Fix non-square matrix derivatives (shader-slang#6282)

Author: saipraveenb25

source/slang/slang-ir-autodiff-transpose.h

@@ -1812,8 +1812,8 @@ struct DiffTransposePass
     {
         List<RevGradient> gradients;
         auto matrixType = as<IRMatrixType>(fwdMakeMatrix->getDataType());
-        auto row = as<IRIntLit>(matrixType->getRowCount());
         auto colCount = matrixType->getColumnCount();
+        auto colCountVal = as<IRIntLit>(matrixType->getColumnCount())->getValue();
         IRType* rowVectorType = nullptr;
         for (UIndex ii = 0; ii < fwdMakeMatrix->getOperandCount(); ii++)
         {
@@ -1828,9 +1828,8 @@ struct DiffTransposePass
             }
             else
             {
-                SLANG_RELEASE_ASSERT(row);
-                UInt rowIndex = ii / (UInt)row->getValue();
-                UInt colIndex = ii % (UInt)row->getValue();
+                UInt rowIndex = ii / (UInt)colCountVal;
+                UInt colIndex = ii % (UInt)colCountVal;
                 if (!rowVectorType)
                     rowVectorType = builder->getVectorType(matrixType->getElementType(), colCount);
                 auto revRow = builder->emitElementExtract(

tests/autodiff/matrix-arithmetic-non-sqr.slang

@@ -0,0 +1,41 @@
+//TEST(compute):COMPARE_COMPUTE_EX(filecheck-buffer=CHECK):-slang -compute -shaderobj -output-using-type
+
+//TEST_INPUT:ubuffer(data=[0 0 0 0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<float> outputBuffer;
+
+[Differentiable]
+float h_outer(float a, float b, float c)
+{
+    const float3x2 m2 = float3x2(2 * a, 0.0,
+        0.0, 3 * b,
+        0.0, 5 * c);
+
+    const float3x3 m1 = float3x3(1.f);
+    return h(m1, m2);
+}
+
+[Differentiable]
+float h(float3x3 x, float3x2 y)
+{
+    let res = mul(x, y);
+    return dot(mul(res, float2(1.0)), float3(1.0));
+}
+
+[numthreads(1, 1, 1)]
+void computeMain(uint3 dispatchThreadID : SV_DispatchThreadID)
+{
+    // Do a bwd_diff test for h_outer
+    var dpa = diffPair(1.f, 0.f);
+    var dpb = diffPair(1.f, 0.f);
+    var dpc = diffPair(1.f, 0.f);
+    bwd_diff(h_outer)(dpa, dpb, dpc, 1.f);
+
+    outputBuffer[1] = dpa.d;
+    outputBuffer[2] = dpb.d;
+    outputBuffer[3] = dpc.d;
+
+    // CHECK: type: float
+    // CHECK: 6.0
+    // CHECK: 9.0
+    // CHECK: 15.0
+}