[libcxx-commits] [libcxx] [libc++] Tiny optimizations for is_permutation (PR #129565)

[Imad Aldij via libcxx-commits]

https://github.com/imdj updated https://github.com/llvm/llvm-project/pull/129565

>From cc9c0b2b876056ccbf5747f60af07b9cb4d4dec9 Mon Sep 17 00:00:00 2001
From: Imad Aldij <69906094+imdj at users.noreply.github.com>
Date: Fri, 7 Mar 2025 06:41:17 +0200
Subject: [PATCH 1/2] [libc++] Tiny optimizations for is_permutation

Replace hand-written loops with vectorized std::mismatch, std::find_if, and std::count_if
---
 libcxx/include/__algorithm/is_permutation.h | 41 +++++++++++----------
 1 file changed, 22 insertions(+), 19 deletions(-)

diff --git a/libcxx/include/__algorithm/is_permutation.h b/libcxx/include/__algorithm/is_permutation.h
index 1afb11596bc6b..b1d597f8e1e40 100644
--- a/libcxx/include/__algorithm/is_permutation.h
+++ b/libcxx/include/__algorithm/is_permutation.h
@@ -11,9 +11,13 @@
 #define _LIBCPP___ALGORITHM_IS_PERMUTATION_H
 
 #include <__algorithm/comp.h>
+#include <__algorithm/count_if.h>
+#include <__algorithm/find_if.h>
 #include <__algorithm/iterator_operations.h>
+#include <__algorithm/mismatch.h>
 #include <__config>
 #include <__functional/identity.h>
+#include <__functional/reference_wrapper.h>
 #include <__iterator/concepts.h>
 #include <__iterator/distance.h>
 #include <__iterator/iterator_traits.h>
@@ -82,28 +86,28 @@ _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 bool __is_permutation_impl(
 
   for (auto __i = __first1; __i != __last1; ++__i) {
     //  Have we already counted the number of *__i in [f1, l1)?
-    auto __match = __first1;
-    for (; __match != __i; ++__match) {
-      if (std::__invoke(__pred, std::__invoke(__proj1, *__match), std::__invoke(__proj1, *__i)))
-        break;
-    }
+    auto __match = std::find_if(__first1, __i, [&](typename iterator_traits<_Iter1>::reference __x) -> bool {
+      return std::__invoke(__pred, std::__invoke(__proj1, __x), std::__invoke(__proj1, *__i));
+    });
 
     if (__match == __i) {
       // Count number of *__i in [f2, l2)
-      _D1 __c2 = 0;
-      for (auto __j = __first2; __j != __last2; ++__j) {
-        if (std::__invoke(__pred, std::__invoke(__proj1, *__i), std::__invoke(__proj2, *__j)))
-          ++__c2;
-      }
+      __identity __ident1;
+      auto __predicate1 = [&](typename iterator_traits<_Iter2>::reference __x) -> bool {
+        return std::__invoke(__pred, std::__invoke(__proj1, *__i), std::__invoke(__proj2, __x));
+      };
+      _D1 __c2 = std::__count_if<_AlgPolicy>(__first2, __last2, __predicate1, __ident1);
       if (__c2 == 0)
         return false;
 
