[WIP][libc++] PSTL dispatching mechanism overhaul

The experimental PSTL's current dispatching mechanism was designed with
flexibility in mind. However, while reviewing the in-progress OpenMP
backend, I realized that the dispatching mechanism based on ADL and
default definitions in the frontend had several downsides. To name a
few:

1. The dispatching of an algorithm to the back-end and its default
   implementation is bundled together via `_LIBCPP_PSTL_CUSTOMIZATION_POINT`.
   This makes the dispatching really confusing and leads to annoyances
   such as variable shadowing and weird lambda captures in the front-end.
2. The distinction between back-end functions and front-end algorithms
   is not as clear as it could be, which led us to call one where we meant
   the other in a few cases. This is bad due to the exception requirements
   of the PSTL.
3. There are two levels of back-end dispatching in the PSTL, which treat
   CPU backends as a special case. This was confusing and not as flexible
   as we'd like. For example, there was no straightforward way to dispatch
   all uses of `unseq` to a specific back-end from the OpenMP backend,
   or for CPU backends to fall back on each other.

This patch rewrites the backend dispatching mechanism to solve these
problems, but doesn't touch any of the actual implementation of
algorithms. Specifically, this rewrite has the following characteristics:

- All back-ends are full top-level backends defining all the basis operations
  required by the PSTL. This is made realistic for CPU backends by providing
  the CPU-based basis operations as simple helpers that can easily be reused
  when defining the PSTL basis operations.

- The default definitions for algorithms are separated from their dispatching
  logic and grouped in families instead, based on the basis operation they
  require for their default implementation.

- The front-end is thus simplified a whole lot and made very consistent
  for all algorithms, which makes it easier to audit the front-end for
  things like exception-correctness.

Fixes #70718

Author: ldionne

libcxx/CMakeLists.txt

@@ -301,9 +301,9 @@ option(LIBCXX_HAS_EXTERNAL_THREAD_API
    This option may only be set to ON when LIBCXX_ENABLE_THREADS=ON." OFF)
 
 if (LIBCXX_ENABLE_THREADS)
-  set(LIBCXX_PSTL_CPU_BACKEND "std_thread" CACHE STRING "Which PSTL CPU backend to use")
+  set(LIBCXX_PSTL_BACKEND "std_thread" CACHE STRING "Which PSTL backend to use")
 else()
-  set(LIBCXX_PSTL_CPU_BACKEND "serial" CACHE STRING "Which PSTL CPU backend to use")
+  set(LIBCXX_PSTL_BACKEND "serial" CACHE STRING "Which PSTL backend to use")
 endif()
 
 # Misc options ----------------------------------------------------------------
@@ -793,14 +793,14 @@ elseif (LIBCXX_HARDENING_MODE STREQUAL "debug")
   config_define(8 _LIBCPP_HARDENING_MODE_DEFAULT)
 endif()
 
-if (LIBCXX_PSTL_CPU_BACKEND STREQUAL "serial")
-  config_define(1 _LIBCPP_PSTL_CPU_BACKEND_SERIAL)
-elseif(LIBCXX_PSTL_CPU_BACKEND STREQUAL "std_thread")
-  config_define(1 _LIBCPP_PSTL_CPU_BACKEND_THREAD)
-elseif(LIBCXX_PSTL_CPU_BACKEND STREQUAL "libdispatch")
-  config_define(1 _LIBCPP_PSTL_CPU_BACKEND_LIBDISPATCH)
+if (LIBCXX_PSTL_BACKEND STREQUAL "serial")
+  config_define(1 _LIBCPP_PSTL_BACKEND_SERIAL)
+elseif(LIBCXX_PSTL_BACKEND STREQUAL "std_thread")
+  config_define(1 _LIBCPP_PSTL_BACKEND_STD_THREAD)
+elseif(LIBCXX_PSTL_BACKEND STREQUAL "libdispatch")
+  config_define(1 _LIBCPP_PSTL_BACKEND_LIBDISPATCH)
 else()
-  message(FATAL_ERROR "LIBCXX_PSTL_CPU_BACKEND is set to ${LIBCXX_PSTL_CPU_BACKEND}, which is not a valid backend.
+  message(FATAL_ERROR "LIBCXX_PSTL_BACKEND is set to ${LIBCXX_PSTL_BACKEND}, which is not a valid backend.
                        Valid backends are: serial, std_thread and libdispatch")
 endif()
 

libcxx/cmake/caches/Apple.cmake

@@ -7,7 +7,7 @@ set(LIBCXX_ENABLE_STATIC ON CACHE BOOL "")
 set(LIBCXX_ENABLE_SHARED ON CACHE BOOL "")
 set(LIBCXX_CXX_ABI libcxxabi CACHE STRING "")
 set(LIBCXX_ENABLE_VENDOR_AVAILABILITY_ANNOTATIONS ON CACHE BOOL "")
-set(LIBCXX_PSTL_CPU_BACKEND libdispatch CACHE STRING "")
+set(LIBCXX_PSTL_BACKEND libdispatch CACHE STRING "")
 
