12 files changed, 454 insertions, 285 deletions

diff --git a/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backend.h b/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backend.h
index e54f331b9430..6980ded189ea 100644
--- a/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backend.h
+++ b/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backend.h
@@ -15,10 +15,11 @@
 
   // _Functor takes a subrange for [__first, __last) that should be executed in serial
   template <class _RandomAccessIterator, class _Functor>
-  void __parallel_for(_RandomAccessIterator __first, _RandomAccessIterator __last, _Functor __func);
+  optional<__empty> __parallel_for(_RandomAccessIterator __first, _RandomAccessIterator __last, _Functor __func);
 
   template <class _Iterator, class _UnaryOp, class _Tp, class _BinaryOp, class _Reduction>
-  _Tp __parallel_transform_reduce(_Iterator __first, _Iterator __last, _UnaryOp, _Tp __init, _BinaryOp, _Reduction);
+  optional<_Tp>
+  __parallel_transform_reduce(_Iterator __first, _Iterator __last, _UnaryOp, _Tp __init, _BinaryOp, _Reduction);
 
   // Cancel the execution of other jobs - they aren't needed anymore
   void __cancel_execution();
@@ -28,7 +29,7 @@
             class _RandomAccessIterator3,
             class _Compare,
             class _LeafMerge>
-  void __parallel_merge(
+  optional<void> __parallel_merge(
       _RandomAccessIterator1 __first1,
       _RandomAccessIterator1 __last1,
       _RandomAccessIterator2 __first2,
@@ -44,6 +45,14 @@
                               _LeafSort __leaf_sort);
 
   TODO: Document the parallel backend
+
+Exception handling
+==================
+
+CPU backends are expected to report errors (i.e. failure to allocate) by returning a disengaged `optional` from their
+implementation. Exceptions shouldn't be used to report an internal failure-to-allocate, since all exceptions are turned
+into a program termination at the front-end level. When a backend returns a disengaged `optional` to the frontend, the
+frontend will turn that into a call to `std::__throw_bad_alloc();` to report the internal failure to the user.
 */
 
 #include <__algorithm/pstl_backends/cpu_backends/any_of.h>
diff --git a/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/any_of.h b/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/any_of.h
index 8fe26797bf15..13dff80086e7 100644
--- a/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/any_of.h
+++ b/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/any_of.h
@@ -16,26 +16,32 @@
 #include <__atomic/memory_order.h>
 #include <__config>
 #include <__functional/operations.h>
-#include <__iterator/iterator_traits.h>
+#include <__iterator/concepts.h>
 #include <__type_traits/is_execution_policy.h>
+#include <__utility/move.h>
 #include <__utility/pair.h>
-#include <__utility/terminate_on_exception.h>
 #include <cstdint>
+#include <optional>
 
 #if !defined(_LIBCPP_HAS_NO_INCOMPLETE_PSTL) && _LIBCPP_STD_VER >= 17
 
+_LIBCPP_PUSH_MACROS
+#  include <__undef_macros>
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _Index, class _Brick>
-_LIBCPP_HIDE_FROM_ABI bool __parallel_or(_Index __first, _Index __last, _Brick __f) {
+_LIBCPP_HIDE_FROM_ABI optional<bool> __parallel_or(_Index __first, _Index __last, _Brick __f) {
   std::atomic<bool> __found(false);
-  __par_backend::__parallel_for(__first, __last, [__f, &__found](_Index __i, _Index __j) {
+  auto __ret = __par_backend::__parallel_for(__first, __last, [__f, &__found](_Index __i, _Index __j) {
     if (!__found.load(std::memory_order_relaxed) && __f(__i, __j)) {
       __found.store(true, std::memory_order_relaxed);
       __par_backend::__cancel_execution();
     }
   });
-  return __found;
+  if (!__ret)
+    return nullopt;
+  return static_cast<bool>(__found);
 }
 
 // TODO: check whether __simd_first() can be used here
@@ -64,19 +70,19 @@ _LIBCPP_HIDE_FROM_ABI bool __simd_or(_Index __first, _DifferenceType __n, _Pred
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _Predicate>
-_LIBCPP_HIDE_FROM_ABI bool
+_LIBCPP_HIDE_FROM_ABI optional<bool>
 __pstl_any_of(__cpu_backend_tag, _ForwardIterator __first, _ForwardIterator __last, _Predicate __pred) {
   if constexpr (__is_parallel_execution_policy_v<_ExecutionPolicy> &&
-                __has_random_access_iterator_category<_ForwardIterator>::value) {
-    return std::__terminate_on_exception([&] {
-      return std::__parallel_or(
-          __first, __last, [&__pred](_ForwardIterator __brick_first, _ForwardIterator __brick_last) {
-            return std::__pstl_any_of<__remove_parallel_policy_t<_ExecutionPolicy>>(
-                __cpu_backend_tag{}, __brick_first, __brick_last, __pred);
-          });
-    });
+                __has_random_access_iterator_category_or_concept<_ForwardIterator>::value) {
+    return std::__parallel_or(
+        __first, __last, [&__pred](_ForwardIterator __brick_first, _ForwardIterator __brick_last) {
+          auto __res = std::__pstl_any_of<__remove_parallel_policy_t<_ExecutionPolicy>>(
+              __cpu_backend_tag{}, __brick_first, __brick_last, __pred);
+          _LIBCPP_ASSERT_INTERNAL(__res, "unseq/seq should never try to allocate!");
+          return *std::move(__res);
+        });
   } else if constexpr (__is_unsequenced_execution_policy_v<_ExecutionPolicy> &&
-                       __has_random_access_iterator_category<_ForwardIterator>::value) {
+                       __has_random_access_iterator_category_or_concept<_ForwardIterator>::value) {
     return std::__simd_or(__first, __last - __first, __pred);
   } else {
     return std::any_of(__first, __last, __pred);
@@ -85,6 +91,8 @@ __pstl_any_of(__cpu_backend_tag, _ForwardIterator __first, _ForwardIterator __la
 
 _LIBCPP_END_NAMESPACE_STD
 
+_LIBCPP_POP_MACROS
+
 #endif // !defined(_LIBCPP_HAS_NO_INCOMPLETE_PSTL) && _LIBCPP_STD_VER >= 17
 
 #endif // _LIBCPP___ALGORITHM_PSTL_BACKENDS_CPU_BACKEND_ANY_OF_H
diff --git a/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/fill.h b/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/fill.h
index 5e5e0a23bf89..64babe9fd2bd 100644
--- a/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/fill.h
+++ b/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/fill.h
@@ -12,9 +12,10 @@
 #include <__algorithm/fill.h>
 #include <__algorithm/pstl_backends/cpu_backends/backend.h>
 #include <__config>
-#include <__iterator/iterator_traits.h>
+#include <__iterator/concepts.h>
 #include <__type_traits/is_execution_policy.h>
-#include <__utility/terminate_on_exception.h>
+#include <__utility/empty.h>
+#include <optional>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
@@ -34,22 +35,23 @@ _LIBCPP_HIDE_FROM_ABI _Index __simd_fill_n(_Index __first, _DifferenceType __n,
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _Tp>
-_LIBCPP_HIDE_FROM_ABI void
+_LIBCPP_HIDE_FROM_ABI optional<__empty>
 __pstl_fill(__cpu_backend_tag, _ForwardIterator __first, _ForwardIterator __last, const _Tp& __value) {
   if constexpr (__is_parallel_execution_policy_v<_ExecutionPolicy> &&
-                __has_random_access_iterator_category<_ForwardIterator>::value) {
-    std::__terminate_on_exception([&] {
-      __par_backend::__parallel_for(
-          __first, __last, [&__value](_ForwardIterator __brick_first, _ForwardIterator __brick_last) {
-            std::__pstl_fill<__remove_parallel_policy_t<_ExecutionPolicy>>(
-                __cpu_backend_tag{}, __brick_first, __brick_last, __value);
-          });
-    });
+                __has_random_access_iterator_category_or_concept<_ForwardIterator>::value) {
+    return __par_backend::__parallel_for(
+        __first, __last, [&__value](_ForwardIterator __brick_first, _ForwardIterator __brick_last) {
+          [[maybe_unused]] auto __res = std::__pstl_fill<__remove_parallel_policy_t<_ExecutionPolicy>>(
+              __cpu_backend_tag{}, __brick_first, __brick_last, __value);
+          _LIBCPP_ASSERT_INTERNAL(__res, "unseq/seq should never try to allocate!");
+        });
   } else if constexpr (__is_unsequenced_execution_policy_v<_ExecutionPolicy> &&
-                       __has_random_access_iterator_category<_ForwardIterator>::value) {
+                       __has_random_access_iterator_category_or_concept<_ForwardIterator>::value) {
     std::__simd_fill_n(__first, __last - __first, __value);
+    return __empty{};
   } else {
     std::fill(__first, __last, __value);
+    return __empty{};
   }
 }
 
