Mooncake 0.5.25 compat by sunxd3 · Pull Request #988 · JuliaDiff/DifferentiationInterface.jl

sunxd3 · 2026-04-04T14:47:35Z

An attempt at addressing #986.

Feel free to make any edits or take over!

codecov · 2026-04-04T14:53:27Z

Codecov Report

❌ Patch coverage is 90.47619% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.28%. Comparing base (a5ecbe0) to head (ce72baf).

Files with missing lines	Patch %	Lines
...e/ext/DifferentiationInterfaceMooncakeExt/utils.jl	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #988      +/-   ##
==========================================
- Coverage   98.21%   97.28%   -0.94%     
==========================================
  Files         135      131       -4     
  Lines        8000     7984      -16     
==========================================
- Hits         7857     7767      -90     
- Misses        143      217      +74

Flag	Coverage Δ
DI	`97.86% <90.47%> (-1.12%)`	⬇️
DIT	`95.76% <ø> (-0.47%)`	⬇️

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sunxd3 · 2026-04-04T20:31:01Z

I think I understand the CI error, there is something I need to patch on the Mooncake side, will come back to this

sunxd3 · 2026-04-09T06:50:22Z

chalk-lab/Mooncake.jl#1129 should unblock this PR, we'll release it as soon as it's available

gdalle · 2026-04-10T06:37:18Z

Thank you for taking a crack at this! I'll wait until the tests pass before reviewing if that's okay

sunxd3 · 2026-04-10T07:05:22Z

totally fine!

Mooncake returns raw Tangent objects instead of friendly arrays for StaticArrays on Julia 1.11. This is an upstream bug — skip the test until it is fixed.

On Julia 1.11, Mooncake may return raw Tangent objects instead of friendly arrays for StaticArrays even with friendly_tangents=true. Add _maybe_to_primal dispatch as a safety net that converts leaked Tangent objects to primal-shaped values, no-op otherwise.

Also convert leaked Mooncake.MutableTangent (e.g. MVector tangents) and apply _maybe_to_primal in forward mode (pushforward) paths.

sunxd3 · 2026-04-10T13:40:32Z

@gdalle the Mooncake CIs are passing (it probably requires Mooncake v0.5.26).

Could you take over? I also won't be offended if you want to start a new PR.

gdalle · 2026-04-10T14:24:27Z

I'll take a look when I can, thanks a bunch! DI's tests are failing on main too because of Mooncake's breaking release so this is a priority for me. Do you know why coverage is not complete?

sunxd3 · 2026-04-10T15:11:17Z

Thanks a lot.

Do you know why coverage is not complete?

I am not certain. A bad guess is that some code changes are more defensive than necessary. Sorry!

gdalle

Thank you for trying to fix what others broke! I added a few remarks to understand the task a bit better, I'll wait for your answers

gdalle · 2026-04-16T12:22:37Z

+@inline maybe_getfield(mod, name::Symbol) =
+    isdefined(mod, name) ? getfield(mod, name) : nothing
+
+const mooncake_tangent_to_friendly = maybe_getfield(
+    Mooncake, Symbol("tangent_to_friendly!!")
+)
+const mooncake_friendly_tangent_cache = maybe_getfield(Mooncake, :FriendlyTangentCache)
+const mooncake_as_primal = maybe_getfield(Mooncake, :AsPrimal)
+const mooncake_no_cache = maybe_getfield(Mooncake, :NoCache)


This doesn't seem to be very robust? I'd rather impose a lower bound for Mooncake at v0.5.25 in Project.toml (that way we're sure we can use all of these symbols)

gdalle · 2026-04-16T12:23:19Z

        )
        y = first(y_and_dy)
-        dy = _copy_output(last(y_and_dy))
+        dy = _maybe_to_primal(last(y_and_dy), y)


Why do we need to ensure that primal conversion happens here? If friendly_tangents is set to true, won't Mooncake's pushforward and pullback already return a primal-like object?

