From f23e36243e9c59c02f66ec2e18b80ba4ea540f45 Mon Sep 17 00:00:00 2001
From: Yong He <yonghe@outlook.com>
Date: Mon, 27 Feb 2023 21:21:39 -0800
Subject: Diagnose on storing differentiable value into non-differentiable
 location. (#2681)

---
 source/slang/slang-ir-check-differentiability.cpp | 62 ++++++++++++++++++++++-
 1 file changed, 60 insertions(+), 2 deletions(-)

(limited to 'source/slang/slang-ir-check-differentiability.cpp')

diff --git a/source/slang/slang-ir-check-differentiability.cpp b/source/slang/slang-ir-check-differentiability.cpp
index 1ee94e67e..21f53fcbd 100644
--- a/source/slang/slang-ir-check-differentiability.cpp
+++ b/source/slang/slang-ir-check-differentiability.cpp
@@ -152,6 +152,40 @@ public:
         return false;
     }
 
+    bool canAddressHoldDerivative(DifferentiableTypeConformanceContext& diffTypeContext, IRInst* addr)
+    {
+        if (!addr)
+            return false;
+
+        while (addr)
+        {
+            switch (addr->getOp())
+            {
+            case kIROp_Var:
+            case kIROp_GlobalVar:
+            case kIROp_Param:
+            case kIROp_GlobalParam:
+                return isDifferentiableType(diffTypeContext, addr->getDataType());
+            case kIROp_FieldAddress:
+                if (!as<IRFieldAddress>(addr)->getField() ||
+                    as<IRFieldAddress>(addr)
+                    ->getField()
+                    ->findDecoration<IRDerivativeMemberDecoration>() == nullptr)
+                    return false;
+                addr = as<IRFieldAddress>(addr)->getBase();
+                break;
+            case kIROp_GetElementPtr:
+                if (!isDifferentiableType(diffTypeContext, as<IRGetElementPtr>(addr)->getBase()->getDataType()))
+                    return false;
+                addr = as<IRGetElementPtr>(addr)->getBase();
+                break;
+            default:
+                return false;
+            }
+        }
+        return false;
+    }
+
     void processFunc(IRGlobalValueWithCode* funcInst)
     {
         if (!_isFuncMarkedForAutoDiff(funcInst))
@@ -197,9 +231,9 @@ public:
                 return inst->findDecoration<IRTreatAsDifferentiableDecoration>() || isDifferentiableFunc(as<IRCall>(inst)->getCallee(), requiredDiffLevel);
             case kIROp_Load:
                 // We don't have more knowledge on whether diff is available at the destination address.
-                // Just assume it is producing diff.
+                // Just assume it is producing diff if the dest address can hold a derivative.
                 //TODO: propagate the info if this is a load of a temporary variable intended to receive result from an `out` parameter.
-                return isDifferentiableType(diffTypeContext, inst->getDataType());
+                return canAddressHoldDerivative(diffTypeContext, as<IRLoad>(inst)->getPtr());
             default:
                 // default case is to assume the inst produces a diff value if any
                 // of its operands produces a diff value.
@@ -224,6 +258,7 @@ public:
                     expectDiffInstWorkList.add(inst);
             }
         };
+
         // Run data flow analysis and generate `produceDiffSet` and an intial `expectDiffSet`.
         Index lastProduceDiffCount = 0;
         do
@@ -373,6 +408,29 @@ public:
                 sink->diagnose(loop->sourceLoc, Diagnostics::loopInDiffFuncRequireUnrollOrMaxIters);
             }
         }
+
+        // Make sure all stores of differentiable values are into addresses that can hold derivatives.
+        for (auto block : funcInst->getBlocks())
+        {
+            for (auto inst : block->getChildren())
+            {
+                if (auto storeInst = as<IRStore>(inst))
+                {
+                    if (produceDiffSet.Contains(storeInst->getVal()) &&
+                        !canAddressHoldDerivative(diffTypeContext, storeInst->getPtr()))
+                    {
+                        switch (storeInst->getVal()->getOp())
+                        {
+                        case kIROp_DetachDerivative:
+                            break;
+                        default:
+                            sink->diagnose(storeInst->sourceLoc, Diagnostics::lossOfDerivativeAssigningToNonDifferentiableLocation);
+                            break;
+                        }
+                    }
+                }
+            }
+        }
     }
 
     void processModule()
-- 
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