Treat user defined backward derivative function as non differentiable. (#2650)

Co-authored-by: Yong He <yhe@nvidia.com>

4 files changed, 33 insertions, 7 deletions

diff --git a/source/slang/slang-ir-check-differentiability.cpp b/source/slang/slang-ir-check-differentiability.cpp
index e7d5a0e5c..a3f7ae7c0 100644
--- a/source/slang/slang-ir-check-differentiability.cpp
+++ b/source/slang/slang-ir-check-differentiability.cpp
@@ -165,7 +165,6 @@ public:
 
         HashSet<IRInst*> produceDiffSet;
         HashSet<IRInst*> expectDiffSet;
-        int differentiableInputs = 0;
         int differentiableOutputs = 0;
         for (auto param : funcInst->getFirstBlock()->getParams())
         {
@@ -173,8 +172,6 @@ public:
             {
                 if (as<IROutTypeBase>(param->getFullType()))
                     differentiableOutputs++;
-                if (!as<IROutType>(param->getFullType()))
-                    differentiableInputs++;
                 produceDiffSet.Add(param);
             }
         }
@@ -186,8 +183,6 @@ public:
 
         if (differentiableOutputs == 0)
             sink->diagnose(funcInst, Diagnostics::differentiableFuncMustHaveOutput);
-        if (differentiableInputs == 0)
-            sink->diagnose(funcInst, Diagnostics::differentiableFuncMustHaveInput);
 
         DifferentiableLevel requiredDiffLevel = DifferentiableLevel::Forward;
         if (isBackwardDifferentiableFunc(funcInst))
diff --git a/source/slang/slang-lower-to-ir.cpp b/source/slang/slang-lower-to-ir.cpp
index 9cf6aedf4..957f74d40 100644
--- a/source/slang/slang-lower-to-ir.cpp
+++ b/source/slang/slang-lower-to-ir.cpp
@@ -8436,8 +8436,6 @@ struct DeclLoweringVisitor : DeclVisitor<DeclLoweringVisitor, LoweredValInfo>
                 else
                 {
                     originalSubBuilder->addUserDefinedBackwardDerivativeDecoration(originalFuncVal, derivativeFuncVal.val);
-                    getBuilder()->addForwardDifferentiableDecoration(irFunc);
-                    getBuilder()->addBackwardDifferentiableDecoration(irFunc);
                 }
             }
             subContext->irBuilder->setInsertInto(irFunc);
diff --git a/tests/autodiff/backward-diff-check.slang b/tests/autodiff/backward-diff-check.slang
new file mode 100644
index 000000000..2718f31f1
--- /dev/null
+++ b/tests/autodiff/backward-diff-check.slang
@@ -0,0 +1,27 @@
+//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 0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<float> outputBuffer;
+
+typedef DifferentialPair<float> dpfloat;
+typedef float.Differential dfloat;
+
+[BackwardDifferentiable]
+float test()
+{
+    return 2.0f;
+}
+
+float noDiffFunc(float x) { return x; }
+[BackwardDerivativeOf(test)]
+void d_test(float dOut)
+{
+    outputBuffer[0] = noDiffFunc(dOut);
+}
+
+[numthreads(1, 1, 1)]
+void computeMain(uint3 dispatchThreadID : SV_DispatchThreadID)
+{
+    __bwd_diff(test)(3.0f);
+}
diff --git a/tests/autodiff/backward-diff-check.slang.expected.txt b/tests/autodiff/backward-diff-check.slang.expected.txt
new file mode 100644
index 000000000..72bbdbbcd
--- /dev/null
+++ b/tests/autodiff/backward-diff-check.slang.expected.txt
@@ -0,0 +1,6 @@
+type: float
+3.000000
+0.000000
+0.000000
+0.000000
+0.000000