diff --git a/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/find_if.h b/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/find_if.h
index 3fa49549e64e..170470e4fb7e 100644
--- a/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/find_if.h
+++ b/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/find_if.h
@@ -14,11 +14,13 @@
 #include <__atomic/atomic.h>
 #include <__config>
 #include <__functional/operations.h>
+#include <__iterator/concepts.h>
 #include <__iterator/iterator_traits.h>
 #include <__type_traits/is_execution_policy.h>
+#include <__utility/move.h>
 #include <__utility/pair.h>
-#include <__utility/terminate_on_exception.h>
 #include <cstddef>
+#include <optional>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
@@ -26,31 +28,37 @@
 
 #if !defined(_LIBCPP_HAS_NO_INCOMPLETE_PSTL) && _LIBCPP_STD_VER >= 17
 
+_LIBCPP_PUSH_MACROS
+#  include <__undef_macros>
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _Index, class _Brick, class _Compare>
-_LIBCPP_HIDE_FROM_ABI _Index
+_LIBCPP_HIDE_FROM_ABI optional<_Index>
 __parallel_find(_Index __first, _Index __last, _Brick __f, _Compare __comp, bool __b_first) {
   typedef typename std::iterator_traits<_Index>::difference_type _DifferenceType;
   const _DifferenceType __n      = __last - __first;
   _DifferenceType __initial_dist = __b_first ? __n : -1;
   std::atomic<_DifferenceType> __extremum(__initial_dist);
   // TODO: find out what is better here: parallel_for or parallel_reduce
-  __par_backend::__parallel_for(__first, __last, [__comp, __f, __first, &__extremum](_Index __i, _Index __j) {
-    // See "Reducing Contention Through Priority Updates", PPoPP '13, for discussion of
-    // why using a shared variable scales fairly well in this situation.
-    if (__comp(__i - __first, __extremum)) {
-      _Index __res = __f(__i, __j);
-      // If not '__last' returned then we found what we want so put this to extremum
-      if (__res != __j) {
-        const _DifferenceType __k = __res - __first;
-        for (_DifferenceType __old = __extremum; __comp(__k, __old); __old = __extremum) {
-          __extremum.compare_exchange_weak(__old, __k);
+  auto __res =
+      __par_backend::__parallel_for(__first, __last, [__comp, __f, __first, &__extremum](_Index __i, _Index __j) {
+        // See "Reducing Contention Through Priority Updates", PPoPP '13, for discussion of
+        // why using a shared variable scales fairly well in this situation.
+        if (__comp(__i - __first, __extremum)) {
+          _Index __result = __f(__i, __j);
+          // If not '__last' returned then we found what we want so put this to extremum
+          if (__result != __j) {
+            const _DifferenceType __k = __result - __first;
+            for (_DifferenceType __old = __extremum; __comp(__k, __old); __old = __extremum) {
+              __extremum.compare_exchange_weak(__old, __k);
+            }
+          }
         }
-      }
-    }
-  });
-  return __extremum != __initial_dist ? __first + __extremum : __last;
+      });
+  if (!__res)
+    return nullopt;
+  return __extremum.load() != __initial_dist ? __first + __extremum.load() : __last;
 }
 
 template <class _Index, class _DifferenceType, class _Compare>
@@ -90,23 +98,23 @@ __simd_first(_Index __first, _DifferenceType __begin, _DifferenceType __end, _Co
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _Predicate>
-_LIBCPP_HIDE_FROM_ABI _ForwardIterator
+_LIBCPP_HIDE_FROM_ABI optional<_ForwardIterator>
 __pstl_find_if(__cpu_backend_tag, _ForwardIterator __first, _ForwardIterator __last, _Predicate __pred) {
   if constexpr (__is_parallel_execution_policy_v<_ExecutionPolicy> &&
-                __has_random_access_iterator_category<_ForwardIterator>::value) {
-    return std::__terminate_on_exception([&] {
-      return std::__parallel_find(
-          __first,
-          __last,
-          [&__pred](_ForwardIterator __brick_first, _ForwardIterator __brick_last) {
-            return std::__pstl_find_if<__remove_parallel_policy_t<_ExecutionPolicy>>(
-                __cpu_backend_tag{}, __brick_first, __brick_last, __pred);
-          },
-          less<>{},
-          true);
-    });
+                __has_random_access_iterator_category_or_concept<_ForwardIterator>::value) {
+    return std::__parallel_find(
+        __first,
+        __last,
+        [&__pred](_ForwardIterator __brick_first, _ForwardIterator __brick_last) {
+          auto __res = std::__pstl_find_if<__remove_parallel_policy_t<_ExecutionPolicy>>(
+              __cpu_backend_tag{}, __brick_first, __brick_last, __pred);
+          _LIBCPP_ASSERT_INTERNAL(__res, "unseq/seq should never try to allocate!");
+          return *std::move(__res);
+        },
+        less<>{},
+        true);
   } else if constexpr (__is_unsequenced_execution_policy_v<_ExecutionPolicy> &&
-                       __has_random_access_iterator_category<_ForwardIterator>::value) {
+                       __has_random_access_iterator_category_or_concept<_ForwardIterator>::value) {
     using __diff_t = __iter_diff_t<_ForwardIterator>;
     return std::__simd_first(__first, __diff_t(0), __last - __first, [&__pred](_ForwardIterator __iter, __diff_t __i) {
       return __pred(__iter[__i]);
@@ -118,6 +126,8 @@ __pstl_find_if(__cpu_backend_tag, _ForwardIterator __first, _ForwardIterator __l
 