 set(LIBCXX_HERMETIC_STATIC_LIBRARY ON CACHE BOOL "")
 set(LIBCXXABI_HERMETIC_STATIC_LIBRARY ON CACHE BOOL "")

libcxx/include/CMakeLists.txt

@@ -73,27 +73,12 @@ set(files
   __algorithm/pop_heap.h
   __algorithm/prev_permutation.h
   __algorithm/pstl_any_all_none_of.h
-  __algorithm/pstl_backend.h
-  __algorithm/pstl_backends/cpu_backend.h
-  __algorithm/pstl_backends/cpu_backends/any_of.h
-  __algorithm/pstl_backends/cpu_backends/backend.h
-  __algorithm/pstl_backends/cpu_backends/fill.h
-  __algorithm/pstl_backends/cpu_backends/find_if.h
-  __algorithm/pstl_backends/cpu_backends/for_each.h
-  __algorithm/pstl_backends/cpu_backends/libdispatch.h
-  __algorithm/pstl_backends/cpu_backends/merge.h
-  __algorithm/pstl_backends/cpu_backends/serial.h
-  __algorithm/pstl_backends/cpu_backends/stable_sort.h
-  __algorithm/pstl_backends/cpu_backends/thread.h
-  __algorithm/pstl_backends/cpu_backends/transform.h
-  __algorithm/pstl_backends/cpu_backends/transform_reduce.h
   __algorithm/pstl_copy.h
   __algorithm/pstl_count.h
   __algorithm/pstl_equal.h
   __algorithm/pstl_fill.h
   __algorithm/pstl_find.h
   __algorithm/pstl_for_each.h
-  __algorithm/pstl_frontend_dispatch.h
   __algorithm/pstl_generate.h
   __algorithm/pstl_is_partitioned.h
   __algorithm/pstl_merge.h
@@ -591,7 +576,28 @@ set(files
   __numeric/transform_exclusive_scan.h
   __numeric/transform_inclusive_scan.h
   __numeric/transform_reduce.h
+  __pstl/backend_fwd.h
+  __pstl/backends/libdispatch.h
+  __pstl/backends/serial.h
+  __pstl/backends/std_thread.h
+  __pstl/configuration.h
+  __pstl/cpu_algos/any_of.h
   __pstl/cpu_algos/cpu_traits.h
+  __pstl/cpu_algos/fill.h
+  __pstl/cpu_algos/find_if.h
+  __pstl/cpu_algos/for_each.h
+  __pstl/cpu_algos/merge.h
+  __pstl/cpu_algos/stable_sort.h
+  __pstl/cpu_algos/transform_reduce.h
+  __pstl/cpu_algos/transform.h
+  __pstl/defaults.h
+  __pstl/defaults/find_if_family.h
+  __pstl/defaults/for_each_family.h
+  __pstl/defaults/merge_family.h
+  __pstl/defaults/stable_sort_family.h
+  __pstl/defaults/transform_family.h
+  __pstl/defaults/transform_reduce_family.h
+  __pstl/run_backend.h
   __random/bernoulli_distribution.h
   __random/binomial_distribution.h
   __random/cauchy_distribution.h

libcxx/include/__algorithm/pstl_any_all_none_of.h

@@ -9,15 +9,15 @@
 #ifndef _LIBCPP___ALGORITHM_PSTL_ANY_ALL_NONE_OF_H
 #define _LIBCPP___ALGORITHM_PSTL_ANY_ALL_NONE_OF_H
 
