Fix crash when swizzling non-differentiable types (#6613)

* Fix crash when swizzling non-differentiable types

* Update slang-ir-autodiff-fwd.cpp

1 files changed, 19 insertions, 11 deletions

diff --git a/source/slang/slang-ir-autodiff-fwd.cpp b/source/slang/slang-ir-autodiff-fwd.cpp
index 0302d9ce7..e146ac3e0 100644
--- a/source/slang/slang-ir-autodiff-fwd.cpp
+++ b/source/slang/slang-ir-autodiff-fwd.cpp
@@ -955,20 +955,28 @@ InstPair ForwardDiffTranscriber::transcribeCall(IRBuilder* builder, IRCall* orig
 InstPair ForwardDiffTranscriber::transcribeSwizzle(IRBuilder* builder, IRSwizzle* origSwizzle)
 {
     IRInst* primalSwizzle = maybeCloneForPrimalInst(builder, origSwizzle);
-
     if (auto diffBase = lookupDiffInst(origSwizzle->getBase(), nullptr))
     {
-        List<IRInst*> swizzleIndices;
-        for (UIndex ii = 0; ii < origSwizzle->getElementCount(); ii++)
-            swizzleIndices.add(origSwizzle->getElementIndex(ii));
+        // `diffBase` may exist even if the type is non-differentiable (e.g. IRCall inst that
+        // creates other differentiable outputs).
+        //
+        // We'll check to see if we can get a differential for the type in order to determine
+        // whether to generate a differential swizzle inst.
+        //
+        if (auto diffType = differentiateType(builder, primalSwizzle->getDataType()))
+        {
+            List<IRInst*> swizzleIndices;
+            for (UIndex ii = 0; ii < origSwizzle->getElementCount(); ii++)
+                swizzleIndices.add(origSwizzle->getElementIndex(ii));
 
-        return InstPair(
-            primalSwizzle,
-            builder->emitSwizzle(
-                differentiateType(builder, primalSwizzle->getDataType()),
-                diffBase,
-                origSwizzle->getElementCount(),
-                swizzleIndices.getBuffer()));
+            return InstPair(
+                primalSwizzle,
+                builder->emitSwizzle(
+                    diffType,
+                    diffBase,
+                    origSwizzle->getElementCount(),
+                    swizzleIndices.getBuffer()));
+        }
     }
 
     return InstPair(primalSwizzle, nullptr);