       // Count number of *__i in [__i, l1) (we can start with 1)
-      _D1 __c1 = 1;
-      for (auto __j = _IterOps<_AlgPolicy>::next(__i); __j != __last1; ++__j) {
-        if (std::__invoke(__pred, std::__invoke(__proj1, *__i), std::__invoke(__proj1, *__j)))
-          ++__c1;
-      }
+      __identity __ident2;
+      auto __predicate2 = [&](typename iterator_traits<_Iter1>::reference __x) -> bool {
+        return std::__invoke(__pred, std::__invoke(__proj1, *__i), std::__invoke(__proj1, __x));
+      };
+      auto __start = _IterOps<_AlgPolicy>::next(__i);
+      _D1 __c1     = std::__count_if<_AlgPolicy>(__start, __last1, __predicate2, __ident2);
+      __c1 += 1;
       if (__c1 != __c2)
         return false;
     }
@@ -117,10 +121,9 @@ template <class _AlgPolicy, class _ForwardIterator1, class _Sentinel1, class _Fo
 [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 bool __is_permutation(
     _ForwardIterator1 __first1, _Sentinel1 __last1, _ForwardIterator2 __first2, _BinaryPredicate&& __pred) {
   // Shorten sequences as much as possible by lopping of any equal prefix.
-  for (; __first1 != __last1; ++__first1, (void)++__first2) {
-    if (!__pred(*__first1, *__first2))
-      break;
-  }
+  auto __result = std::mismatch(__first1, __last1, __first2, std::ref(__pred));
+  __first1      = __result.first;
+  __first2      = __result.second;
 
   if (__first1 == __last1)
     return true;

>From c82043d59011ff52effbac8a4dbc7c0b53d141ea Mon Sep 17 00:00:00 2001
From: Imad Aldij <69906094+imdj at users.noreply.github.com>
Date: Sun, 9 Mar 2025 12:34:21 +0200
Subject: [PATCH 2/2] Adjust for readability and performance

---
 libcxx/include/__algorithm/is_permutation.h | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/libcxx/include/__algorithm/is_permutation.h b/libcxx/include/__algorithm/is_permutation.h
index b1d597f8e1e40..19006b76c4728 100644
--- a/libcxx/include/__algorithm/is_permutation.h
+++ b/libcxx/include/__algorithm/is_permutation.h
@@ -83,31 +83,31 @@ _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 bool __is_permutation_impl(
     _Proj1&& __proj1,
     _Proj2&& __proj2) {
   using _D1 = __iter_diff_t<_Iter1>;
+  using _Ref1 = typename iterator_traits<_Iter1>::reference;
+  using _Ref2 = typename iterator_traits<_Iter2>::reference;
+  __identity __ident;
 
-  for (auto __i = __first1; __i != __last1; ++__i) {
+  for (auto __i = __first1; __i != __last1; ++__i) {    
     //  Have we already counted the number of *__i in [f1, l1)?
-    auto __match = std::find_if(__first1, __i, [&](typename iterator_traits<_Iter1>::reference __x) -> bool {
+    auto __match = std::find_if(__first1, __i, [&](_Ref1 __x) -> bool {
       return std::__invoke(__pred, std::__invoke(__proj1, __x), std::__invoke(__proj1, *__i));
     });
 
     if (__match == __i) {
       // Count number of *__i in [f2, l2)
-      __identity __ident1;
-      auto __predicate1 = [&](typename iterator_traits<_Iter2>::reference __x) -> bool {
+      auto __predicate2 = [&](_Ref2 __x) -> bool {
         return std::__invoke(__pred, std::__invoke(__proj1, *__i), std::__invoke(__proj2, __x));
       };
-      _D1 __c2 = std::__count_if<_AlgPolicy>(__first2, __last2, __predicate1, __ident1);
+      _D1 __c2 = std::__count_if<_AlgPolicy>(__first2, __last2, __predicate2, __ident);
       if (__c2 == 0)
         return false;
 
       // Count number of *__i in [__i, l1) (we can start with 1)
-      __identity __ident2;
-      auto __predicate2 = [&](typename iterator_traits<_Iter1>::reference __x) -> bool {
+      auto __predicate1 = [&](_Ref1 __x) -> bool {
         return std::__invoke(__pred, std::__invoke(__proj1, *__i), std::__invoke(__proj1, __x));
       };
       auto __start = _IterOps<_AlgPolicy>::next(__i);
-      _D1 __c1     = std::__count_if<_AlgPolicy>(__start, __last1, __predicate2, __ident2);
-      __c1 += 1;
+      _D1 __c1     = 1 + std::__count_if<_AlgPolicy>(__start, __last1, __predicate1, __ident);
       if (__c1 != __c2)
         return false;
     }