Add forward-mode AD support for std::thread. by a-elrawy · Pull Request #1891 · vgvassilev/clad

a-elrawy · 2026-07-14T17:39:26Z

Adds forward-mode custom derivatives for std::thread (constructors, join, joinable, detach) and extends ForwardMode/Concurrency.C. The worker runs the callable’s pushforward so mutations through std::ref update derivatives (free functions and named functors; lambdas not yet supported). Also generalizes isCopyable and uses std::move for non-copyable custom constructor pushforward returns so differentiated code compiles.

github-actions

clang-tidy made some suggestions

codecov · 2026-07-14T18:13:32Z

Codecov Report

❌ Patch coverage is 88.05970% with 8 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
lib/Differentiator/BaseForwardModeVisitor.cpp	85.96%	8 Missing ⚠️

📢 Thoughts on this report? Let us know!

github-actions

clang-tidy made some suggestions

github-actions · 2026-07-21T23:44:28Z

clang-tidy review says "All clean, LGTM! 👍"

Copilot

Pull request overview

Adds forward-mode (pushforward) custom-derivative coverage needed for std::thread-based code paths to compile under Clad, and introduces/extends forward-mode tests around concurrency patterns using std::reference_wrapper and std::thread. Also tightens copyability detection and updates forward-mode constructor handling to move out non-copyable ValueAndPushforward results.

Changes:

Add forward-mode custom derivatives for std::thread construction and basic member ops (join, joinable) in STLBuiltins.h.
Generalize copyability detection (isCopyable) to treat deleted/inaccessible copy ctors as non-copyable.
Update forward-mode constructor handling to std::move non-copyable value/pushforward members out of custom-derivative constructor returns.
Add/extend ForwardMode/Concurrency.C tests covering reference_wrapper and std::thread usage.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
test/ForwardMode/Concurrency.C	Adds forward-mode tests exercising `reference_wrapper` and `std::thread` constructs.
lib/Differentiator/CladUtils.cpp	Expands `isCopyable` to detect inaccessible/deleted copy constructors.
lib/Differentiator/BaseForwardModeVisitor.cpp	Moves non-copyable constructor pushforward results to avoid copy attempts.
include/clad/Differentiator/STLBuiltins.h	Introduces `std::thread` pushforwards and additional `reference_wrapper` pushforwards.

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github-actions · 2026-07-22T13:36:51Z

clang-tidy review says "All clean, LGTM! 👍"

vgvassilev · 2026-07-23T07:25:14Z

+  if (!clonedArgs.empty() && !derivedArgs.empty()) {
+    QualType recTy = CE->getType().getNonReferenceType().getCanonicalType();
+    if (const auto* RD = recTy->getAsCXXRecordDecl()) {
+      if (RD->getName() == "thread" && RD->isInStdNamespace() &&


Why do we have to special case here?

Constructing std::thread doesn’t call the callable, so VisitCallExpr never nest-diffs it. The callable’s tangent is just {}/nullptr, not F_pushforward, which the custom derivative needs to run AD through the worker. We can generalize this later if you prefer.

github-actions

clang-tidy made some suggestions

github-actions · 2026-07-25T20:48:46Z

clang-tidy review says "All clean, LGTM! 👍"

vgvassilev

This adds forward-mode differentiation for std::thread: it finds the
function a thread was started with, generates that function's derivative,
and gives the thread a value/derivative pair. The approach looks right.
I would split it up before landing it.

Scope: this is three changes in one pull request.
The title says std::thread, but about half the diff is
std::reference_wrapper -- the get_pushforward and
operator_T_amp_pushforward overloads in STLBuiltins.h, and the
f_ref, f_cref, f_implicit, f_twice_ref and mul_refs tests. The
isCopyable edit is a third. Each is useful and each can be reviewed on
its own. Three commits would be easier to review and easier to bisect.

Three more findings are on the lines they concern.

What I could not check: I read the change rather than building it, so I do
not know whether the generated code compiles. The test checks 20 runtime
results but only 5 lines of generated code, so a future change to how the
thread pair is built would only be noticed if the numbers moved. I also
could not tell whether the constructor_pushforward overload taking
F&& f, ..., F&& d_f is ever used: F is deduced from both arguments, so
they must have the same type, and I found no test that reaches it.

These comments come from ai-vgvassilev: written by an AI reviewer and posted after @vgvassilev read them.

vgvassilev · 2026-08-25T13:48:28Z

+      // thread).
+      for (const clang::CXXConstructorDecl* Ctor : RD->ctors())
+        if (Ctor->isCopyConstructor() &&
+            (Ctor->isDeleted() || Ctor->getAccess() != clang::AS_public))


This changes a shared function, and nothing in this PR tests the change.

isCopyable now answers "no" for any class whose copy constructor is
private or protected. That is a reasonable answer. The problem is who else
asks: six places in ReverseModeVisitor call isCopyable, and they now
take a different path for every class with a private copy constructor.
This PR only adds forward-mode tests, so nothing here checks reverse mode.

Please move this to its own commit with a reverse-mode test.

vgvassilev · 2026-08-25T13:48:28Z

+  LookupResult R(SemaRef, DN, noLoc, Sema::LookupMemberName);
+  R.suppressDiagnostics();
+  SemaRef.LookupQualifiedName(R, RD);
+  if (!R.isSingleResult())


clad stays silent when it cannot find the thread's function.

resolveThreadCallable returns nothing for a generic lambda, an
overloaded operator(), a std::function, or a std::bind result. When
that happens the code just skips ahead. No derivative is generated and
nothing is reported. The user gets a wrong number back and no reason to
suspect it.