gdalle · 2026-04-16T12:25:26Z

+    backend = AutoMooncake(; config = Mooncake.Config(; friendly_tangents = true))
+    inputs = (
+        Symmetric([2.0 1.0; 1.0 3.0]),
+        Hermitian(ComplexF64[2 1 + im; 1 - im 3]),


Do you know which convention Mooncake uses for gradients of functions with complex inputs and real outputs? There are two possible choices, see e.g. https://arxiv.org/abs/2409.06752

See: chalk-lab/Mooncake.jl#773

gdalle · 2026-04-16T12:26:12Z

+    @testset "$(typeof(x))" for x in inputs
+        grad = gradient(f, backend, x)
+        y, grad2 = value_and_gradient(f, backend, x)
+        pb = only(pullback(identity, backend, x, (x,)))


This is not a strong enough test, the function is too simple

gdalle · 2026-04-16T12:26:25Z

+        @test grad isa Matrix
+        @test grad2 isa Matrix
+        @test pb isa Matrix
+        @test grad == grad2


grad and grad2 are never compared against the ground truth

gdalle · 2026-04-16T12:27:16Z

+            !isnothing(mooncake_as_primal) &&
+            !isnothing(mooncake_no_cache)
+        dest = mooncake_friendly_tangent_cache{mooncake_as_primal}(_copy_output(x))
+        cache = isbitstype(typeof(x)) ? mooncake_no_cache() : IdDict{Any, Any}()


Why is a type-unstable dictionary needed here?
Does this make every tangent-to-primal conversion outside of bitstypes slow?

Regardless of why, we may want to allocate this dictionary in the preparation phase

yup this will regress performance on hot paths. This should be allocated once during the prepare phase and stored in the extras cache.

gdalle · 2026-04-16T12:29:40Z

@AstitvaAggarwal @Technici4n could you maybe take a look too?

AstitvaAggarwal

also we might want to keep track of future possible tangent_types: _maybe_to_primal(x, _) = x will silently pass through any tangent type not yet accounted for (e.g. a future Mooncake.SparseTangent), making failures invisible.

AstitvaAggarwal · 2026-04-17T09:03:42Z

+            !isnothing(mooncake_as_primal) &&
+            !isnothing(mooncake_no_cache)
+        dest = mooncake_friendly_tangent_cache{mooncake_as_primal}(_copy_output(x))
+        cache = isbitstype(typeof(x)) ? mooncake_no_cache() : IdDict{Any, Any}()


yup this will regress performance on hot paths. This should be allocated once during the prepare phase and stored in the extras cache.

sunxd3 added 3 commits April 4, 2026 15:37

fix Mooncake friendly_tangents compatibility

75d3c05

format Mooncake fix

f420234

remove Mooncake test formatting churn

8235e6c

sunxd3 requested a review from gdalle as a code owner April 4, 2026 14:47

gdalle marked this pull request as draft April 5, 2026 08:36

sunxd3 added 5 commits April 10, 2026 08:12

ci: retrigger after Mooncake v0.5.26 release

a478578

style: apply Runic formatting to utils.jl

65997c4

test: skip friendly_tangents static_scenarios on Julia 1.11

106f50f

Mooncake returns raw Tangent objects instead of friendly arrays for StaticArrays on Julia 1.11. This is an upstream bug — skip the test until it is fixed.

fix: handle MutableTangent and forward mode tangent leaks

ce72baf

Also convert leaked Mooncake.MutableTangent (e.g. MVector tangents) and apply _maybe_to_primal in forward mode (pushforward) paths.

This was referenced Apr 11, 2026

Mooncake ext: unwrap struct tangents back to primal type #989

Closed

Changes in Mooncake 0.5.25 that may affect DI users #986

Open

gdalle requested changes Apr 16, 2026

View reviewed changes

AstitvaAggarwal reviewed Apr 17, 2026

View reviewed changes

This was referenced Apr 20, 2026

chore(deps): bump julia-actions/setup-julia from 2 to 3 #994

Merged

chore: upper-bound Mooncake version (coherence with General) #995

Merged

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sunxd3 commented Apr 4, 2026

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sunxd3 commented Apr 4, 2026

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