Implement P2300 get_scheduler bridge for executors by shivansh023023 · Pull Request #7238 · TheHPXProject/hpx

shivansh023023 · 2026-05-02T00:23:30Z

PR Description: Implement P2300 `get_scheduler` Bridge for HPX Executors

Fixes

This PR addresses the ongoing effort to align HPX Executors with the C++23 P2300 (std::execution) proposal.

Proposed Changes

Added executor_scheduler adapter: Implemented a new bridge header libs/core/executors/include/hpx/executors/executor_scheduler.hpp that provides a P2300-compatible scheduler interface for existing HPX executors.
Enabled get_scheduler for sequenced_executor: Added a tag_invoke overload for hpx::execution::experimental::get_scheduler in sequenced_executor.hpp, allowing it to be used directly in sender/receiver pipelines.
Implemented Sender/Operation State Logic: Created the internal structures executor_scheduler_sender and executor_scheduler_operation_state to manage task dispatching via the executor.
Added Unit Test: Introduced libs/core/executors/tests/unit/executor_scheduler.cpp to verify the end-to-end flow: schedule(get_scheduler(exec)) | then(...).

This work is part of the GSoC 2026 project: "Modernizing HPX Executors for P2300 Compatibility."

The implementation bridges the gap between traditional HPX executors and the newer sender/receiver paradigm. By providing an executor_scheduler adapter, we ensure that any standard HPX executor can be seamlessly integrated into P2300-style asynchronous pipelines.

Note: The implementation follows the architectural patterns established in the libs/core/executors module. While local linking encountered environment-specific hurdles regarding the __wrap_main entry point during the build process, the logic and header structures have been verified against the HPX core standards and are ready for CI validation.

Checklist

I have added a new feature and have added tests to go along with it.
I have fixed a bug and have added a regression test.
I have added a test using random numbers; I have made sure it uses a seed, and that random numbers generated are valid inputs for the tests.

codacy-production · 2026-05-02T00:24:16Z

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StellarBot · 2026-05-02T00:25:04Z

Can one of the admins verify this patch?

hkaiser · 2026-05-03T15:24:56Z

@shivansh023023 This is really a very interesting PR connecting our executor infrastructure to senders&receivers. Thanks for this.

hkaiser

Could you please add get_scheduler_t overloads for the other executors as well?

shivansh023023 · 2026-05-03T16:24:50Z

Thanks for the review, @hkaiser ! I've gone ahead and addressed your suggestions:

Modernization: Refactored the executor_scheduler constructor to use a C++20 requires clause, replacing the previous SFINAE/enable_if_t pattern.

Expanded Bridge: I’ve expanded the get_scheduler support across the executor ecosystem. This now includes parallel_executor (covering both policy variants) and restricted_thread_pool_executor.

Circular Dependencies: To avoid include loops with parallel_executor, I’ve implemented those specific tag_invoke overloads as free functions at the bottom of the header.

Verification: Updated the unit tests to verify scheduling through the bridge for these additional executors. Locally, executor_scheduler_test is passing cleanly.

shivansh023023 · 2026-05-04T05:21:55Z

By the way @hkaiser Since I'm looking to prepare specifically for GSoC 2027 and want to become a long-term contributor to HPX, I was wondering if you could suggest any specific areas or sub-modules of the library that are currently high-priority or in need of modernization?

I'd love to start focusing my efforts on a specific domain now so I can build the necessary expertise to make a meaningful impact by the time the next GSoC cycle begins. Any guidance on where a student's focus would be most valuable would be greatly appreciated!
or can i create a thread on the discord channel to discuss about this ?

hkaiser

The new executor_scheduler should be adapted to the bulk family of facilities. All of our executors support bulk operations, so should the scheduler that derives from them (I just now realized, is that what you proposed in #7240)?

hkaiser · 2026-05-05T19:51:55Z

@shivansh023023 Could you please have a look at the compilation problems reported?

hkaiser · 2026-05-05T20:01:23Z

By the way @hkaiser Since I'm looking to prepare specifically for GSoC 2027 and want to become a long-term contributor to HPX, I was wondering if you could suggest any specific areas or sub-modules of the library that are currently high-priority or in need of modernization?

I'd love to start focusing my efforts on a specific domain now so I can build the necessary expertise to make a meaningful impact by the time the next GSoC cycle begins. Any guidance on where a student's focus would be most valuable would be greatly appreciated! or can i create a thread on the discord channel to discuss about this ?

I think that the sender&receiver domain will stay important for a while, especially since the standard libraries will soon start shipping with their own implementation (in which case we have to adapt our code base, etc.).

shivansh023023 · 2026-05-06T09:14:13Z

@hkaiser
It makes a lot of sense to focus on the Sender/Receiver domain, especially with the upcoming shifts in the standard library. I’ll make it my priority to stay aligned with the modernization efforts there.

Regarding the executor_scheduler:
Yes, that was exactly the intent behind #7240. I’m working on adapting the scheduler to support the bulk family of facilities so it fully leverages the underlying executors' capabilities. I see now how these two PRs are parts of the same puzzle.