-#include <__algorithm/pstl_find.h>
-#include <__algorithm/pstl_frontend_dispatch.h>
 #include <__config>
 #include <__iterator/cpp17_iterator_concepts.h>
+#include <__pstl/configuration.h>
+#include <__pstl/run_backend.h>
 #include <__type_traits/enable_if.h>
 #include <__type_traits/is_execution_policy.h>
 #include <__type_traits/remove_cvref.h>
+#include <__utility/forward.h>
 #include <__utility/move.h>
-#include <optional>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
@@ -30,29 +30,6 @@ _LIBCPP_PUSH_MACROS
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-template <class>
-void __pstl_any_of(); // declaration needed for the frontend dispatch below
-
-template <class _ExecutionPolicy,
-          class _ForwardIterator,
-          class _Predicate,
-          class _RawPolicy                                    = __remove_cvref_t<_ExecutionPolicy>,
-          enable_if_t<is_execution_policy_v<_RawPolicy>, int> = 0>
-[[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<bool> __any_of(
-    _ExecutionPolicy&& __policy, _ForwardIterator&& __first, _ForwardIterator&& __last, _Predicate&& __pred) noexcept {
-  return std::__pstl_frontend_dispatch(
-      _LIBCPP_PSTL_CUSTOMIZATION_POINT(__pstl_any_of, _RawPolicy),
-      [&](_ForwardIterator __g_first, _ForwardIterator __g_last, _Predicate __g_pred) -> optional<bool> {
-        auto __res = std::__find_if(__policy, __g_first, __g_last, __g_pred);
-        if (!__res)
-          return nullopt;
-        return *__res != __g_last;
-      },
-      std::move(__first),
-      std::move(__last),
-      std::move(__pred));
-}
-
 template <class _ExecutionPolicy,
           class _ForwardIterator,
           class _Predicate,
@@ -61,35 +38,9 @@ template <class _ExecutionPolicy,
 _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI bool
 any_of(_ExecutionPolicy&& __policy, _ForwardIterator __first, _ForwardIterator __last, _Predicate __pred) {
   _LIBCPP_REQUIRE_CPP17_FORWARD_ITERATOR(_ForwardIterator);
-  auto __res = std::__any_of(__policy, std::move(__first), std::move(__last), std::move(__pred));
-  if (!__res)
-    std::__throw_bad_alloc();
-  return *std::move(__res);
-}
-
-template <class>
-void __pstl_all_of(); // declaration needed for the frontend dispatch below
-
-template <class _ExecutionPolicy,
-          class _ForwardIterator,
-          class _Pred,
-          class _RawPolicy                                    = __remove_cvref_t<_ExecutionPolicy>,
-          enable_if_t<is_execution_policy_v<_RawPolicy>, int> = 0>
-[[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<bool>
-__all_of(_ExecutionPolicy&& __policy, _ForwardIterator&& __first, _ForwardIterator&& __last, _Pred&& __pred) noexcept {
-  return std::__pstl_frontend_dispatch(
-      _LIBCPP_PSTL_CUSTOMIZATION_POINT(__pstl_all_of, _RawPolicy),
-      [&](_ForwardIterator __g_first, _ForwardIterator __g_last, _Pred __g_pred) -> optional<bool> {
-        auto __res = std::__any_of(__policy, __g_first, __g_last, [&](__iter_reference<_ForwardIterator> __value) {
-          return !__g_pred(__value);
-        });
-        if (!__res)
-          return nullopt;
-        return !*__res;
-      },
-      std::move(__first),
-      std::move(__last),
-      std::move(__pred));
+  using _Implementation = __pstl::__any_of<__pstl::__configured_backend, _RawPolicy>;
+  return __pstl::__run_backend<_Implementation>(
+      std::forward<_ExecutionPolicy>(__policy), std::move(__first), std::move(__last), std::move(__pred));
 }
 
 template <class _ExecutionPolicy,
@@ -100,33 +51,9 @@ template <class _ExecutionPolicy,
 _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI bool
 all_of(_ExecutionPolicy&& __policy, _ForwardIterator __first, _ForwardIterator __last, _Pred __pred) {
   _LIBCPP_REQUIRE_CPP17_FORWARD_ITERATOR(_ForwardIterator);
-  auto __res = std::__all_of(__policy, std::move(__first), std::move(__last), std::move(__pred));
-  if (!__res)
-    std::__throw_bad_alloc();
-  return *std::move(__res);
-}
-
-template <class>
-void __pstl_none_of(); // declaration needed for the frontend dispatch below
-
-template <class _ExecutionPolicy,
-          class _ForwardIterator,
-          class _Pred,
-          class _RawPolicy                                    = __remove_cvref_t<_ExecutionPolicy>,
-          enable_if_t<is_execution_policy_v<_RawPolicy>, int> = 0>
-[[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<bool>
-__none_of(_ExecutionPolicy&& __policy, _ForwardIterator&& __first, _ForwardIterator&& __last, _Pred&& __pred) noexcept {
-  return std::__pstl_frontend_dispatch(
-      _LIBCPP_PSTL_CUSTOMIZATION_POINT(__pstl_none_of, _RawPolicy),
-      [&](_ForwardIterator __g_first, _ForwardIterator __g_last, _Pred __g_pred) -> optional<bool> {
-        auto __res = std::__any_of(__policy, __g_first, __g_last, __g_pred);
-        if (!__res)
-          return nullopt;
-        return !*__res;
-      },
-      std::move(__first),
-      std::move(__last),
-      std::move(__pred));
+  using _Implementation = __pstl::__all_of<__pstl::__configured_backend, _RawPolicy>;
+  return __pstl::__run_backend<_Implementation>(
+      std::forward<_ExecutionPolicy>(__policy), std::move(__first), std::move(__last), std::move(__pred));
 }
 
 template <class _ExecutionPolicy,
@@ -137,10 +64,9 @@ template <class _ExecutionPolicy,
 _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI bool
 none_of(_ExecutionPolicy&& __policy, _ForwardIterator __first, _ForwardIterator __last, _Pred __pred) {
   _LIBCPP_REQUIRE_CPP17_FORWARD_ITERATOR(_ForwardIterator);
-  auto __res = std::__none_of(__policy, std::move(__first), std::move(__last), std::move(__pred));
-  if (!__res)
-    std::__throw_bad_alloc();
-  return *std::move(__res);
+  using _Implementation = __pstl::__none_of<__pstl::__configured_backend, _RawPolicy>;
+  return __pstl::__run_backend<_Implementation>(
+      std::forward<_ExecutionPolicy>(__policy), std::move(__first), std::move(__last), std::move(__pred));
 }
 
 _LIBCPP_END_NAMESPACE_STD