Correct IR generation for no-diff pointer type (shader-slang#5976)

* Correct IR generation for no-diff pointer type

Close shader-slang#5805 

There is an issue on checking whether a pointer type parameter
is no_diff, we should first check whether this parameter is
an Attribute type first, then check the data type.

In the back-propagate pass, for the pointer type parameter, we should
load this parameter to a temp variable, then pass it to the primal
function call. Otherwise, the temp variable will no be initialized,
which will cause the following calculation wrong.

source/slang/slang-ir-autodiff-rev.cpp

@@ -512,11 +512,12 @@ InstPair BackwardDiffTranscriber::transcribeFuncHeader(IRBuilder* inBuilder, IRF
             {
                 // If primal parameter is mutable, we need to pass in a temp var.
                 auto tempVar = builder.emitVar(primalParamPtrType->getValueType());
-                if (primalParamPtrType->getOp() == kIROp_InOutType)
-                {
-                    // If the primal parameter is inout, we need to set the initial value.
-                    builder.emitStore(tempVar, primalArg);
-                }
+
+                // We also need to setup the initial value of the temp var, otherwise
+                // the temp var will be uninitialized which could cause undefined behavior
+                // in the primal function.
+                builder.emitStore(tempVar, primalArg);
+
                 primalArgs.add(tempVar);
             }
             else

source/slang/slang-ir-autodiff-transcriber-base.cpp

@@ -565,6 +565,9 @@ IRType* AutoDiffTranscriberBase::tryGetDiffPairType(IRBuilder* builder, IRType*
     // If this is a PtrType (out, inout, etc..), then create diff pair from
     // value type and re-apply the appropropriate PtrType wrapper.
     //
+    if (isNoDiffType(originalType))
+        return nullptr;
+
     if (auto origPtrType = as<IRPtrTypeBase>(originalType))
     {
         if (auto diffPairValueType = tryGetDiffPairType(builder, origPtrType->getValueType()))

source/slang/slang-ir-autodiff.cpp

@@ -126,13 +126,22 @@ static IRInst* _getDiffTypeWitnessFromPairType(
 
 bool isNoDiffType(IRType* paramType)
 {
-    while (auto ptrType = as<IRPtrTypeBase>(paramType))
-        paramType = ptrType->getValueType();
-    while (auto attrType = as<IRAttributedType>(paramType))
+    while (paramType)
     {
-        if (attrType->findAttr<IRNoDiffAttr>())
+        if (auto attrType = as<IRAttributedType>(paramType))
         {
-            return true;
+            if (attrType->findAttr<IRNoDiffAttr>())
+                return true;
+
+            paramType = attrType->getBaseType();
+        }
+        else if (auto ptrType = as<IRPtrTypeBase>(paramType))
+        {
+            paramType = ptrType->getValueType();
+        }
+        else
+        {
+            return false;
         }
     }
     return false;

tests/autodiff/nodiff-ptr.slang

@@ -0,0 +1,40 @@
+
+[Differentiable]
+float sumOfSquares(float x, float y, no_diff float4* test)
+{
+    return x * x + y * y * (test->x + test->y + test->z);
+}
+
+//TEST(compute, vulkan):COMPARE_COMPUTE_EX:-vk -compute -shaderobj -output-using-type -compile-arg -skip-spirv-validation -emit-spirv-directly
+
+//TEST_INPUT: set ptr = ubuffer(data=[1.0 2.0 3.0], stride=4)
+uniform float* ptr;
+
+//TEST_INPUT:ubuffer(data=[0.0 0.0 0.0 0.0 0.0], stride=4):out, name outputBuffer
+RWStructuredBuffer<float> outputBuffer;
+
+[shader("compute")]
+[numthreads(1, 1, 1)]
+void computeMain()
+{
+    float4* testPtr = (float4*)ptr;
+
+    let result = sumOfSquares(2.0, 3.0, testPtr);
+
+    // Use forward differentiation to compute the gradient of the output w.r.t. x only.
+    let diffX = fwd_diff(sumOfSquares)(diffPair(2.0, 1.0), diffPair(3.0, 0.0), testPtr);
+
+    // Create a differentiable pair to pass in the primal value and to receive the gradient.
+    var dpX = diffPair(2.0);
+    var dpY = diffPair(3.0);
+
+    // Propagate the gradient of the output (1.0f) to the input parameters.
+    bwd_diff(sumOfSquares)(dpX, dpY, testPtr, 1.0);
+
+    outputBuffer[0] = result;     // 2^2 + 3^2 * (1 + 2 + 3) = 58
+    outputBuffer[1] = diffX.d;    // 2*x * dx + 2*y * dy * (1 + 2 + 3) = 4
+    outputBuffer[2] = diffX.p;    // 2^2 + 3^2 * (1 + 2 + 3) = 58
+    outputBuffer[3] = dpX.d;      // 2*x = 4
+
+    outputBuffer[4] = dpY.d;      // 2*y * (1 + 2 +3) = 36
+}

tests/autodiff/nodiff-ptr.slang.expected.txt

@@ -0,0 +1,6 @@
+type: float
+58.000000
+4.000000
+58.000000
+4.000000
+36.000000