/vector/cons/89164.cc: Likewise. * testsuite/23_containers/vector/cons/89164_c++17.cc: Likewise. Reviewed-by: Patrick Palka <ppa...@redhat.com>
