libstdc++: Extend memcmp optimization in std::lexicographical_compare
Jonathan Wakely
jwakely@redhat.com
Tue Jun 9 10:24:37 GMT 2020
On 08/06/20 19:20 +0100, Jonathan Wakely wrote:
>On 05/06/20 22:24 +0200, François Dumont via Libstdc++ wrote:
>>Hi
>>
>>Â Â Â Here is the last of my algo patches this time to extend the
>>memcmp optimization to std::deque iterators and _GLIBCXX_DEBUG mode.
>>
>>Â Â Â To do so I had to return int in implementation details of
>>lexicographical_compare to make sure we can treat a deque iterator
>>range by chunk in a performant way.
>>
>>Tested under Linux x86_64 normal and debug modes.
>>
>>Â Â Â Â Â Â Â Â Â Â Â * include/bits/stl_algobase.h
>>Â Â Â Â Â Â Â Â Â Â Â (__lexicographical_compare_impl): Return int.
>>Â Â Â Â Â Â Â Â Â Â Â (__lexicographical_compare::__lc): Likewise.
>>Â Â Â Â Â Â Â Â Â Â Â (__lexicographical_compare_aux1(_II1, _II1, _II2, _II2)): New.
>>(__lexicographical_compare_aux1(_Deque_iterator<>, _Deque_iterator<>,
>>Â Â Â Â Â Â Â Â Â Â Â _II2, _II2)): Declare.
>>Â Â Â Â Â Â Â Â Â Â Â (__lexicographical_compare_aux1(_II1, _II1,
>>Â Â Â Â Â Â Â Â Â Â Â _Deque_iterator<>, _Deque_iterator<>)): Declare.
>>(__lexicographical_compare_aux1(_Deque_iterator<>, _Deque_iterator<>,
>>Â Â Â Â Â Â Â Â Â Â Â _Deque_iterator<>, _Deque_iterator<>)): Declare.
>>Â Â Â Â Â Â Â Â Â Â Â (__lexicographical_compare_aux): Adapt, call later.
>
>Is this meant to say "latter"? That's still not correct grammar
>though. I think it would be better to name the function it calls
>explicitly: "Call __lexicographical_compare_aux1."
>
>>Â Â Â Â Â Â Â Â Â Â Â (__lexicographical_compare_aux(_Safe_iterator<>,
>>_Safe_iterator<>,
>>Â Â Â Â Â Â Â Â Â Â Â _II2, _II2)): Declare.
>>Â Â Â Â Â Â Â Â Â Â Â (__lexicographical_compare_aux(_II1, _II1,
>>Â Â Â Â Â Â Â Â Â Â Â _Safe_iterator<>, _Safe_iterator<>)): Declare.
>>Â Â Â Â Â Â Â Â Â Â Â (__lexicographical_compare_aux(_Safe_iterator<>,
>>_Safe_iterator<>,
>>Â Â Â Â Â Â Â Â Â Â Â _Safe_iterator<>, _Safe_iterator<>)): Declare.
>>Â Â Â Â Â Â Â Â Â Â Â (std::lexicographical_compare): Adapt, call later
>>without __niter_base
>>Â Â Â Â Â Â Â Â Â Â Â usage.
>>Â Â Â Â Â Â Â Â Â Â Â * include/bits/deque.tcc (__lex_cmp_dit): New.
>>Â Â Â Â Â Â Â Â Â Â Â (__lexicographical_compare_aux1): New.
>>Â Â Â Â Â Â Â Â Â Â Â * include/debug/safe_iterator.tcc
>>Â Â Â Â Â Â Â Â Â Â Â (__lexicographical_compare_aux(const _Safe_iterator<>&,
>>Â Â Â Â Â Â Â Â Â Â Â const _Safe_iterator<>&, _II2, _II2)): New.
>>Â Â Â Â Â Â Â Â Â Â Â (__lexicographical_compare_aux(
>>Â Â Â Â Â Â Â Â Â Â Â const _Safe_iterator<>&, const_Safe_iterator<>&,
>>Â Â Â Â Â Â Â Â Â Â Â const _Safe_iterator<>&, const _Safe_iterator<>&)): New.
