Support `IDifferentiablePtrType` (#5031)

* initial diff-ref-type interface

* Initial support for `IDifferentiablePtrType`

* Fix unused vars

* More tests + fix switch case fallthrough.

* Update slang-ir-autodiff.cpp

* Update diff-ptr-type-loop.slang

* Add optimization to allow more complex pair types

* Update slang-ir-autodiff-primal-hoist.cpp

* Update diff-ptr-type-loop.slang

* Update slang-ir-autodiff-primal-hoist.cpp

* More fixes to address reviews

* Update slang-check-expr.cpp

* Optimizations + rename `differentiableRefInterfaceType` -> `differentiablePtrInterfaceType`

* Move pair logic to ir-builder, unify the type dictionaries.

---------

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

3 files changed, 171 insertions, 0 deletions

diff --git a/tests/autodiff/diff-ptr-type-call.slang b/tests/autodiff/diff-ptr-type-call.slang
new file mode 100644
index 000000000..258a4477b
--- /dev/null
+++ b/tests/autodiff/diff-ptr-type-call.slang
@@ -0,0 +1,57 @@
+//TEST(compute):COMPARE_COMPUTE(filecheck-buffer=CHECK): -shaderobj -output-using-type
+//TEST(compute):COMPARE_COMPUTE(filecheck-buffer=CHECK): -vk -shaderobj -output-using-type
+
+//TEST_INPUT:ubuffer(data=[0 0 0 0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<float> outputBuffer;
+
+// ----- MyPtrType definition -----
+struct MyPtrType : IDifferentiablePtrType
+{
+    typealias Differential = MyPtrType;
+
+    RWStructuredBuffer<float> buffer;
+    uint offset;
+
+    float load(uint idx) { return buffer[offset + idx]; }
+    void accumulate(uint idx, float value) { buffer[offset + idx] += value; }
+}
+
+[BackwardDerivative(load_bwd)]
+float load(MyPtrType b, uint idx)
+{
+    return b.load(idx);
+}
+
+void load_bwd(DifferentialPtrPair<MyPtrType> b, uint idx, float grad)
+{
+    b.d.accumulate(idx, grad);
+}
+
+// ------
+[Differentiable]
+float reduce(MyPtrType a)
+{
+    return load(a, 0) + load(a, 1);
+}
+
+[Differentiable]
+float test(MyPtrType b)
+{
+    return reduce(b);
+}
+
+[numthreads(1, 1, 1)]
+void computeMain(uint id : SV_DispatchThreadID)
+{
+    outputBuffer[0] = 1; // CHECK: 1
+    outputBuffer[1] = 2; // CHECK: 2
+
+    // Denote the first two elements in the buffer as the primal buffer and the last two elements for the derivative.
+    var b = DifferentialPtrPair<MyPtrType>( { outputBuffer, 0 }, { outputBuffer, 2 } );
+
+    bwd_diff(test)(b, 1.5f);
+
+    // Check locations [2] and [3] in the buffer
+    // CHECK: 1.5
+    // CHECK: 1.5
+}
\ No newline at end of file
diff --git a/tests/autodiff/diff-ptr-type-loop.slang b/tests/autodiff/diff-ptr-type-loop.slang
new file mode 100644
index 000000000..a57c69b76
--- /dev/null
+++ b/tests/autodiff/diff-ptr-type-loop.slang
@@ -0,0 +1,65 @@
+//TEST(compute):COMPARE_COMPUTE(filecheck-buffer=CHECK): -shaderobj -output-using-type
+//TEST(compute, vulkan):COMPARE_COMPUTE(filecheck-buffer=CHECK): -vk -shaderobj -output-using-type
+
+//TEST_INPUT:ubuffer(data=[0 0 0 0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<float> outputBuffer;
+
+// ----- MyPtrType definition -----
+struct MyPtrType : IDifferentiablePtrType
+{
+    typealias Differential = MyPtrType;
+
+    RWStructuredBuffer<float> buffer;
+    uint offset;
+
+    float load(uint idx) { return buffer[offset + idx]; }
+    void accumulate(uint idx, float value) { buffer[offset + idx] += value; }
+}
+
+[BackwardDerivative(load_bwd)]
+float load(MyPtrType b, uint idx)
+{
+    return b.load(idx);
+}
+
+void load_bwd(DifferentialPtrPair<MyPtrType> b, uint idx, float grad)
+{
+    b.d.accumulate(idx, grad);
+}
+
+
+// ------
+[Differentiable]
+float reduce(MyPtrType a, uint num)
+{   
+    float sum = 0;
+    [MaxIters(3)]
+    for (uint i = 0; i < num; i++)
+    {
+        sum += load(a, i);
+    }
+
+    return sum;
+}
+
+[Differentiable]
+float test(MyPtrType b, uint num)
+{
+    return reduce(b, num);
+}
+
+[numthreads(1, 1, 1)]
+void computeMain(uint id : SV_DispatchThreadID)
+{
+    outputBuffer[0] = 1; // CHECK: 1
+    outputBuffer[1] = 2; // CHECK: 2
+
+    // Denote the first two elements in the buffer as the primal buffer and the last two elements for the derivative.
+    var b = DifferentialPtrPair<MyPtrType>( { outputBuffer, 0 }, { outputBuffer, 2 } );
+
+    bwd_diff(test)(b, 2, 1.5f);
+
+    // Check locations [2] and [3] in the buffer
+    // CHECK: 1.5
+    // CHECK: 1.5
+}
\ No newline at end of file
diff --git a/tests/autodiff/diff-ptr-type-smoke.slang b/tests/autodiff/diff-ptr-type-smoke.slang
new file mode 100644
index 000000000..e7e03c5e3
--- /dev/null
+++ b/tests/autodiff/diff-ptr-type-smoke.slang
@@ -0,0 +1,49 @@
+//TEST(compute):COMPARE_COMPUTE(filecheck-buffer=CHECK): -shaderobj -output-using-type
+//TEST(compute):COMPARE_COMPUTE(filecheck-buffer=CHECK): -vk -shaderobj -output-using-type
+
+//TEST_INPUT:ubuffer(data=[0 0 0 0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<float> outputBuffer;
+
+struct MyPtrType : IDifferentiablePtrType
+{
+    typealias Differential = MyPtrType;
+
+    RWStructuredBuffer<float> buffer;
+    uint offset;
+
+    float load(uint idx) { return buffer[offset + idx]; }
+    void accumulate(uint idx, float value) { buffer[offset + idx] += value; }
+}
+
+[BackwardDerivative(load_bwd)]
+float load(MyPtrType b, uint idx)
+{
+    return b.load(idx);
+}
+
+void load_bwd(DifferentialPtrPair<MyPtrType> b, uint idx, float grad)
+{
+    b.d.accumulate(idx, grad);
+}
+
+[BackwardDifferentiable]
+float test(MyPtrType b, uint idx)
+{
+    return load(b, idx) + load(b, idx + 1);
+}
+
+[numthreads(1, 1, 1)]
+void computeMain(uint id : SV_DispatchThreadID)
+{
+    outputBuffer[0] = 1; // CHECK: 1
+    outputBuffer[1] = 2; // CHECK: 2
+
+    // Denote the first two elements in the buffer as the primal buffer and the last two elements for the derivative.
+    var b = DifferentialPtrPair<MyPtrType>( { outputBuffer, 0 }, { outputBuffer, 2 } );
+
+    bwd_diff(test)(b, id, 1.5f);
+
+    // Check locations [2] and [3] in the buffer
+    // CHECK: 1.5
+    // CHECK: 1.5
+}
\ No newline at end of file