On 17/07/25 15:31 +0200, Tomasz Kamiński wrote:
From: Jonathan Wakely <jwak...@redhat.com>
Implement std::inplace_vector as specified in P0843R14, without follow
up papers, in particular P3074R7 (trivial unions). In consequence
inplace_vector<T, N> can be used inside constant evaluations only
if T is trivial of N is equal to zero.
We provide a separate specialization for inplace_vector<T, 0> to meet
the requirements of N5008 [inplace.vector.overview] p5. In particular
objects of such types needs to be empty.
To allow contexpr variable of inplace_vector v, where v.size() < v.capacity(),
constexpr
we need to guaranteed that all elements of the storage array are initialized,
even ones in range [v.data() + v.size(), v.data() + v.capacity()). This is
perfomed by _M_init function, that is alled by each constructored. By storing
performed ... called ... constructor
the array in animous union, we can perform this intialization in constant
anonymous
evaluation, avoiding the impact on runtime path.
The size() function conveys the information that _M_size <= _Nm to compiler,
by calling __builtin_unreachable(). In particular this allows us to eliminate
FP warnings by using _Nm - size() instead of _Nm - _M_size, when computing
Please spell 'false positive' out in full
available elements.
However, we still have one -Waggressive-loop-optimizations (to best of our
knowledge false-positive warning produced in cons/from_range.cc and
Missing ')' after knowledge?
cons/throws.cc. Currently it is pruned using dg-prune-output and tracked by
PR121143.
The included test cover almost all code paths at runtime, however some
compile time evaluation test are not yet implemented:
* operations on range, they depenend on making testsuite_iterators constexpr
depend
* negative test for invoking operations with preconditions at compile time,
especially for zero size specialization.
