Register `IDifferentiable` witnesses for the differential associated types of registered types. (#2777)

* Create high-order-forward-diff-struct.slang

* Add a small pass to register differentials for differentials of registered types

* Remove type reg pass and apply logic in differentiable type loading instead

* Update slang-ir-autodiff.cpp

* Fix comments.

---------

Co-authored-by: Yong He <yonghe@outlook.com>

3 files changed, 65 insertions, 0 deletions

diff --git a/source/slang/slang-ir-autodiff.cpp b/source/slang/slang-ir-autodiff.cpp
index 024d31fd8..3257ee102 100644
--- a/source/slang/slang-ir-autodiff.cpp
+++ b/source/slang/slang-ir-autodiff.cpp
@@ -399,6 +399,16 @@ void DifferentiableTypeConformanceContext::setFunc(IRGlobalValueWithCode* func)
             else
             {
                 differentiableWitnessDictionary.Add((IRType*)item->getConcreteType(), item->getWitness());
+
+                // Also register the type's differential type with the same witness.
+                IRBuilder subBuilder(item->getConcreteType());
+                if (!as<IRInterfaceType>(item->getConcreteType()))
+                {
+                    differentiableWitnessDictionary.AddIfNotExists(
+                        (IRType*)_lookupWitness(&subBuilder, item->getWitness(), sharedContext->differentialAssocTypeStructKey), 
+                        item->getWitness());
+                }
+
                 if (auto diffPairType = as<IRDifferentialPairTypeBase>(item->getConcreteType()))
                 {
                     // For differential pair types, register the differential type as well.
diff --git a/tests/autodiff/high-order-forward-diff-struct.slang b/tests/autodiff/high-order-forward-diff-struct.slang
new file mode 100644
index 000000000..7e39640a1
--- /dev/null
+++ b/tests/autodiff/high-order-forward-diff-struct.slang
@@ -0,0 +1,50 @@
+//TEST(compute):COMPARE_COMPUTE_EX:-slang -compute -shaderobj -output-using-type
+//TEST(compute, vulkan):COMPARE_COMPUTE_EX:-vk -compute -shaderobj -output-using-type
+
+//TEST_INPUT:ubuffer(data=[0 0 0 0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<float> outputBuffer;
+
+struct MyStruct : IDifferentiable
+{
+    float x;
+    uint i;
+
+    [ForwardDifferentiable]
+    __init(float x, uint i)
+    {
+        this.x = x;
+        this.i = i;
+    }
+};
+
+[ForwardDifferentiable]
+float mySqr(MyStruct s)
+{
+    if (s.i >= 1)
+        return s.x * s.x;
+    else
+        return 0.f;
+}
+
+[ForwardDifferentiable]
+float f(float x)
+{
+    MyStruct ms = MyStruct(x * x, 1);
+    return mySqr(ms);
+}
+
+[ForwardDifferentiable]
+float df(float x)
+{
+    return __fwd_diff(f)(DifferentialPair<float>(x, 1.0)).d;
+}
+
+[numthreads(1, 1, 1)]
+void computeMain(uint3 dispatchThreadID : SV_DispatchThreadID)
+{
+    // Given f(x) = x^4,
+    // f''(x) = 12 * x^2
+    // Expect f''(4) = 192
+    outputBuffer[0] = __fwd_diff(df)(DifferentialPair<float>(4.0, 1.0)).d;
+}
+
diff --git a/tests/autodiff/high-order-forward-diff-struct.slang.expected.txt b/tests/autodiff/high-order-forward-diff-struct.slang.expected.txt
new file mode 100644
index 000000000..0f08247f0
--- /dev/null
+++ b/tests/autodiff/high-order-forward-diff-struct.slang.expected.txt
@@ -0,0 +1,5 @@
+type: float
+192.000000
+0.000000
+0.000000
+0.000000