 _LIBCPP_END_NAMESPACE_STD
 
+_LIBCPP_POP_MACROS
+
 #endif // !defined(_LIBCPP_HAS_NO_INCOMPLETE_PSTL) && _LIBCPP_STD_VER >= 17
 
 #endif // _LIBCPP___ALGORITHM_PSTL_BACKENDS_CPU_BACKENDS_FIND_IF_H
diff --git a/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/for_each.h b/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/for_each.h
index 36d0ac238e24..81fd4526b8db 100644
--- a/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/for_each.h
+++ b/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/for_each.h
@@ -12,9 +12,10 @@
 #include <__algorithm/for_each.h>
 #include <__algorithm/pstl_backends/cpu_backends/backend.h>
 #include <__config>
-#include <__iterator/iterator_traits.h>
+#include <__iterator/concepts.h>
 #include <__type_traits/is_execution_policy.h>
-#include <__utility/terminate_on_exception.h>
+#include <__utility/empty.h>
+#include <optional>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
@@ -25,7 +26,7 @@
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _Iterator, class _DifferenceType, class _Function>
-_LIBCPP_HIDE_FROM_ABI _Iterator __simd_walk_1(_Iterator __first, _DifferenceType __n, _Function __f) noexcept {
+_LIBCPP_HIDE_FROM_ABI _Iterator __simd_walk(_Iterator __first, _DifferenceType __n, _Function __f) noexcept {
   _PSTL_PRAGMA_SIMD
   for (_DifferenceType __i = 0; __i < __n; ++__i)
     __f(__first[__i]);
@@ -34,22 +35,23 @@ _LIBCPP_HIDE_FROM_ABI _Iterator __simd_walk_1(_Iterator __first, _DifferenceType
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _Functor>
-_LIBCPP_HIDE_FROM_ABI void
+_LIBCPP_HIDE_FROM_ABI optional<__empty>
 __pstl_for_each(__cpu_backend_tag, _ForwardIterator __first, _ForwardIterator __last, _Functor __func) {
   if constexpr (__is_parallel_execution_policy_v<_ExecutionPolicy> &&
-                __has_random_access_iterator_category<_ForwardIterator>::value) {
-    std::__terminate_on_exception([&] {
-      std::__par_backend::__parallel_for(
-          __first, __last, [__func](_ForwardIterator __brick_first, _ForwardIterator __brick_last) {
-            std::__pstl_for_each<__remove_parallel_policy_t<_ExecutionPolicy>>(
-                __cpu_backend_tag{}, __brick_first, __brick_last, __func);
-          });
-    });
+                __has_random_access_iterator_category_or_concept<_ForwardIterator>::value) {
+    return std::__par_backend::__parallel_for(
+        __first, __last, [__func](_ForwardIterator __brick_first, _ForwardIterator __brick_last) {
+          [[maybe_unused]] auto __res = std::__pstl_for_each<__remove_parallel_policy_t<_ExecutionPolicy>>(
+              __cpu_backend_tag{}, __brick_first, __brick_last, __func);
+          _LIBCPP_ASSERT_INTERNAL(__res, "unseq/seq should never try to allocate!");
+        });
   } else if constexpr (__is_unsequenced_execution_policy_v<_ExecutionPolicy> &&
-                       __has_random_access_iterator_category<_ForwardIterator>::value) {
-    std::__simd_walk_1(__first, __last - __first, __func);
+                       __has_random_access_iterator_category_or_concept<_ForwardIterator>::value) {
+    std::__simd_walk(__first, __last - __first, __func);
+    return __empty{};
   } else {
     std::for_each(__first, __last, __func);
+    return __empty{};
   }
 }
 
diff --git a/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/libdispatch.h b/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/libdispatch.h
index bab6a3639bd0..e885e7f22517 100644
--- a/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/libdispatch.h
+++ b/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/libdispatch.h
@@ -9,8 +9,10 @@
 #ifndef _LIBCPP___ALGORITHM_PSTL_BACKENDS_CPU_BACKENDS_LIBDISPATCH_H
 #define _LIBCPP___ALGORITHM_PSTL_BACKENDS_CPU_BACKENDS_LIBDISPATCH_H
 
+#include <__algorithm/inplace_merge.h>
 #include <__algorithm/lower_bound.h>
 #include <__algorithm/max.h>
+#include <__algorithm/merge.h>
 #include <__algorithm/upper_bound.h>
 #include <__atomic/atomic.h>
 #include <__config>
@@ -21,12 +23,13 @@
 #include <__memory/construct_at.h>
 #include <__memory/unique_ptr.h>
 #include <__numeric/reduce.h>
+#include <__utility/empty.h>
 #include <__utility/exception_guard.h>
 #include <__utility/move.h>
 #include <__utility/pair.h>
-#include <__utility/terminate_on_exception.h>
 #include <cstddef>
 #include <new>
+#include <optional>
 
 _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
@@ -57,13 +60,11 @@ struct __chunk_partitions {
   ptrdiff_t __first_chunk_size_;
 };
 
-[[__gnu__::__const__]] _LIBCPP_EXPORTED_FROM_ABI __chunk_partitions __partition_chunks(ptrdiff_t __size);
+[[__gnu__::__const__]] _LIBCPP_EXPORTED_FROM_ABI __chunk_partitions __partition_chunks(ptrdiff_t __size) noexcept;
 
 template <class _RandomAccessIterator, class _Functor>
-_LIBCPP_HIDE_FROM_ABI void
-__parallel_for(_RandomAccessIterator __first, _RandomAccessIterator __last, _Functor __func) {
-  auto __partitions = __libdispatch::__partition_chunks(__last - __first);
-
+_LIBCPP_HIDE_FROM_ABI optional<__empty>
+__dispatch_parallel_for(__chunk_partitions __partitions, _RandomAccessIterator __first, _Functor __func) {
   // Perform the chunked execution.
   __libdispatch::__dispatch_apply(__partitions.__chunk_count_, [&](size_t __chunk) {
     auto __this_chunk_size = __chunk == 0 ? __partitions.__first_chunk_size_ : __partitions.__chunk_size_;
@@ -73,6 +74,15 @@ __parallel_for(_RandomAccessIterator __first, _RandomAccessIterator __last, _Fun
             : (__chunk * __partitions.__chunk_size_) + (__partitions.__first_chunk_size_ - __partitions.__chunk_size_);
     __func(__first + __index, __first + __index + __this_chunk_size);
   });
+
+  return __empty{};
+}
+
+template <class _RandomAccessIterator, class _Functor>
+_LIBCPP_HIDE_FROM_ABI optional<__empty>
+__parallel_for(_RandomAccessIterator __first, _RandomAccessIterator __last, _Functor __func) {
+  return __libdispatch::__dispatch_parallel_for(
+      __libdispatch::__partition_chunks(__last - __first), std::move(__first), std::move(__func));
 }
 
 template <class _RandomAccessIterator1, class _RandomAccessIterator2, class _RandomAccessIteratorOut>
@@ -90,23 +100,23 @@ template <typename _RandomAccessIterator1,
           typename _RandomAccessIterator3,
           typename _Compare,
           typename _LeafMerge>
-_LIBCPP_HIDE_FROM_ABI void __parallel_merge(
+_LIBCPP_HIDE_FROM_ABI optional<__empty> __parallel_merge(
     _RandomAccessIterator1 __first1,
     _RandomAccessIterator1 __last1,
     _RandomAccessIterator2 __first2,
     _RandomAccessIterator2 __last2,
     _RandomAccessIterator3 __result,
     _Compare __comp,
-    _LeafMerge __leaf_merge) {
+    _LeafMerge __leaf_merge) noexcept {
   __chunk_partitions __partitions =
       __libdispatch::__partition_chunks(std::max<ptrdiff_t>(__last1 - __first1, __last2 - __first2));
 
   if (__partitions.__chunk_count_ == 0)
-    return;
+    return __empty{};
 
   if (__partitions.__chunk_count_ == 1) {
     __leaf_merge(__first1, __last1, __first2, __last2, __result, __comp);
-    return;
+    return __empty{};
   }
 
   using __merge_range_t = __merge_range<_RandomAccessIterator1, _RandomAccessIterator2, _RandomAccessIterator3>;
@@ -117,61 +127,76 @@ _LIBCPP_HIDE_FROM_ABI void __parallel_merge(
     std::destroy_n(__ptr, __n_ranges);
     std::allocator<__merge_range_t>().deallocate(__ptr, __n_ranges);
   };
-  unique_ptr<__merge_range_t[], decltype(__destroy)> __ranges(
-      std::allocator<__merge_range_t>().allocate(__n_ranges), __destroy);
 
-  // TODO: Improve the case where the smaller range is merged into just a few (or even one) chunks of the larger case
-  std::__terminate_on_exception([&] {
-    __merge_range_t* __r = __ranges.get();
-    std::__construct_at(__r++, __first1, __first2, __result);
-
-    bool __iterate_first_range = __last1 - __first1 > __last2 - __first2;
-
-    auto __compute_chunk = [&](size_t __chunk_size) -> __merge_range_t {
-      auto [__mid1, __mid2] = [&] {
-        if (__iterate_first_range) {
-          auto __m1 = __first1 + __chunk_size;
-          auto __m2 = std::lower_bound(__first2, __last2, __m1[-1], __comp);
-          return std::make_pair(__m1, __m2);
-        } else {
-          auto __m2 = __first2 + __chunk_size;
-          auto __m1 = std::lower_bound(__first1, __last1, __m2[-1], __comp);
-          return std::make_pair(__m1, __m2);
+  unique_ptr<__merge_range_t[], decltype(__destroy)> __ranges(
+      [&]() -> __merge_range_t* {
+#  ifndef _LIBCPP_HAS_NO_EXCEPTIONS
+        try {
+#  endif
+          return std::allocator<__merge_range_t>().allocate(__n_ranges);
+#  ifndef _LIBCPP_HAS_NO_EXCEPTIONS
+        } catch (const std::bad_alloc&) {
+          return nullptr;
         }
-      }();
+#  endif
+      }(),
+      __destroy);
 