>>Â Â Â Â Â Â Â Â Â Â Â * testsuite/25_algorithms/lexicographical_compare/1.cc
>>(test6, test7):
>>Â Â Â Â Â Â Â Â Â Â Â New.
>>Â Â Â Â Â Â Â Â Â Â Â *
>>testsuite/25_algorithms/lexicographical_compare/deque_iterators/1.cc:
>>Â Â Â Â Â Â Â Â Â Â Â New test.
>>
>>Ok to commit ?
>>
>>François
>>
>>
>
>>diff --git a/libstdc++-v3/include/bits/deque.tcc b/libstdc++-v3/include/bits/deque.tcc
>>index 1d32a1691c7..d7dbe64f3e1 100644
>>--- a/libstdc++-v3/include/bits/deque.tcc
>>+++ b/libstdc++-v3/include/bits/deque.tcc
>>@@ -1261,6 +1261,98 @@ _GLIBCXX_END_NAMESPACE_CONTAINER
>> return true;
>> }
>>
>>+ template<typename _Tp, typename _Ref, typename _Ptr, typename _II>
>
>_II is the wrong name here, it mean InputIterator. All callers of this
>function are constrained for random access iterators. This cannot be
>called with an input iterator. Please use _RAIter.
>
>>+ int
>>+ __lex_cmp_dit(
>>+ const _GLIBCXX_STD_C::_Deque_iterator<_Tp, _Ref, _Ptr>& __first1,
>>+ const _GLIBCXX_STD_C::_Deque_iterator<_Tp, _Ref, _Ptr>& __last1,
>>+ _II __first2, _II __last2)
>>+ {
>>+ typedef _GLIBCXX_STD_C::_Deque_iterator<_Tp, _Ref, _Ptr> _Iter;
>>+ typedef typename _Iter::difference_type difference_type;
>>+
>>+ if (__first1._M_node != __last1._M_node)
>>+ {
>>+ difference_type __len = __last2 - __first2;
>>+ difference_type __flen
>
>What does "flen" mean? Why not use len2 for last2 - first2, as we do
>elsewhere? And then use len for min(len1, len2)?
>
>
>>+ = std::min(__len, __first1._M_last - __first1._M_cur);
>>+ if (int __ret = std::__lexicographical_compare_aux1(
>
>This call (and the three later in this function) will do overload
>resolution again on the full set of __lexicographical_compare_aux1
>overloads, which includes the ones for deque iterators. But we know
>that __first1._M_cur and __first1._M_last are not deque iterators.
>
>__first2 could be a deque iterator, but I'm not sure if we really want
>one function that has to handle that case anyway.
>
>>+ __first1._M_cur, __first1._M_last, __first2, __first2 + __flen))
>>+ return __ret;
>>+
>>+ __first2 += __flen;
>>+ __len -= __flen;
>>+ __flen = std::min<size_t>(__len, _Iter::_S_buffer_size());
>>+ for (typename _Iter::_Map_pointer __node = __first1._M_node + 1;
>>+ __node != __last1._M_node;
>>+ __first2 += __flen, __len -= __flen,
>>+ __flen = std::min<size_t>(__len, _Iter::_S_buffer_size()),
>>+ ++__node)
>>+ if (int __ret = std::__lexicographical_compare_aux1(
>>+ *__node, *__node + _Iter::_S_buffer_size(),
>>+ __first2, __first2 + __flen))
>>+ return __ret;
>>+
>>+ return std::__lexicographical_compare_aux1(
>>+ __last1._M_first, __last1._M_cur, __first2, __last2);
>>+ }
>>+
>>+ return std::__lexicographical_compare_aux1(
>>+ __first1._M_cur, __last1._M_cur, __first2, __last2);
>>+ }
>>+
>>+ template<typename _Tp1, typename _Ref1, typename _Ptr1,
>>+ typename _II2>
>
>This is not an input iterator either.
>
>>+ typename __gnu_cxx::__enable_if<
>>+ __is_random_access_iter<_II2>::__value, int>::__type
>
>This should be inline.
