Propagate exception sets for assignments (dotnet#64882)

* Propagate exception sets for block assignments

* Propagate exception sets for simple assignments

* Fix fgValueNumberCastHelper

To not rely on VNs of setup/placeholder args.

* Enable the checker

src/coreclr/jit/valuenum.cpp

@@ -7480,59 +7480,29 @@ void Compiler::fgValueNumberAssignment(GenTreeOp* tree)
 {
     assert(tree->OperIs(GT_ASG) && varTypeIsEnregisterable(tree));
 
-    GenTree* lhs = tree->AsOp()->gtOp1;
-    GenTree* rhs = tree->AsOp()->gtOp2;
+    GenTree* lhs = tree->gtGetOp1();
+    GenTree* rhs = tree->gtGetOp2();
 
-    ValueNumPair rhsVNPair = rhs->gtVNPair;
+    // Only normal values are to be stored in SSA defs, VN maps, etc.
+    ValueNumPair rhsExcSet;
+    ValueNumPair rhsVNPair;
+    vnStore->VNPUnpackExc(rhs->gtVNPair, &rhsVNPair, &rhsExcSet);
 
     // Is the type being stored different from the type computed by the rhs?
-    if ((rhs->TypeGet() != lhs->TypeGet()) && (lhs->OperGet() != GT_BLK))
-    {
-        // This means that there is an implicit cast on the rhs value
-        //
-        // We will add a cast function to reflect the possible narrowing of the rhs value
-        //
-        var_types castToType   = lhs->TypeGet();
-        var_types castFromType = rhs->TypeGet();
-        bool      isUnsigned   = varTypeIsUnsigned(castFromType);
-
-        rhsVNPair = vnStore->VNPairForCast(rhsVNPair, castToType, castFromType, isUnsigned);
-    }
-
-    if (tree->TypeGet() != TYP_VOID)
-    {
-        // Assignment operators, as expressions, return the value of the RHS.
-        tree->gtVNPair = rhsVNPair;
-    }
-
-    // Now that we've labeled the assignment as a whole, we don't care about exceptions.
-    rhsVNPair = vnStore->VNPNormalPair(rhsVNPair);
-
-    // Record the exception set for this 'tree' in vnExcSet.
-    // First we'll record the exception set for the rhs and
-    // later we will union in the exception set for the lhs.
-    //
-    ValueNum vnExcSet;
-
-    // Unpack, Norm,Exc for 'rhsVNPair'
-    ValueNum vnRhsLibNorm;
-    vnStore->VNUnpackExc(rhsVNPair.GetLiberal(), &vnRhsLibNorm, &vnExcSet);
-
-    // Now that we've saved the rhs exeception set, we we will use the normal values.
-    rhsVNPair = ValueNumPair(vnRhsLibNorm, vnStore->VNNormalValue(rhsVNPair.GetConservative()));
-
-    // If the types of the rhs and lhs are different then we
-    //  may want to change the ValueNumber assigned to the lhs.
-    //
     if (rhs->TypeGet() != lhs->TypeGet())
     {
         if (rhs->TypeGet() == TYP_REF)
         {
             // If we have an unsafe IL assignment of a TYP_REF to a non-ref (typically a TYP_BYREF)
-            // then don't propagate this ValueNumber to the lhs, instead create a new unique VN
-            //
+            // then don't propagate this ValueNumber to the lhs, instead create a new unique VN.
             rhsVNPair.SetBoth(vnStore->VNForExpr(compCurBB, lhs->TypeGet()));
         }
+        else if (lhs->OperGet() != GT_BLK)
+        {
+            // This means that there is an implicit cast on the rhs value
+            // We will add a cast function to reflect the possible narrowing of the rhs value
+            rhsVNPair = vnStore->VNPairForCast(rhsVNPair, lhs->TypeGet(), rhs->TypeGet());
+        }
     }
 
     // We have to handle the case where the LHS is a comma.  In that case, we don't evaluate the comma,
@@ -8002,6 +7972,12 @@ void Compiler::fgValueNumberAssignment(GenTreeOp* tree)
         default:
             unreached();
     }
+
+    // For exception sets, we need the contribution from COMMAs on the
+    // LHS. ASGs produce no values, and as such are given the "Void" VN.
+    ValueNumPair lhsExcSet = vnStore->VNPExceptionSet(tree->gtGetOp1()->gtVNPair);
+    ValueNumPair asgExcSet = vnStore->VNPExcSetUnion(lhsExcSet, rhsExcSet);
+    tree->gtVNPair         = vnStore->VNPWithExc(vnStore->VNPForVoid(), asgExcSet);
 }
 
 //------------------------------------------------------------------------
@@ -8133,8 +8109,11 @@ void Compiler::fgValueNumberBlockAssignment(GenTree* tree)
         fgMutateGcHeap(tree DEBUGARG("INITBLK/COPYBLK - non local"));
     }
 
-    // Assignments produce no values so we give them the "Void" VN.
-    tree->gtVNPair = vnStore->VNPForVoid();
+    // Propagate the exception sets. Assignments produce no values so we give them the "Void" VN.
+    ValueNumPair vnpExcSet = ValueNumStore::VNPForEmptyExcSet();
+    vnpExcSet              = vnStore->VNPUnionExcSet(lhs->gtVNPair, vnpExcSet);
+    vnpExcSet              = vnStore->VNPUnionExcSet(rhs->gtVNPair, vnpExcSet);
+    tree->gtVNPair         = vnStore->VNPWithExc(vnStore->VNPForVoid(), vnpExcSet);
 }
 
 //------------------------------------------------------------------------
@@ -10036,7 +10015,7 @@ void Compiler::fgValueNumberCastHelper(GenTreeCall* call)
             unreached();
     }
 
-    ValueNumPair argVNP  = call->gtCallArgs->GetNode()->gtVNPair;
+    ValueNumPair argVNP  = call->fgArgInfo->GetArgNode(0)->gtVNPair;
     ValueNumPair castVNP = vnStore->VNPairForCast(argVNP, castToType, castFromType, srcIsUnsigned, hasOverflowCheck);
 
     call->SetVNs(castVNP);
@@ -10952,9 +10931,8 @@ void Compiler::fgDebugCheckExceptionSets()
                 return GenTree::VisitResult::Continue;
             });
 
-            // Currently, we fail to properly maintain the exception sets for trees with user
-            // calls or assignments.
-            if ((tree->gtFlags & (GTF_ASG | GTF_CALL)) != 0)
+            // Currently, we fail to properly maintain the exception sets for trees with user calls.
+            if ((tree->gtFlags & GTF_CALL) != 0)
             {
                 return;
             }