-      __result += (__mid1 - __first1) + (__mid2 - __first2);
-      __first1 = __mid1;
-      __first2 = __mid2;
-      return {std::move(__mid1), std::move(__mid2), __result};
-    };
+  if (!__ranges)
+    return nullopt;
+
+  // TODO: Improve the case where the smaller range is merged into just a few (or even one) chunks of the larger case
+  __merge_range_t* __r = __ranges.get();
+  std::__construct_at(__r++, __first1, __first2, __result);
+
+  bool __iterate_first_range = __last1 - __first1 > __last2 - __first2;
+
+  auto __compute_chunk = [&](size_t __chunk_size) -> __merge_range_t {
+    auto [__mid1, __mid2] = [&] {
+      if (__iterate_first_range) {
+        auto __m1 = __first1 + __chunk_size;
+        auto __m2 = std::lower_bound(__first2, __last2, __m1[-1], __comp);
+        return std::make_pair(__m1, __m2);
+      } else {
+        auto __m2 = __first2 + __chunk_size;
+        auto __m1 = std::lower_bound(__first1, __last1, __m2[-1], __comp);
+        return std::make_pair(__m1, __m2);
+      }
+    }();
+
+    __result += (__mid1 - __first1) + (__mid2 - __first2);
+    __first1 = __mid1;
+    __first2 = __mid2;
+    return {std::move(__mid1), std::move(__mid2), __result};
+  };
 
-    // handle first chunk
-    std::__construct_at(__r++, __compute_chunk(__partitions.__first_chunk_size_));
-
-    // handle 2 -> N - 1 chunks
-    for (ptrdiff_t __i = 0; __i != __partitions.__chunk_count_ - 2; ++__i)
-      std::__construct_at(__r++, __compute_chunk(__partitions.__chunk_size_));
-
-    // handle last chunk
-    std::__construct_at(__r, __last1, __last2, __result);
-
-    __libdispatch::__dispatch_apply(__partitions.__chunk_count_, [&](size_t __index) {
-      auto __first_iters = __ranges[__index];
-      auto __last_iters  = __ranges[__index + 1];
-      __leaf_merge(
-          __first_iters.__mid1_,
-          __last_iters.__mid1_,
-          __first_iters.__mid2_,
-          __last_iters.__mid2_,
-          __first_iters.__result_,
-          __comp);
-    });
+  // handle first chunk
+  std::__construct_at(__r++, __compute_chunk(__partitions.__first_chunk_size_));
+
+  // handle 2 -> N - 1 chunks
+  for (ptrdiff_t __i = 0; __i != __partitions.__chunk_count_ - 2; ++__i)
+    std::__construct_at(__r++, __compute_chunk(__partitions.__chunk_size_));
+
+  // handle last chunk
+  std::__construct_at(__r, __last1, __last2, __result);
+
+  __libdispatch::__dispatch_apply(__partitions.__chunk_count_, [&](size_t __index) {
+    auto __first_iters = __ranges[__index];
+    auto __last_iters  = __ranges[__index + 1];
+    __leaf_merge(
+        __first_iters.__mid1_,
+        __last_iters.__mid1_,
+        __first_iters.__mid2_,
+        __last_iters.__mid2_,
+        __first_iters.__result_,
+        __comp);
   });
+
+  return __empty{};
 }
 
 template <class _RandomAccessIterator, class _Transform, class _Value, class _Combiner, class _Reduction>
-_LIBCPP_HIDE_FROM_ABI _Value __parallel_transform_reduce(
+_LIBCPP_HIDE_FROM_ABI optional<_Value> __parallel_transform_reduce(
     _RandomAccessIterator __first,
     _RandomAccessIterator __last,
     _Transform __transform,
@@ -211,20 +236,101 @@ _LIBCPP_HIDE_FROM_ABI _Value __parallel_transform_reduce(
     }
   });
 
-  return std::__terminate_on_exception([&] {
-    return std::reduce(
-        std::make_move_iterator(__values.get()),
-        std::make_move_iterator(__values.get() + __partitions.__chunk_count_),
-        std::move(__init),
-        __combiner);
-  });
+  return std::reduce(
+      std::make_move_iterator(__values.get()),
+      std::make_move_iterator(__values.get() + __partitions.__chunk_count_),
+      std::move(__init),
+      __combiner);
 }
 
-// TODO: parallelize this
 template <class _RandomAccessIterator, class _Comp, class _LeafSort>
-_LIBCPP_HIDE_FROM_ABI void __parallel_stable_sort(
+_LIBCPP_HIDE_FROM_ABI optional<__empty> __parallel_stable_sort(
     _RandomAccessIterator __first, _RandomAccessIterator __last, _Comp __comp, _LeafSort __leaf_sort) {
-  __leaf_sort(__first, __last, __comp);
+  const auto __size = __last - __first;
+  auto __partitions = __libdispatch::__partition_chunks(__size);
+
+  if (__partitions.__chunk_count_ == 0)
+    return __empty{};
+
+  if (__partitions.__chunk_count_ == 1) {
+    __leaf_sort(__first, __last, __comp);
+    return __empty{};
+  }
+
+  using _Value = __iter_value_type<_RandomAccessIterator>;
+
+  auto __destroy = [__size](_Value* __ptr) {
+    std::destroy_n(__ptr, __size);
+    std::allocator<_Value>().deallocate(__ptr, __size);
+  };
+
+  // TODO: use __uninitialized_buffer
+  unique_ptr<_Value[], decltype(__destroy)> __values(std::allocator<_Value>().allocate(__size), __destroy);
+
+  // Initialize all elements to a moved-from state
+  // TODO: Don't do this - this can be done in the first merge - see https://llvm.org/PR63928
+  std::__construct_at(__values.get(), std::move(*__first));
+  for (__iter_diff_t<_RandomAccessIterator> __i = 1; __i != __size; ++__i) {
+    std::__construct_at(__values.get() + __i, std::move(__values.get()[__i - 1]));
+  }
+  *__first = std::move(__values.get()[__size - 1]);
+
+  __libdispatch::__dispatch_parallel_for(
+      __partitions,
+      __first,
+      [&__leaf_sort, &__comp](_RandomAccessIterator __chunk_first, _RandomAccessIterator __chunk_last) {
+        __leaf_sort(std::move(__chunk_first), std::move(__chunk_last), __comp);
+      });
+
+  bool __objects_are_in_buffer = false;
+  do {
+    const auto __old_chunk_size = __partitions.__chunk_size_;
+    if (__partitions.__chunk_count_ % 2 == 1) {
+      auto __inplace_merge_chunks = [&__comp, &__partitions](auto __first_chunk_begin) {
+        std::inplace_merge(
+            __first_chunk_begin,
+            __first_chunk_begin + __partitions.__first_chunk_size_,
+            __first_chunk_begin + __partitions.__first_chunk_size_ + __partitions.__chunk_size_,
+            __comp);
+      };
+      if (__objects_are_in_buffer)
+        __inplace_merge_chunks(__values.get());
+      else
+        __inplace_merge_chunks(__first);
+      __partitions.__first_chunk_size_ += 2 * __partitions.__chunk_size_;
+    } else {
+      __partitions.__first_chunk_size_ += __partitions.__chunk_size_;
+    }
+
+    __partitions.__chunk_size_ *= 2;
+    __partitions.__chunk_count_ /= 2;
+
+    auto __merge_chunks = [__partitions, __old_chunk_size, &__comp](auto __from_first, auto __to_first) {
+      __libdispatch::__dispatch_parallel_for(
+          __partitions,
+          __from_first,
+          [__old_chunk_size, &__from_first, &__to_first, &__comp](auto __chunk_first, auto __chunk_last) {
+            std::merge(std::make_move_iterator(__chunk_first),
+                       std::make_move_iterator(__chunk_last - __old_chunk_size),
+                       std::make_move_iterator(__chunk_last - __old_chunk_size),
+                       std::make_move_iterator(__chunk_last),
+                       __to_first + (__chunk_first - __from_first),
+                       __comp);
+          });
+    };
+
+    if (__objects_are_in_buffer)
+      __merge_chunks(__values.get(), __first);
+    else
+      __merge_chunks(__first, __values.get());
+    __objects_are_in_buffer = !__objects_are_in_buffer;
+  } while (__partitions.__chunk_count_ > 1);
+
+  if (__objects_are_in_buffer) {
+    std::move(__values.get(), __values.get() + __size, __first);
+  }
+
+  return __empty{};
 }
 
