Robustness fixes around reverse-mode differentiation of variables & inout parameters (#2985)

* Add a new test case for checking loop in the reverse mode

* Create duplicate var for primal value to avoid inconsistent inputs to backward call

Fixes an issue with inout parameters where the backprop call may use the post-call value of the var instead of the pre-call value.

* `IRStore`s transpose to `IRLoad` and an `IRStore(0)` to clear differential.

Fixes some subtle issues around transposing

* Simplify test

* Delete out-edited.hlsl

---------

Co-authored-by: Lifan Wu <lifanw@nvidia.com>
Co-authored-by: Yong He <yonghe@outlook.com>

2 files changed, 23 insertions, 1 deletions

diff --git a/source/slang/slang-ir-autodiff-transpose.h b/source/slang/slang-ir-autodiff-transpose.h
index 843612df9..739ce6553 100644
--- a/source/slang/slang-ir-autodiff-transpose.h
+++ b/source/slang/slang-ir-autodiff-transpose.h
@@ -1560,6 +1560,14 @@ struct DiffTransposePass
     TranspositionResult transposeStore(IRBuilder* builder, IRStore* fwdStore, IRInst*)
     {
         IRInst* revVal = builder->emitLoad(fwdStore->getPtr());
+
+        auto primalType = tryGetPrimalTypeFromDiffInst(fwdStore->getVal());
+        SLANG_ASSERT(primalType);
+
+        // Clear the value at the differential address, by setting to 0.
+        IRInst* emptyVal = emitDZeroOfDiffInstType(builder, primalType);
+        builder->emitStore(fwdStore->getPtr(), emptyVal);
+
         if (auto diffPairType = as<IRDifferentialPairType>(revVal->getDataType()))
         {
             revVal = builder->emitDifferentialPairGetDifferential(
diff --git a/source/slang/slang-ir-autodiff-unzip.h b/source/slang/slang-ir-autodiff-unzip.h
index 1d05a8081..2a5d73a01 100644
--- a/source/slang/slang-ir-autodiff-unzip.h
+++ b/source/slang/slang-ir-autodiff-unzip.h
@@ -340,7 +340,21 @@ struct DiffUnzipPass
                     // the remerged diff pair.
                     auto diffPairType = as<IRDifferentialPairType>(as<IRPtrTypeBase>(arg->getDataType())->getValueType());
                     auto primalValueType = diffPairType->getValueType();
-                    auto diffPairRef = diffBuilder->emitReverseGradientDiffPairRef(arg->getDataType(), primalArg, diffArg);
+                    
+                    // We can't simply reuse primalArg for an inout parameter since this will represent the value 
+                    // after the primal call which can potentially alter primalArg. Therefore, we will find the
+                    // first store into primalArg, and create a temp var holding that value (i.e. value prior to primal call)
+                    // 
+                    auto storeUse = findUniqueStoredVal(cast<IRVar>(primalArg));
+                    auto storeInst = cast<IRStore>(storeUse->getUser());
+
+                    auto storedVal = storeInst->getVal();
+
+                    // Emit the temp var into the primal blocks since it's holding a primal value.
+                    auto tempPrimalVar = primalBuilder->emitVar(primalValueType);
+                    primalBuilder->emitStore(tempPrimalVar, storedVal);
+                    
+                    auto diffPairRef = diffBuilder->emitReverseGradientDiffPairRef(arg->getDataType(), tempPrimalVar, diffArg);
                     diffBuilder->markInstAsDifferential(diffPairRef, primalValueType);
                     diffArgs.add(diffPairRef);
                 }