https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127510
--- Comment #6 from Raghesh Aloor <raghesh.aloor at amd dot com> ---
Thanks Richard. From your comments, here is our understanding; please correct
us
if we are wrong.
To your question in comment on libstdc++, the multi-pointer == comes
from the user's iterator (Zip<3> in the testcase), but the "loop until
last" comes from the random-access branch of std::for_each_n in
libstdc++-v3/include/bits/stl_algo.h:
template<typename _InputIterator, typename _Size, typename _Function>
_GLIBCXX20_CONSTEXPR
_InputIterator
for_each_n(_InputIterator __first, _Size __n, _Function __f)
{
auto __n2 = std::__size_to_integer(__n);
using _Cat = decltype(std::__iter_concept_or_category<_InputIterator>());
if constexpr (is_base_of_v<random_access_iterator_tag, _Cat>)
{
if (__n2 <= 0)
return __first;
typename iterator_traits<_InputIterator>::difference_type __d = __n2;
auto __last = __first + __d;
std::for_each(__first, __last, std::move(__f));
return __last;
}
else
{
while (__n2-->0)
{
__f(*__first);
++__first;
}
return __first;
}
Are you suggesting a libstdc++ fix that makes the random-access branch a
decremented counted loop, as in the non-random-access case? Something like:
if constexpr (is_base_of_v<random_access_iterator_tag, _Cat>)
{
if (__n2 <= 0)
return __first;
typename iterator_traits<_InputIterator>::difference_type __d = __n2;
for (; __d > 0; --__d, ++__first)
__f(*__first);
return __first;
}
We are not sure the libstdc++ maintainers would accept this, though, as
forwarding to std::for_each may be intentional.
For the compiler side tasks, we understand the following.
As a first step, we need to get the same output as with -fno-vect-cost-model,
by teaching niter analysis to handle the &-combined EQ/NE exit condition.
As a follow-up optimization, we would then do what you suggested in your final
comment: rewrite the IVs with the same step in terms of a single IV bound by
_14
(with versioning where termination cannot be proven), so the redundant compares
and extra IVs go away.