 _LIBCPP_HIDE_FROM_ABI inline void __cancel_execution() {}
diff --git a/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/merge.h b/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/merge.h
index d5be1e302d35..b0db70f58b2e 100644
--- a/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/merge.h
+++ b/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/merge.h
@@ -12,10 +12,10 @@
 #include <__algorithm/merge.h>
 #include <__algorithm/pstl_backends/cpu_backends/backend.h>
 #include <__config>
-#include <__iterator/iterator_traits.h>
+#include <__iterator/concepts.h>
 #include <__type_traits/is_execution_policy.h>
 #include <__utility/move.h>
-#include <__utility/terminate_on_exception.h>
+#include <optional>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
@@ -23,6 +23,9 @@
 
 #if !defined(_LIBCPP_HAS_NO_INCOMPLETE_PSTL) && _LIBCPP_STD_VER >= 17
 
+_LIBCPP_PUSH_MACROS
+#  include <__undef_macros>
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _ExecutionPolicy,
@@ -30,7 +33,7 @@ template <class _ExecutionPolicy,
           class _ForwardIterator2,
           class _ForwardOutIterator,
           class _Comp>
-_LIBCPP_HIDE_FROM_ABI _ForwardOutIterator __pstl_merge(
+_LIBCPP_HIDE_FROM_ABI optional<_ForwardOutIterator> __pstl_merge(
     __cpu_backend_tag,
     _ForwardIterator1 __first1,
     _ForwardIterator1 __last1,
@@ -39,34 +42,35 @@ _LIBCPP_HIDE_FROM_ABI _ForwardOutIterator __pstl_merge(
     _ForwardOutIterator __result,
     _Comp __comp) {
   if constexpr (__is_parallel_execution_policy_v<_ExecutionPolicy> &&
-                __has_random_access_iterator_category<_ForwardIterator1>::value &&
-                __has_random_access_iterator_category<_ForwardIterator2>::value &&
-                __has_random_access_iterator_category<_ForwardOutIterator>::value) {
-    return std::__terminate_on_exception([&] {
-      __par_backend::__parallel_merge(
-          __first1,
-          __last1,
-          __first2,
-          __last2,
-          __result,
-          __comp,
-          [](_ForwardIterator1 __g_first1,
-             _ForwardIterator1 __g_last1,
-             _ForwardIterator2 __g_first2,
-             _ForwardIterator2 __g_last2,
-             _ForwardOutIterator __g_result,
-             _Comp __g_comp) {
-            return std::__pstl_merge<__remove_parallel_policy_t<_ExecutionPolicy>>(
-                __cpu_backend_tag{},
-                std::move(__g_first1),
-                std::move(__g_last1),
-                std::move(__g_first2),
-                std::move(__g_last2),
-                std::move(__g_result),
-                std::move(__g_comp));
-          });
-      return __result + (__last1 - __first1) + (__last2 - __first2);
-    });
+                __has_random_access_iterator_category_or_concept<_ForwardIterator1>::value &&
+                __has_random_access_iterator_category_or_concept<_ForwardIterator2>::value &&
+                __has_random_access_iterator_category_or_concept<_ForwardOutIterator>::value) {
+    auto __res = __par_backend::__parallel_merge(
+        __first1,
+        __last1,
+        __first2,
+        __last2,
+        __result,
+        __comp,
+        [](_ForwardIterator1 __g_first1,
+           _ForwardIterator1 __g_last1,
+           _ForwardIterator2 __g_first2,
+           _ForwardIterator2 __g_last2,
+           _ForwardOutIterator __g_result,
+           _Comp __g_comp) {
+          [[maybe_unused]] auto __g_res = std::__pstl_merge<__remove_parallel_policy_t<_ExecutionPolicy>>(
+              __cpu_backend_tag{},
+              std::move(__g_first1),
+              std::move(__g_last1),
+              std::move(__g_first2),
+              std::move(__g_last2),
+              std::move(__g_result),
+              std::move(__g_comp));
+          _LIBCPP_ASSERT_INTERNAL(__g_res, "unsed/sed should never try to allocate!");
+        });
+    if (!__res)
+      return nullopt;
+    return __result + (__last1 - __first1) + (__last2 - __first2);
   } else {
     return std::merge(__first1, __last1, __first2, __last2, __result, __comp);
   }
@@ -74,6 +78,8 @@ _LIBCPP_HIDE_FROM_ABI _ForwardOutIterator __pstl_merge(
 
 _LIBCPP_END_NAMESPACE_STD
 
+_LIBCPP_POP_MACROS
+
 #endif // !defined(_LIBCPP_HAS_NO_INCOMPLETE_PSTL) && _LIBCPP_STD_VER >= 17
 
 #endif // _LIBCPP___ALGORITHM_PSTL_BACKENDS_CPU_BACKENDS_MERGE_H
diff --git a/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/serial.h b/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/serial.h
index f151c3b098f6..afcc7ffb2661 100644
--- a/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/serial.h
+++ b/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/serial.h
@@ -11,8 +11,10 @@
 #define _LIBCPP___ALGORITHM_PSTL_BACKENDS_CPU_BACKENDS_SERIAL_H
 
 #include <__config>
+#include <__utility/empty.h>
 #include <__utility/move.h>
 #include <cstddef>
+#include <optional>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
@@ -20,26 +22,32 @@
 
 #if !defined(_LIBCPP_HAS_NO_INCOMPLETE_PSTL) && _LIBCPP_STD_VER >= 17
 
+_LIBCPP_PUSH_MACROS
+#  include <__undef_macros>
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 namespace __par_backend {
 inline namespace __serial_cpu_backend {
 
 template <class _RandomAccessIterator, class _Fp>
-_LIBCPP_HIDE_FROM_ABI void __parallel_for(_RandomAccessIterator __first, _RandomAccessIterator __last, _Fp __f) {
+_LIBCPP_HIDE_FROM_ABI optional<__empty>
+__parallel_for(_RandomAccessIterator __first, _RandomAccessIterator __last, _Fp __f) {
   __f(__first, __last);
+  return __empty{};
 }
 
 template <class _Index, class _UnaryOp, class _Tp, class _BinaryOp, class _Reduce>
-_LIBCPP_HIDE_FROM_ABI _Tp
+_LIBCPP_HIDE_FROM_ABI optional<_Tp>
 __parallel_transform_reduce(_Index __first, _Index __last, _UnaryOp, _Tp __init, _BinaryOp, _Reduce __reduce) {
   return __reduce(std::move(__first), std::move(__last), std::move(__init));
 }
 
 template <class _RandomAccessIterator, class _Compare, class _LeafSort>
-_LIBCPP_HIDE_FROM_ABI void __parallel_stable_sort(
+_LIBCPP_HIDE_FROM_ABI optional<__empty> __parallel_stable_sort(
     _RandomAccessIterator __first, _RandomAccessIterator __last, _Compare __comp, _LeafSort __leaf_sort) {
   __leaf_sort(__first, __last, __comp);
+  return __empty{};
 }
 
