Re: [PATCH] PR libstdc++/79254 fix exception-safety in std::string::operator=

[Jonathan Wakely <jwakely at redhat dot com>]

On 01/02/17 11:42 +0000, Jonathan Wakely wrote:
> On 27/01/17 16:16 +0000, Jonathan Wakely wrote:
> > This implements the strong exception-safety guarantee that is required
> > by [string.require] p2, which the new string can fail to meet when
> > propagate_on_container_copy_assignment (POCCA) is true.
> >
> > The solution is to define a helper that takes ownership of the
> > string's memory (and also the associated allocator, length and
> > capacity) and either deallocates it after the assignment, or swaps it
> > back in if an exception happens (i.e. commit or rollback).
> >
> > 	PR libstdc++/79254
> > 	* config/abi/pre/gnu.ver: Add new symbols.
> > 	* include/bits/basic_string.h [_GLIBCXX_USE_CXX11_ABI]
> > 	(basic_string::_M_copy_assign): New overloaded functions to perform
> > 	copy assignment.
> > 	(basic_string::operator=(const basic_string&)): Dispatch to
> > 	_M_copy_assign.
> > 	* include/bits/basic_string.tcc [_GLIBCXX_USE_CXX11_ABI]
> > 	(basic_string::_M_copy_assign(const basic_string&, true_type)):
> > 	Define, performing rollback on exception.
> > 	* testsuite/21_strings/basic_string/allocator/char/copy_assign.cc:
> > 	Test exception-safety guarantee.
> > 	* testsuite/21_strings/basic_string/allocator/wchar_t/copy_assign.cc:
> > 	Likewise.
> > 	* testsuite/util/testsuite_allocator.h (uneq_allocator::swap): Make
> > 	std::swap visible.
> >
> > The backports for the branches will be a bit different, as we can't
> > add new exports to closed symbol versions, so I'll keep everything in
> > operator= instead of tag dispatching. The POCCA code path will still
> > be dependent on a constant expression, so should be optimized away for
> > most allocators.
>
> Whlie working on the backport of this I realised the RAII
> commit-and-rollback approach is a lot more complicated than simply
> doing the new allocation before making any changes to *this.

Here's the backport for the branches, which shows that the new
approach is much closer to the original code and much simpler.

Tested x86_64-linux, committed to gcc-6-branch and gcc-5-branch.

Attachment: gcc6.txt
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