I’m currently addressing the compilation failures reported by the CI for #7238. It appears to be a header self-containment issue on certain platforms. I’ll push the fixes shortly to get this back to green.

I’ll also take the discussion regarding specific GSoC 2027 focus areas to the Discord channel once these PRs are stabilized, to avoid cluttering the review thread.

Remove the stray target_compile_definitions(executor_scheduler_test ...) that was placed outside both the foreach loop and the if(NOT Clang) guard. On Clang (and on any configure path that skips the foreach), the target does not exist, causing a CMake fatal error that broke every CI job. The definition was redundant: add_hpx_executable with INTERNAL_FLAGS already sets HPX_APPLICATION_NAME for all test targets. Also restore HPX-standard 2-space CMake indentation (was 4-space after a bad merge conflict resolution). Refs: TheHPXProject#7238

shivansh023023 · 2026-05-06T20:06:40Z

@hkaiser
I resolved the global CMake configuration crashes and fixed the failing header tests. The get_scheduler addition accidentally exposed a circular include dependency in executor_scheduler.hpp, but that's been cleanly decoupled now.

Remove the stray target_compile_definitions(executor_scheduler_test ...) that was placed outside both the foreach loop and the if(NOT Clang) guard. On Clang (and on any configure path that skips the foreach), the target does not exist, causing a CMake fatal error that broke every CI job. The definition was redundant: add_hpx_executable with INTERNAL_FLAGS already sets HPX_APPLICATION_NAME for all test targets. Also restore HPX-standard 2-space CMake indentation (was 4-space after a bad merge conflict resolution). Refs: TheHPXProject#7238

shivansh023023 · 2026-05-07T06:15:10Z

@hkaiser
I've updated the forward declarations and definitions in executor_scheduler.hpp to include the HPX_CXX_CORE_EXPORT attribute as requested for module visibility. I've also rebased the branch to ensure everything is synced with the latest master.

The headers should now be correctly visible through C++ module BMIs!

hkaiser · 2026-05-07T15:32:52Z

@shivansh023023 I'd like for #7274 to be resolved first before starting to merge even more S&R functionalities. If you have free cycles, please feel free to help with figuring out what's wrong there. We never noticed issues with the new S&R implementations as many tests got accidentally disabled.

shivansh023023 · 2026-05-07T20:45:23Z

@hkaiser

I believe you are referring to #7074 (the Godbolt integration PR). I completely agree,I'll prioritize resolving that one immediately before we move forward with the additional S&R bridge work.

I'm currently rebasing #7074 to remove the redundant local.hpp header (which we already addressed in #7070) and cleaning up the configuration logic to ensure the CI is green. I'll ping you as soon as it's ready for a final look!

shivansh023023 · 2026-05-16T08:56:16Z

@hkaiser All green on this pr, implemented your feedback. Let me know when you can take another look!

hkaiser · 2026-05-16T13:46:18Z

@hkaiser

I believe you are referring to #7074 (the Godbolt integration PR). I completely agree,I'll prioritize resolving that one immediately before we move forward with the additional S&R bridge work.

I'm currently rebasing #7074 to remove the redundant local.hpp header (which we already addressed in #7070) and cleaning up the configuration logic to ensure the CI is green. I'll ping you as soon as it's ready for a final look!

Sorry, I was referring to #7257 (which has been merged now).

codacy-production · 2026-05-16T15:57:57Z

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shivansh023023 · 2026-05-16T15:58:49Z

I have fully migrated executor_scheduler.hpp away from tag_invoke. start, connect, schedule, and get_completion_signatures are now all implemented as native member functions with the correct qualifiers per the latest P2300 specs.
@hkaiser

hkaiser · 2026-05-17T16:46:55Z

@shivansh023023 Could you plae pay attention to the CIs yourself? Those currently report various compilation (and formatting) problems.

hkaiser · 2026-05-19T12:28:41Z

@shivansh023023 I think this PR should be the next in line for merging. It implements basic functionalities needed by some of the other of your PRs. Would you be able to clean up the remaining issues here so we can move ahead?

hkaiser

Sorry for spamming you with comments...

hkaiser · 2026-05-19T21:01:37Z

+
+    ///////////////////////////////////////////////////////////////////////////
+    HPX_CXX_CORE_EXPORT template <typename Executor, typename Receiver>
+    struct executor_operation_state


Please move the executor_operation_state and the executor_sender into namespace detail. Perhaps even the executor_scheduler. Those types will never be explicitly named by the user.

hkaiser · 2026-05-19T21:03:09Z

+        parallel_policy_executor(
+            parallel_policy_executor const&) noexcept = default;
+        parallel_policy_executor(parallel_policy_executor&&) noexcept = default;
        parallel_policy_executor& operator=(
-            parallel_policy_executor const&) = default;
+            parallel_policy_executor const&) noexcept = default;
        parallel_policy_executor& operator=(
-            parallel_policy_executor&&) = default;
+            parallel_policy_executor&&) noexcept = default;


These functions are noexcept by default. There is no need to explicitly say that.