 _LIBCPP_HIDE_FROM_ABI inline void __cancel_execution() {}
@@ -49,7 +57,7 @@ template <class _RandomAccessIterator1,
           class _RandomAccessIterator3,
           class _Compare,
           class _LeafMerge>
-_LIBCPP_HIDE_FROM_ABI void __parallel_merge(
+_LIBCPP_HIDE_FROM_ABI optional<__empty> __parallel_merge(
     _RandomAccessIterator1 __first1,
     _RandomAccessIterator1 __last1,
     _RandomAccessIterator2 __first2,
@@ -58,6 +66,7 @@ _LIBCPP_HIDE_FROM_ABI void __parallel_merge(
     _Compare __comp,
     _LeafMerge __leaf_merge) {
   __leaf_merge(__first1, __last1, __first2, __last2, __outit, __comp);
+  return __empty{};
 }
 
 // TODO: Complete this list
@@ -67,6 +76,8 @@ _LIBCPP_HIDE_FROM_ABI void __parallel_merge(
 
 _LIBCPP_END_NAMESPACE_STD
 
+_LIBCPP_POP_MACROS
+
 #endif // !defined(_LIBCPP_HAS_NO_INCOMPLETE_PSTL) && && _LIBCPP_STD_VER >= 17
 
 #endif // _LIBCPP___ALGORITHM_PSTL_BACKENDS_CPU_BACKENDS_SERIAL_H
diff --git a/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/stable_sort.h b/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/stable_sort.h
index 0a701443b3c4..34c423586c4b 100644
--- a/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/stable_sort.h
+++ b/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/stable_sort.h
@@ -13,7 +13,8 @@
 #include <__algorithm/stable_sort.h>
 #include <__config>
 #include <__type_traits/is_execution_policy.h>
-#include <__utility/terminate_on_exception.h>
+#include <__utility/empty.h>
+#include <optional>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
@@ -24,17 +25,16 @@
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _ExecutionPolicy, class _RandomAccessIterator, class _Comp>
-_LIBCPP_HIDE_FROM_ABI void
+_LIBCPP_HIDE_FROM_ABI optional<__empty>
 __pstl_stable_sort(__cpu_backend_tag, _RandomAccessIterator __first, _RandomAccessIterator __last, _Comp __comp) {
   if constexpr (__is_parallel_execution_policy_v<_ExecutionPolicy>) {
-    std::__terminate_on_exception([&] {
-      __par_backend::__parallel_stable_sort(
-          __first, __last, __comp, [](_RandomAccessIterator __g_first, _RandomAccessIterator __g_last, _Comp __g_comp) {
-            std::stable_sort(__g_first, __g_last, __g_comp);
-          });
-    });
+    return __par_backend::__parallel_stable_sort(
+        __first, __last, __comp, [](_RandomAccessIterator __g_first, _RandomAccessIterator __g_last, _Comp __g_comp) {
+          std::stable_sort(__g_first, __g_last, __g_comp);
+        });
   } else {
     std::stable_sort(__first, __last, __comp);
+    return __empty{};
   }
 }
 
diff --git a/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/thread.h b/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/thread.h
index 30eb0ae3626d..eb11a961b760 100644
--- a/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/thread.h
+++ b/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/thread.h
@@ -11,8 +11,10 @@
 
 #include <__assert>
 #include <__config>
+#include <__utility/empty.h>
 #include <__utility/move.h>
 #include <cstddef>
+#include <optional>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
@@ -32,20 +34,23 @@ namespace __par_backend {
 inline namespace __thread_cpu_backend {
 
 template <class _RandomAccessIterator, class _Fp>
-_LIBCPP_HIDE_FROM_ABI void __parallel_for(_RandomAccessIterator __first, _RandomAccessIterator __last, _Fp __f) {
+_LIBCPP_HIDE_FROM_ABI optional<__empty>
+__parallel_for(_RandomAccessIterator __first, _RandomAccessIterator __last, _Fp __f) {
   __f(__first, __last);
+  return __empty{};
 }
 
 template <class _Index, class _UnaryOp, class _Tp, class _BinaryOp, class _Reduce>
-_LIBCPP_HIDE_FROM_ABI _Tp
+_LIBCPP_HIDE_FROM_ABI optional<_Tp>
 __parallel_transform_reduce(_Index __first, _Index __last, _UnaryOp, _Tp __init, _BinaryOp, _Reduce __reduce) {
   return __reduce(std::move(__first), std::move(__last), std::move(__init));
 }
 
 template <class _RandomAccessIterator, class _Compare, class _LeafSort>
-_LIBCPP_HIDE_FROM_ABI void __parallel_stable_sort(
+_LIBCPP_HIDE_FROM_ABI optional<__empty> __parallel_stable_sort(
     _RandomAccessIterator __first, _RandomAccessIterator __last, _Compare __comp, _LeafSort __leaf_sort) {
   __leaf_sort(__first, __last, __comp);
+  return __empty{};
 }
 
 _LIBCPP_HIDE_FROM_ABI inline void __cancel_execution() {}
@@ -55,7 +60,7 @@ template <class _RandomAccessIterator1,
           class _RandomAccessIterator3,
           class _Compare,
           class _LeafMerge>
-_LIBCPP_HIDE_FROM_ABI void __parallel_merge(
+_LIBCPP_HIDE_FROM_ABI optional<__empty> __parallel_merge(
     _RandomAccessIterator1 __first1,
     _RandomAccessIterator1 __last1,
     _RandomAccessIterator2 __first2,
@@ -64,6 +69,7 @@ _LIBCPP_HIDE_FROM_ABI void __parallel_merge(
     _Compare __comp,
     _LeafMerge __leaf_merge) {
   __leaf_merge(__first1, __last1, __first2, __last2, __outit, __comp);
+  return __empty{};
 }
 
 } // namespace __thread_cpu_backend
diff --git a/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/transform.h b/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/transform.h
index ef25ff0238da..fdf1a2e78dad 100644
--- a/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/transform.h
+++ b/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/transform.h
@@ -12,11 +12,12 @@
 #include <__algorithm/pstl_backends/cpu_backends/backend.h>
 #include <__algorithm/transform.h>
 #include <__config>
+#include <__iterator/concepts.h>
 #include <__iterator/iterator_traits.h>
 #include <__type_traits/enable_if.h>
 #include <__type_traits/is_execution_policy.h>
 #include <__type_traits/remove_cvref.h>
-#include <__utility/terminate_on_exception.h>
+#include <optional>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
@@ -24,11 +25,14 @@
 