>
>Also, I'm curious why these overloads use __enable_if rather than the
>conventional approach of tag dispatching using iterator_category.
>(The same question applies to th __equal_aux1 and __copy_move_a1
>overloads for deque iterators as well). It doesn't really matter
>though.
>
>>+ __lexicographical_compare_aux1(
>>+ _GLIBCXX_STD_C::_Deque_iterator<_Tp1, _Ref1, _Ptr1> __first1,
>>+ _GLIBCXX_STD_C::_Deque_iterator<_Tp1, _Ref1, _Ptr1> __last1,
>>+ _II2 __first2, _II2 __last2)
>>+ { return std::__lex_cmp_dit(__first1, __last1, __first2, __last2); }
>>+
>>+ template<typename _Tp1, typename _Ref1, typename _Ptr1,
>>+ typename _Tp2, typename _Ref2, typename _Ptr2>
>>+ int
>
>This should be inline.
>
>>+ __lexicographical_compare_aux1(
>>+ _GLIBCXX_STD_C::_Deque_iterator<_Tp1, _Ref1, _Ptr1> __first1,
>>+ _GLIBCXX_STD_C::_Deque_iterator<_Tp1, _Ref1, _Ptr1> __last1,
>>+ _GLIBCXX_STD_C::_Deque_iterator<_Tp2, _Ref2, _Ptr2> __first2,
>>+ _GLIBCXX_STD_C::_Deque_iterator<_Tp2, _Ref2, _Ptr2> __last2)
>>+ { return std::__lex_cmp_dit(__first1, __last1, __first2, __last2); }
>
>I'm not convinced it's useful for this function and the one above it
>to share the same logic.
>
>>+ template<typename _II1,
>>+ typename _Tp2, typename _Ref2, typename _Ptr2>
>>+ typename __gnu_cxx::__enable_if<
>>+ __is_random_access_iter<_II1>::__value, int>::__type
>>+ __lexicographical_compare_aux1(
>>+ _II1 __first1, _II1 __last1,
>>+ _GLIBCXX_STD_C::_Deque_iterator<_Tp2, _Ref2, _Ptr2> __first2,
>>+ _GLIBCXX_STD_C::_Deque_iterator<_Tp2, _Ref2, _Ptr2> __last2)
>>+ {
>>+ typedef _GLIBCXX_STD_C::_Deque_iterator<_Tp2, _Ref2, _Ptr2> _Iter;
>>+ typedef typename _Iter::difference_type difference_type;
>>+
>>+ difference_type __len = __last1 - __first1;
>>+ while (__len > 0)
>>+ {
>>+ const difference_type __flen = __first2._M_node == __last2._M_node
>>+ ? __last2._M_cur - __first2._M_cur
>>+ : __first2._M_last - __first2._M_cur;
>>+ const difference_type __clen = std::min(__len, __flen);
>
>"flen"? "clen"?
>
>>+ if (int __ret = std::__lexicographical_compare_aux1(
>>+ __first1, __first1 + __clen,
>>+ __first2._M_cur, __first2._M_cur + __flen))
>>+ return __ret;
>>+
>>+ __first1 += __clen;
>>+ __len -= __clen;
>>+ __first2 += __clen;
>>+ }
>>+
>>+ return __first1 == __last1 ? (__first2 != __last2 ? -1 : 0) : 1;
>
>Isn't this function doing basically the same as __lex_cmp_dit? Why
>does it not use the same logic?
>
>Can this whole function just negate the result of __lex_cmp_dit called
>with the arguments switched?
>
> return -std::__lex_cmp_dit(__first2, __last2, __first1, __last1);
>
>That would also fix the bug that makes the following loop forever:
>
> std::deque<int> d;
> int i = 0;
> std::lexicographical_compare(&i, &i + 1, d.begin(), d.end());
Same question for __equal_aux1, why is it implemented twice? Why
doesn't __equal_aux1(_II f1, _II l1, _Deque_iterator<> f2) just call
__equal_aux1(f2, f2 + (l1 - f1), f1) ?
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