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libstdc++: Reuse predicates in std::search and std::is_permutation

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Hoist construction of the call wrappers out of the loop when we're
repeatedly creating a call wrapper with the same bound arguments.

We need to be careful about iterators that return proxy references,
because bind1st(pred, *first) could bind a reference to a prvalue proxy
reference returned by *first. That would then be an invalid reference by
the time we invoked the call wrapper.

If we dereference the iterator first and store the result of that on the
stack, then we don't have a prvalue proxy reference, and can bind it (or
the value it refers to) into the call wrapper:

  auto&& val = *first; // lifetime extension
  auto wrapper = bind1st(pred, val);
  for (;;)
    /* use wrapper */;

This ensures that the reference returned from *first outlives the call
wrapper, whether it's a proxy reference or not.

For C++98 compatibility in __search we can use __decltype(expr) instead
of auto&&.

libstdc++-v3/ChangeLog:

	* include/bits/stl_algobase.h (__search, __is_permutation):
	Reuse predicate instead of creating a new one each time.
	* include/bits/stl_algo.h (__is_permutation): Likewise.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
This commit is contained in:
Jonathan Wakely
2025-09-10 10:10:07 +01:00
committed by Jonathan Wakely
parent b83c2e52a2
commit 1cf6cda055
2 changed files with 18 additions and 25 deletions
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14
libstdc++-v3/include/bits/stl_algo.h
View File

@@ -3531,18 +3531,14 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
for (_ForwardIterator1 __scan = __first1; __scan != __last1; ++__scan)
{
if (__scan != std::__find_if(__first1, __scan,
__gnu_cxx::__ops::bind1st(__pred,
*__scan)))
auto&& __scan_val = *__scan;
auto __scaneq = __gnu_cxx::__ops::bind1st(__pred, __scan_val);
if (__scan != std::__find_if(__first1, __scan, __scaneq))
continue; // We've seen this one before.
auto __matches = std::__count_if(__first2, __last2,
__gnu_cxx::__ops::bind1st(__pred,
*__scan));
auto __matches = std::__count_if(__first2, __last2, __scaneq);
if (0 == __matches
|| std::__count_if(__scan, __last1,
__gnu_cxx::__ops::bind1st(__pred, *__scan))
!= __matches)
|| std::__count_if(__scan, __last1, __scaneq) != __matches)
return false;
}
return true;

29
libstdc++-v3/include/bits/stl_algobase.h
View File

@@ -2149,21 +2149,21 @@ _GLIBCXX_END_NAMESPACE_ALGO
if (__first1 == __last1 || __first2 == __last2)
return __first1;
__decltype(*__first2) __first2_val(*__first2);
__decltype(__gnu_cxx::__ops::bind2nd(__predicate, __first2_val))
__match_first = __gnu_cxx::__ops::bind2nd(__predicate, __first2_val);
// Test for a pattern of length 1.
_ForwardIterator2 __p1(__first2);
if (++__p1 == __last2)
return std::__find_if(__first1, __last1,
__gnu_cxx::__ops::bind2nd(__predicate,
*__first2));
return std::__find_if(__first1, __last1, __match_first);
// General case.
_ForwardIterator1 __current = __first1;
for (;;)
{
__first1 =
std::__find_if(__first1, __last1,
__gnu_cxx::__ops::bind2nd(__predicate, *__first2));
__first1 = std::__find_if(__first1, __last1, __match_first);
if (__first1 == __last1)
return __last1;
@@ -2184,6 +2184,7 @@ _GLIBCXX_END_NAMESPACE_ALGO
}
return __first1;
}
#undef __match_first
#if __cplusplus >= 201103L
template<typename _ForwardIterator1, typename _ForwardIterator2,
@@ -2208,18 +2209,14 @@ _GLIBCXX_END_NAMESPACE_ALGO
std::advance(__last2, std::distance(__first1, __last1));
for (_ForwardIterator1 __scan = __first1; __scan != __last1; ++__scan)
{
if (__scan != std::__find_if(__first1, __scan,
__gnu_cxx::__ops::bind1st(__pred,
*__scan)))
auto&& __scan_val = *__scan;
auto __scaneq = __gnu_cxx::__ops::bind1st(__pred, __scan_val);
if (__scan != std::__find_if(__first1, __scan, __scaneq))
continue; // We've seen this one before.
auto __matches
= std::__count_if(__first2, __last2,
__gnu_cxx::__ops::bind1st(__pred, *__scan));
if (0 == __matches ||
std::__count_if(__scan, __last1,
__gnu_cxx::__ops::bind1st(__pred, *__scan))
!= __matches)
auto __matches = std::__count_if(__first2, __last2, __scaneq);
if (0 == __matches
|| std::__count_if(__scan, __last1, __scaneq) != __matches)
return false;
}
return true;