 #if !defined(_LIBCPP_HAS_NO_INCOMPLETE_PSTL) && _LIBCPP_STD_VER >= 17
 
+_LIBCPP_PUSH_MACROS
+#  include <__undef_macros>
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _Iterator1, class _DifferenceType, class _Iterator2, class _Function>
 _LIBCPP_HIDE_FROM_ABI _Iterator2
-__simd_walk_2(_Iterator1 __first1, _DifferenceType __n, _Iterator2 __first2, _Function __f) noexcept {
+__simd_walk(_Iterator1 __first1, _DifferenceType __n, _Iterator2 __first2, _Function __f) noexcept {
   _PSTL_PRAGMA_SIMD
   for (_DifferenceType __i = 0; __i < __n; ++__i)
     __f(__first1[__i], __first2[__i]);
@@ -36,27 +40,27 @@ __simd_walk_2(_Iterator1 __first1, _DifferenceType __n, _Iterator2 __first2, _Fu
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _ForwardOutIterator, class _UnaryOperation>
-_LIBCPP_HIDE_FROM_ABI _ForwardOutIterator __pstl_transform(
+_LIBCPP_HIDE_FROM_ABI optional<_ForwardOutIterator> __pstl_transform(
     __cpu_backend_tag,
     _ForwardIterator __first,
     _ForwardIterator __last,
     _ForwardOutIterator __result,
     _UnaryOperation __op) {
   if constexpr (__is_parallel_execution_policy_v<_ExecutionPolicy> &&
-                __has_random_access_iterator_category<_ForwardIterator>::value &&
-                __has_random_access_iterator_category<_ForwardOutIterator>::value) {
-    std::__terminate_on_exception([&] {
-      std::__par_backend::__parallel_for(
-          __first, __last, [__op, __first, __result](_ForwardIterator __brick_first, _ForwardIterator __brick_last) {
-            return std::__pstl_transform<__remove_parallel_policy_t<_ExecutionPolicy>>(
-                __cpu_backend_tag{}, __brick_first, __brick_last, __result + (__brick_first - __first), __op);
-          });
-    });
+                __has_random_access_iterator_category_or_concept<_ForwardIterator>::value &&
+                __has_random_access_iterator_category_or_concept<_ForwardOutIterator>::value) {
+    std::__par_backend::__parallel_for(
+        __first, __last, [__op, __first, __result](_ForwardIterator __brick_first, _ForwardIterator __brick_last) {
+          auto __res = std::__pstl_transform<__remove_parallel_policy_t<_ExecutionPolicy>>(
+              __cpu_backend_tag{}, __brick_first, __brick_last, __result + (__brick_first - __first), __op);
+          _LIBCPP_ASSERT_INTERNAL(__res, "unseq/seq should never try to allocate!");
+          return *std::move(__res);
+        });
     return __result + (__last - __first);
   } else if constexpr (__is_unsequenced_execution_policy_v<_ExecutionPolicy> &&
-                       __has_random_access_iterator_category<_ForwardIterator>::value &&
-                       __has_random_access_iterator_category<_ForwardOutIterator>::value) {
-    return std::__simd_walk_2(
+                       __has_random_access_iterator_category_or_concept<_ForwardIterator>::value &&
+                       __has_random_access_iterator_category_or_concept<_ForwardOutIterator>::value) {
+    return std::__simd_walk(
         __first,
         __last - __first,
         __result,
@@ -69,7 +73,7 @@ _LIBCPP_HIDE_FROM_ABI _ForwardOutIterator __pstl_transform(
 }
 
 template <class _Iterator1, class _DifferenceType, class _Iterator2, class _Iterator3, class _Function>
-_LIBCPP_HIDE_FROM_ABI _Iterator3 __simd_walk_3(
+_LIBCPP_HIDE_FROM_ABI _Iterator3 __simd_walk(
     _Iterator1 __first1, _DifferenceType __n, _Iterator2 __first2, _Iterator3 __first3, _Function __f) noexcept {
   _PSTL_PRAGMA_SIMD
   for (_DifferenceType __i = 0; __i < __n; ++__i)
@@ -82,7 +86,7 @@ template <class _ExecutionPolicy,
           class _ForwardOutIterator,
           class _BinaryOperation,
           enable_if_t<is_execution_policy_v<__remove_cvref_t<_ExecutionPolicy>>, int> = 0>
-_LIBCPP_HIDE_FROM_ABI _ForwardOutIterator __pstl_transform(
+_LIBCPP_HIDE_FROM_ABI optional<_ForwardOutIterator> __pstl_transform(
     __cpu_backend_tag,
     _ForwardIterator1 __first1,
     _ForwardIterator1 __last1,
@@ -90,29 +94,29 @@ _LIBCPP_HIDE_FROM_ABI _ForwardOutIterator __pstl_transform(
     _ForwardOutIterator __result,
     _BinaryOperation __op) {
   if constexpr (__is_parallel_execution_policy_v<_ExecutionPolicy> &&
-                __has_random_access_iterator_category<_ForwardIterator1>::value &&
-                __has_random_access_iterator_category<_ForwardIterator2>::value &&
-                __has_random_access_iterator_category<_ForwardOutIterator>::value) {
-    std::__terminate_on_exception([&] {
-      std::__par_backend::__parallel_for(
-          __first1,
-          __last1,
-          [__op, __first1, __first2, __result](_ForwardIterator1 __brick_first, _ForwardIterator1 __brick_last) {
-            return std::__pstl_transform<__remove_parallel_policy_t<_ExecutionPolicy>>(
-                __cpu_backend_tag{},
-                __brick_first,
-                __brick_last,
-                __first2 + (__brick_first - __first1),
-                __result + (__brick_first - __first1),
-                __op);
-          });
-    });
+                __has_random_access_iterator_category_or_concept<_ForwardIterator1>::value &&
+                __has_random_access_iterator_category_or_concept<_ForwardIterator2>::value &&
+                __has_random_access_iterator_category_or_concept<_ForwardOutIterator>::value) {
+    auto __res = std::__par_backend::__parallel_for(
+        __first1,
+        __last1,
+        [__op, __first1, __first2, __result](_ForwardIterator1 __brick_first, _ForwardIterator1 __brick_last) {
+          return std::__pstl_transform<__remove_parallel_policy_t<_ExecutionPolicy>>(
+              __cpu_backend_tag{},
+              __brick_first,
+              __brick_last,
+              __first2 + (__brick_first - __first1),
+              __result + (__brick_first - __first1),
+              __op);
+        });
+    if (!__res)
+      return nullopt;
     return __result + (__last1 - __first1);
   } else if constexpr (__is_unsequenced_execution_policy_v<_ExecutionPolicy> &&
-                       __has_random_access_iterator_category<_ForwardIterator1>::value &&
-                       __has_random_access_iterator_category<_ForwardIterator2>::value &&
-                       __has_random_access_iterator_category<_ForwardOutIterator>::value) {
-    return std::__simd_walk_3(
+                       __has_random_access_iterator_category_or_concept<_ForwardIterator1>::value &&
+                       __has_random_access_iterator_category_or_concept<_ForwardIterator2>::value &&
+                       __has_random_access_iterator_category_or_concept<_ForwardOutIterator>::value) {
+    return std::__simd_walk(
         __first1,
         __last1 - __first1,
         __first2,
@@ -127,6 +131,8 @@ _LIBCPP_HIDE_FROM_ABI _ForwardOutIterator __pstl_transform(
 
 _LIBCPP_END_NAMESPACE_STD
 
+_LIBCPP_POP_MACROS
+
 #endif // !defined(_LIBCPP_HAS_NO_INCOMPLETE_PSTL) && _LIBCPP_STD_VER >= 17
 
 #endif // _LIBCPP___ALGORITHM_PSTL_BACKENDS_CPU_BACKENDS_TRANSFORM_H
diff --git a/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/transform_reduce.h b/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/transform_reduce.h
index c51c312d938a..ab2e3172b8b6 100644
--- a/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/transform_reduce.h
+++ b/contrib/llvm-project/libcxx/include/__algorithm/pstl_backends/cpu_backends/transform_reduce.h
@@ -11,14 +11,15 @@
 
 #include <__algorithm/pstl_backends/cpu_backends/backend.h>
 #include <__config>
+#include <__iterator/concepts.h>
 #include <__iterator/iterator_traits.h>
 #include <__numeric/transform_reduce.h>
 #include <__type_traits/is_arithmetic.h>
 #include <__type_traits/is_execution_policy.h>
 #include <__type_traits/operation_traits.h>
 #include <__utility/move.h>
-#include <__utility/terminate_on_exception.h>
 #include <new>
+#include <optional>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
@@ -28,12 +29,14 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-template <
-    typename _DifferenceType,
-    typename _Tp,
-    typename _BinaryOperation,
-    typename _UnaryOperation,
-    __enable_if_t<__is_trivial_plus_operation<_BinaryOperation, _Tp, _Tp>::value && is_arithmetic_v<_Tp>, int> = 0>
+template <typename _DifferenceType,
+          typename _Tp,
+          typename _BinaryOperation,
+          typename _UnaryOperation,
+          typename _UnaryResult = invoke_result_t<_UnaryOperation, _DifferenceType>,
+          __enable_if_t<__desugars_to<__plus_tag, _BinaryOperation, _Tp, _UnaryResult>::value && is_arithmetic_v<_Tp> &&
+                            is_arithmetic_v<_UnaryResult>,
+                        int>    = 0>
 _LIBCPP_HIDE_FROM_ABI _Tp
 __simd_transform_reduce(_DifferenceType __n, _Tp __init, _BinaryOperation, _UnaryOperation __f) noexcept {
   _PSTL_PRAGMA_SIMD_REDUCTION(+ : __init)
@@ -42,12 +45,14 @@ __simd_transform_reduce(_DifferenceType __n, _Tp __init, _BinaryOperation, _Unar
   return __init;
 }
 