A wrong answer is worse than an error here. Report it with
DiagnosticsEngine::Report, pointing at the constructor and naming the
callable that could not be resolved.

vgvassilev · 2026-08-25T13:48:28Z

+          pushforwardFnRequest.Functor = MD->getParent();
+        FunctionDecl* pushforwardFD =
+            m_Builder.HandleNestedDiffRequest(pushforwardFnRequest);
+        if (pushforwardFD && !isa<CXXMethodDecl>(callableFD))


A derivative is generated and then thrown away.

HandleNestedDiffRequest runs for every callable it resolves, but the
result is only used when the callable is not a class method. For a functor
it is dropped. I think that is on purpose: the functor's derivative is
reached later through operator_call_pushforward, so the call is only
here to make clad emit that method.

If so, please say that in a comment. As written it looks like a bug, and
whoever reads it next will either "fix" it or delete it.

Add custom pushforwards in STLBuiltins.h and forward-mode tests.

github-actions

clang-tidy made some suggestions

github-actions

clang-tidy made some suggestions

vgvassilev · 2026-08-25T19:16:56Z

We still need the reverse of this.

github-actions

clang-tidy made some suggestions

github-actions · 2026-08-25T19:36:23Z

clang-tidy review says "All clean, LGTM! 👍"

a-elrawy · 2026-08-25T19:50:04Z

We still need the reverse of this.

I've started working on it. Should I make a new PR for the reverse mode or extend the current one?

github-actions · 2026-08-25T19:51:28Z

clang-tidy review says "All clean, LGTM! 👍"

vgvassilev · 2026-08-26T05:14:45Z

We still need the reverse of this.

I've started working on it. Should I make a new PR for the reverse mode or extend the current one?

I’d prefer to be a single PR.

a-elrawy force-pushed the gsoc/concurrency-thread-forward branch 2 times, most recently from 0bf129e to 9d1e866 Compare July 14, 2026 17:52

github-actions Bot reviewed Jul 14, 2026

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Comment thread include/clad/Differentiator/STLBuiltins.h

a-elrawy force-pushed the gsoc/concurrency-thread-forward branch 3 times, most recently from 3b26d88 to 29be48d Compare July 21, 2026 23:25

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Comment thread lib/Differentiator/CladUtils.cpp Outdated

a-elrawy force-pushed the gsoc/concurrency-thread-forward branch from 29be48d to 2a51726 Compare July 21, 2026 23:37

a-elrawy marked this pull request as ready for review July 22, 2026 02:57

Copilot AI review requested due to automatic review settings July 22, 2026 02:57

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Comment thread include/clad/Differentiator/STLBuiltins.h

Comment thread include/clad/Differentiator/STLBuiltins.h Outdated

Comment thread include/clad/Differentiator/STLBuiltins.h Outdated

Comment thread test/ForwardMode/Concurrency.C Outdated

a-elrawy force-pushed the gsoc/concurrency-thread-forward branch from 2a51726 to 1973f4e Compare July 22, 2026 13:28

vgvassilev reviewed Jul 23, 2026

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a-elrawy force-pushed the gsoc/concurrency-thread-forward branch 2 times, most recently from 6207920 to 4b781ab Compare July 25, 2026 20:30

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Comment thread include/clad/Differentiator/STLBuiltins.h

Comment thread include/clad/Differentiator/STLBuiltins.h

Comment thread include/clad/Differentiator/STLBuiltins.h

Comment thread lib/Differentiator/BaseForwardModeVisitor.cpp

a-elrawy force-pushed the gsoc/concurrency-thread-forward branch from 4b781ab to 26285b3 Compare July 25, 2026 20:41

vgvassilev reviewed Aug 25, 2026

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Add forward-mode AD for std::ref, std::cref, and reference_wrapper.

81f6db3

Add custom pushforwards in STLBuiltins.h and forward-mode tests.

a-elrawy force-pushed the gsoc/concurrency-thread-forward branch from 26285b3 to e1af565 Compare August 25, 2026 18:05

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Comment thread lib/Differentiator/BaseForwardModeVisitor.cpp Outdated

Comment thread lib/Differentiator/BaseForwardModeVisitor.cpp Outdated

Comment thread lib/Differentiator/BaseForwardModeVisitor.cpp Outdated

Treat inaccessible copy constructors as non-copyable.

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Comment thread include/clad/Differentiator/STLBuiltins.h Outdated

Comment thread include/clad/Differentiator/STLBuiltins.h Outdated

Comment thread include/clad/Differentiator/STLBuiltins.h Outdated

a-elrawy force-pushed the gsoc/concurrency-thread-forward branch from 5eb9914 to e2a508b Compare August 25, 2026 19:15

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Comment thread include/clad/Differentiator/STLBuiltins.h

Comment thread include/clad/Differentiator/STLBuiltins.h

Comment thread include/clad/Differentiator/STLBuiltins.h

a-elrawy force-pushed the gsoc/concurrency-thread-forward branch from e2a508b to 4d91545 Compare August 25, 2026 19:30

Add forward-mode AD for std::thread.

51dd1ab

a-elrawy force-pushed the gsoc/concurrency-thread-forward branch from 4d91545 to 51dd1ab Compare August 25, 2026 19:45

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Pull request overview

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