Decompose GT_FIELD_LIST nodes that use a GT_LONG.

These nodes are decomposed by splitting the appropriate field list
element into two elements: one each for the low and high halves of
the GT_LONG.

2 files changed, 34 insertions, 4 deletions

diff --git a/src/jit/decomposelongs.cpp b/src/jit/decomposelongs.cpp
index 017fa62c3e..04f48aa6fb 100644
--- a/src/jit/decomposelongs.cpp
+++ b/src/jit/decomposelongs.cpp
@@ -260,6 +260,39 @@ GenTree* DecomposeLongs::DecomposeNode(GenTree* tree)
         }
     }
 
+    // If we replaced the argument to a GT_FIELD_LIST element with a GT_LONG node, split that field list
+    // element into two elements: one for each half of the GT_LONG.
+    if ((use.Def()->OperGet() == GT_LONG) && !use.IsDummyUse() && (use.User()->OperGet() == GT_FIELD_LIST))
+    {
+        GenTreeOp* value = use.Def()->AsOp();
+        Range().Remove(value);
+
+        // The node returned by `use.User()` is the head of the field list. We need to find the actual node that uses
+        // the `GT_LONG` so that we can split it.
+        GenTreeFieldList* listNode = use.User()->AsFieldList();
+        for ( ; listNode != nullptr; listNode = listNode->Rest())
+        {
+            if (listNode->Current() == value)
+            {
+                break;
+            }
+        }
+
+        assert(listNode != nullptr);
+        GenTree* rest = listNode->gtOp2;
+
+        GenTreeFieldList* loNode = listNode;
+        loNode->gtOp1            = value->gtOp1;
+        loNode->gtFieldType      = TYP_INT;
+
+        GenTreeFieldList* hiNode = new (m_compiler, GT_FIELD_LIST) GenTreeFieldList(value->gtOp2,
+                                                                                    loNode->gtFieldOffset + 4,
+                                                                                    TYP_INT,
+                                                                                    loNode);
+
+        hiNode->gtOp2 = rest;
+    }
+
 #ifdef DEBUG
     if (m_compiler->verbose)
     {
diff --git a/src/jit/lowerxarch.cpp b/src/jit/lowerxarch.cpp
index 3a745bb3bc..235d7bed6e 100644
--- a/src/jit/lowerxarch.cpp
+++ b/src/jit/lowerxarch.cpp
@@ -1915,10 +1915,7 @@ void Lowering::TreeNodeInfoInitPutArgStk(GenTree* tree)
         for (; fieldListPtr; fieldListPtr = fieldListPtr->Rest())
         {
             GenTree* fieldNode = fieldListPtr->Current();
-            if (fieldNode->OperGet() == GT_LONG)
-            {
-                NYI_X86("Promoted long field of TYP_STRUCT");
-            }
+            assert(fieldNode->TypeGet() != TYP_LONG);
             if (varTypeIsByte(fieldNode))
             {
                 fieldNode->gtLsraInfo.setSrcCandidates(l, l->allRegs(TYP_INT) & ~RBM_NON_BYTE_REGS);