-template <
-    typename _Size,
-    typename _Tp,
-    typename _BinaryOperation,
-    typename _UnaryOperation,
-    __enable_if_t<!(__is_trivial_plus_operation<_BinaryOperation, _Tp, _Tp>::value && is_arithmetic_v<_Tp>), int> = 0>
+template <typename _Size,
+          typename _Tp,
+          typename _BinaryOperation,
+          typename _UnaryOperation,
+          typename _UnaryResult = invoke_result_t<_UnaryOperation, _Size>,
+          __enable_if_t<!(__desugars_to<__plus_tag, _BinaryOperation, _Tp, _UnaryResult>::value &&
+                          is_arithmetic_v<_Tp> && is_arithmetic_v<_UnaryResult>),
+                        int>    = 0>
 _LIBCPP_HIDE_FROM_ABI _Tp
 __simd_transform_reduce(_Size __n, _Tp __init, _BinaryOperation __binary_op, _UnaryOperation __f) noexcept {
   const _Size __block_size = __lane_size / sizeof(_Tp);
@@ -97,7 +102,7 @@ template <class _ExecutionPolicy,
           class _Tp,
           class _BinaryOperation1,
           class _BinaryOperation2>
-_LIBCPP_HIDE_FROM_ABI _Tp __pstl_transform_reduce(
+_LIBCPP_HIDE_FROM_ABI optional<_Tp> __pstl_transform_reduce(
     __cpu_backend_tag,
     _ForwardIterator1 __first1,
     _ForwardIterator1 __last1,
@@ -106,32 +111,30 @@ _LIBCPP_HIDE_FROM_ABI _Tp __pstl_transform_reduce(
     _BinaryOperation1 __reduce,
     _BinaryOperation2 __transform) {
   if constexpr (__is_parallel_execution_policy_v<_ExecutionPolicy> &&
-                __has_random_access_iterator_category<_ForwardIterator1>::value &&
-                __has_random_access_iterator_category<_ForwardIterator2>::value) {
-    return std::__terminate_on_exception([&] {
-      return __par_backend::__parallel_transform_reduce(
-          __first1,
-          std::move(__last1),
-          [__first1, __first2, __transform](_ForwardIterator1 __iter) {
-            return __transform(*__iter, *(__first2 + (__iter - __first1)));
-          },
-          std::move(__init),
-          std::move(__reduce),
-          [__first1, __first2, __reduce, __transform](
-              _ForwardIterator1 __brick_first, _ForwardIterator1 __brick_last, _Tp __brick_init) {
-            return std::__pstl_transform_reduce<__remove_parallel_policy_t<_ExecutionPolicy>>(
-                __cpu_backend_tag{},
-                __brick_first,
-                std::move(__brick_last),
-                __first2 + (__brick_first - __first1),
-                std::move(__brick_init),
-                std::move(__reduce),
-                std::move(__transform));
-          });
-    });
+                __has_random_access_iterator_category_or_concept<_ForwardIterator1>::value &&
+                __has_random_access_iterator_category_or_concept<_ForwardIterator2>::value) {
+    return __par_backend::__parallel_transform_reduce(
+        __first1,
+        std::move(__last1),
+        [__first1, __first2, __transform](_ForwardIterator1 __iter) {
+          return __transform(*__iter, *(__first2 + (__iter - __first1)));
+        },
+        std::move(__init),
+        std::move(__reduce),
+        [__first1, __first2, __reduce, __transform](
+            _ForwardIterator1 __brick_first, _ForwardIterator1 __brick_last, _Tp __brick_init) {
+          return *std::__pstl_transform_reduce<__remove_parallel_policy_t<_ExecutionPolicy>>(
+              __cpu_backend_tag{},
+              __brick_first,
+              std::move(__brick_last),
+              __first2 + (__brick_first - __first1),
+              std::move(__brick_init),
+              std::move(__reduce),
+              std::move(__transform));
+        });
   } else if constexpr (__is_unsequenced_execution_policy_v<_ExecutionPolicy> &&
-                       __has_random_access_iterator_category<_ForwardIterator1>::value &&
-                       __has_random_access_iterator_category<_ForwardIterator2>::value) {
+                       __has_random_access_iterator_category_or_concept<_ForwardIterator1>::value &&
+                       __has_random_access_iterator_category_or_concept<_ForwardIterator2>::value) {
     return std::__simd_transform_reduce(
         __last1 - __first1, std::move(__init), std::move(__reduce), [&](__iter_diff_t<_ForwardIterator1> __i) {
           return __transform(__first1[__i], __first2[__i]);
@@ -148,7 +151,7 @@ _LIBCPP_HIDE_FROM_ABI _Tp __pstl_transform_reduce(
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _Tp, class _BinaryOperation, class _UnaryOperation>
-_LIBCPP_HIDE_FROM_ABI _Tp __pstl_transform_reduce(
+_LIBCPP_HIDE_FROM_ABI optional<_Tp> __pstl_transform_reduce(
     __cpu_backend_tag,
     _ForwardIterator __first,
     _ForwardIterator __last,
@@ -156,26 +159,26 @@ _LIBCPP_HIDE_FROM_ABI _Tp __pstl_transform_reduce(
     _BinaryOperation __reduce,
     _UnaryOperation __transform) {
   if constexpr (__is_parallel_execution_policy_v<_ExecutionPolicy> &&
-                __has_random_access_iterator_category<_ForwardIterator>::value) {
-    return std::__terminate_on_exception([&] {
-      return __par_backend::__parallel_transform_reduce(
-          std::move(__first),
-          std::move(__last),
-          [__transform](_ForwardIterator __iter) { return __transform(*__iter); },
-          std::move(__init),
-          __reduce,
-          [__transform, __reduce](auto __brick_first, auto __brick_last, _Tp __brick_init) {
-            return std::__pstl_transform_reduce<__remove_parallel_policy_t<_ExecutionPolicy>>(
-                __cpu_backend_tag{},
-                std::move(__brick_first),
-                std::move(__brick_last),
-                std::move(__brick_init),
-                std::move(__reduce),
-                std::move(__transform));
-          });
-    });
+                __has_random_access_iterator_category_or_concept<_ForwardIterator>::value) {
+    return __par_backend::__parallel_transform_reduce(
+        std::move(__first),
+        std::move(__last),
+        [__transform](_ForwardIterator __iter) { return __transform(*__iter); },
+        std::move(__init),
+        __reduce,
+        [__transform, __reduce](auto __brick_first, auto __brick_last, _Tp __brick_init) {
+          auto __res = std::__pstl_transform_reduce<__remove_parallel_policy_t<_ExecutionPolicy>>(
+              __cpu_backend_tag{},
+              std::move(__brick_first),
+              std::move(__brick_last),
+              std::move(__brick_init),
+              std::move(__reduce),
+              std::move(__transform));
+          _LIBCPP_ASSERT_INTERNAL(__res, "unseq/seq should never try to allocate!");
+          return *std::move(__res);
+        });
   } else if constexpr (__is_unsequenced_execution_policy_v<_ExecutionPolicy> &&
-                       __has_random_access_iterator_category<_ForwardIterator>::value) {
+                       __has_random_access_iterator_category_or_concept<_ForwardIterator>::value) {
     return std::__simd_transform_reduce(
         __last - __first,
         std::move(__init),