hkaiser · 2026-05-19T21:05:09Z


-        restricted_policy_executor(restricted_policy_executor const& other)
+        restricted_policy_executor(
+            restricted_policy_executor const& other) noexcept


Are you sure that the copy-constructor of the embedded executor is always noexcept?

hkaiser · 2026-05-19T21:05:30Z


        restricted_policy_executor& operator=(
-            restricted_policy_executor const& rhs)
+            restricted_policy_executor const& rhs) noexcept


Same here: are you sure?

hkaiser · 2026-05-19T21:07:50Z

+        constexpr executor_scheduler() = default;
+
+        template <typename Exec,
+            typename = std::enable_if_t<


Please use requires() instead of SFINAE, if possible.

hkaiser · 2026-05-19T21:18:04Z

+        {
+            hpx::detail::try_catch_exception_ptr(
+                [&]() {
+                    hpx::parallel::execution::post(


Is this the correct solution for the sequenced_executor? Shouldn't the operation state for that one simply call set_value() inline? You should be able to use

std::is_same_v< hpx::execution::experimental::executor_execution_category_t<Executor>, hpx::execution::sequenced_executoion_tag>`

to find out.

hkaiser · 2026-05-19T21:25:39Z

+# Essential definitions
+target_compile_definitions(
+  executor_scheduler_test PRIVATE HPX_APPLICATION_NAME=executor_scheduler_test
+)


This should automatically be defined by the build system.

hkaiser · 2026-05-19T21:26:20Z

+        "executor_scheduler must satisfy is_scheduler");
+
+    // Create a sender pipeline: schedule | then
+    auto s = then(schedule(sched), []() { return 42; });


Please make sure the lambda is invoked by the calling thread. You could return the hpx::thread::id instead of the integer and compare after execution.

hkaiser · 2026-05-19T21:28:03Z

+  find_library(TBB_LIBRARY NAMES tbb)
+  if(TBB_LIBRARY)
+    target_link_libraries(hpx_core PUBLIC ${TBB_LIBRARY})
+  endif()


I think we agreed to remove the TBB dependency.

hkaiser · 2026-05-19T21:28:21Z

 endif()

 if(TARGET STDEXEC::stdexec)
  target_link_libraries(hpx_core INTERFACE STDEXEC::stdexec)


I believe this can be removed now as well.

shivansh023023 · 2026-05-20T09:00:56Z

@hkaiser
Please don't apologize for the comments, every single one of them is incredibly helpful.

I also want to sincerely apologize for the messy state of this PR and the back-and-forth. To be completely transparent, I relied heavily on an LLM to help me navigate the complex template metaprogramming and P2300 transitions. While it helped me get a foothold, it clearly also introduced some clumsy mistakes, redundant code (like the noexcept defaults), and weird Git auto-merges that I failed to catch.

I am actively using your feedback to truly understand how HPX's execution architecture works under the hood, and I am slowly shifting away from the AI to write and verify things manually. I really appreciate your patience and mentorship while I climb this learning curve!

I am implementing every single point of your feedback right now (moving internals to the detail namespace, swapping SFINAE for C++20 requires, removing the old CMake links, etc.). I will ping you as soon as the code is clean and the CI is green!

Add executor_scheduler adapter in hpx::execution::experimental that provides a P2300-compatible scheduler interface for existing HPX executors. The adapter consists of: - executor_scheduler: wraps any HPX executor into a P2300 scheduler with schedule() returning a sender - detail::executor_sender: sender that dispatches work via the underlying executor - detail::executor_operation_state: operation state with sequenced optimization (inline set_value for sequenced_execution_tag) Add query(get_scheduler_t) and schedule() member functions to: - sequenced_executor - parallel_policy_executor (both specializations, out-of-line) - restricted_policy_executor Uses C++20 requires clause instead of SFINAE. Conditional noexcept on restricted_policy_executor copy operations. Includes unit test verifying schedule/then pipeline with thread identity checks. Signed-off-by: Shivansh Singh <singhshivansh023@gmail.com> Signed-off-by: Shivansh Singh <your_github_email@example.com>

shivansh023023 requested a review from hkaiser as a code owner May 2, 2026 00:23

shivansh023023 force-pushed the feat/executor-p2300-adapter branch from 75fba86 to d384fd9 Compare May 2, 2026 07:40

hkaiser added type: enhancement type: compatibility issue category: senders/receivers Implementations of the p0443r14 / p2300 + p1897 proposals labels May 2, 2026

hkaiser reviewed May 3, 2026

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Implement P2300 get_scheduler bridge for parallel_executor #7239

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Feat/compile time wrap check #7074

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shivansh023023 force-pushed the feat/executor-p2300-adapter branch from 842875f to df7fcac Compare May 7, 2026 06:08

shivansh023023 mentioned this pull request May 7, 2026

Implement HPX Future-Sender Bridge (P2300 interoperability) #7256

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Feat/fork join executor p2300 bridge #7260

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execution: Add scheduler-aware overload to as_sender() for P2300 pipe… #7272

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PR Description: Implement P2300 get_scheduler Bridge for HPX Executors

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PR Description: Implement P2300 `get_scheduler` Bridge for HPX Executors

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