1 files changed, 37 insertions, 2 deletions

diff --git a/source/slang/slang-ir-autodiff-transcriber-base.cpp b/source/slang/slang-ir-autodiff-transcriber-base.cpp
index 73d9b6ba6..ed122c862 100644
--- a/source/slang/slang-ir-autodiff-transcriber-base.cpp
+++ b/source/slang/slang-ir-autodiff-transcriber-base.cpp
@@ -72,6 +72,14 @@ bool AutoDiffTranscriberBase::shouldUseOriginalAsPrimal(IRInst* currentParent, I
         return true;
     if (isChildInstOf(currentParent, origInst->getParent()))
         return true;
+
+    // If origInst is defined in the first block of the same function as current inst (e.g. a param),
+    // we can use it as primal.
+    // More generally, we should test if origInst dominates currentParent, but that requires calculating
+    // a dom tree on the fly. Right now just testing if it is first block for parameters seems sufficient.
+    auto parentFunc = getParentFunc(currentParent);
+    if (parentFunc && origInst->parent == parentFunc->getFirstBlock())
+        return true;
     return false;
 }
 
@@ -802,6 +810,7 @@ static void _markGenericChildrenWithoutRelaventUse(IRGeneric* origGeneric, HashS
                 case kIROp_BackwardDerivativePrimalContextDecoration:
                 case kIROp_BackwardDerivativePrimalDecoration:
                 case kIROp_BackwardDerivativePropagateDecoration:
+                case kIROp_PrimalSubstituteDecoration:
                     break;
                 default:
                     if (!outInstsToSkip.Contains(use->getUser()))
@@ -876,6 +885,32 @@ InstPair AutoDiffTranscriberBase::transcribeGeneric(IRBuilder* inBuilder, IRGene
     return InstPair(primalGeneric, diffGeneric);
 }
 
+IRInst* getActualInstToTranscribe(IRInst* inst)
+{
+    if (auto gen = as<IRGeneric>(inst))
+    {
+        auto retVal = findGenericReturnVal(gen);
+        if (retVal->getOp() != kIROp_Func)
+            return inst;
+        if (auto primalSubst = retVal->findDecoration<IRPrimalSubstituteDecoration>())
+        {
+            auto spec = as<IRSpecialize>(primalSubst->getPrimalSubstituteFunc());
+            SLANG_RELEASE_ASSERT(spec);
+            return spec->getBase();
+        }
+    }
+    else if (auto func = as<IRFunc>(inst))
+    {
+        if (auto primalSubst = func->findDecoration<IRPrimalSubstituteDecoration>())
+        {
+            auto actualFunc = as<IRFunc>(primalSubst->getPrimalSubstituteFunc());
+            SLANG_RELEASE_ASSERT(actualFunc);
+            return actualFunc;
+        }
+    }
+    return inst;
+}
+
 IRInst* AutoDiffTranscriberBase::transcribe(IRBuilder* builder, IRInst* origInst)
 {
     // If a differential instruction is already mapped for 
@@ -891,8 +926,8 @@ IRInst* AutoDiffTranscriberBase::transcribe(IRBuilder* builder, IRInst* origInst
     // depending on the op-code.
     // 
     instsInProgress.Add(origInst);
-    
-    InstPair pair = transcribeInst(builder, origInst);
+    auto actualInstToTranscribe = getActualInstToTranscribe(origInst);
+    InstPair pair = transcribeInst(builder, actualInstToTranscribe);
 
     instsInProgress.Remove(origInst);