diff --git a/src/coreclr/src/jit/compiler.h b/src/coreclr/src/jit/compiler.h
index 8f9b8e7535a89b..f35f6f769ed630 100644
--- a/src/coreclr/src/jit/compiler.h
+++ b/src/coreclr/src/jit/compiler.h
@@ -3496,19 +3496,21 @@ class Compiler
         assert(varDsc->lvSize() == 16);
 #endif // defined(TARGET_64BIT)
 
-        // We make local variable SIMD12 types 16 bytes instead of just 12. lvSize()
-        // already does this calculation. However, we also need to prevent mapping types if the var is a
-        // dependently promoted struct field, which must remain its exact size within its parent struct.
-        // However, we don't know this until late, so we may have already pretended the field is bigger
-        // before that.
-        if ((varDsc->lvSize() == 16) && !lvaIsFieldOfDependentlyPromotedStruct(varDsc))
+        // We make local variable SIMD12 types 16 bytes instead of just 12.
+        // lvSize() will return 16 bytes for SIMD12, even for fields.
+        // However, we can't do that mapping if the var is a dependently promoted struct field.
+        // Such a field must remain its exact size within its parent struct unless it is a single
+        // field *and* it is the only field in a struct of 16 bytes.
+        if (varDsc->lvSize() != 16)
         {
-            return true;
+            return false;
         }
-        else
+        if (lvaIsFieldOfDependentlyPromotedStruct(varDsc))
         {
-            return false;
+            LclVarDsc* parentVarDsc = lvaGetDesc(varDsc->lvParentLcl);
+            return (parentVarDsc->lvFieldCnt == 1) && (parentVarDsc->lvSize() == 16);
         }
+        return true;
     }
 #endif // defined(FEATURE_SIMD)
 
diff --git a/src/coreclr/src/jit/morph.cpp b/src/coreclr/src/jit/morph.cpp
index fc64cf09877f92..379cf107b62f02 100644
--- a/src/coreclr/src/jit/morph.cpp
+++ b/src/coreclr/src/jit/morph.cpp
@@ -3665,31 +3665,21 @@ GenTreeCall* Compiler::fgMorphArgs(GenTreeCall* call)
 #else  // UNIX_AMD64_ABI
                     // On Unix, structs are always passed by value.
                     // We only need a copy if we have one of the following:
-                    // - We have a lclVar that has been promoted and is passed in registers.
                     // - The sizes don't match for a non-lclVar argument.
                     // - We have a known struct type (e.g. SIMD) that requires multiple registers.
-                    // TODO-Amd64-Unix-CQ: The first case could and should be handled without copies.
                     // TODO-Amd64-Unix-Throughput: We don't need to keep the structDesc in the argEntry if it's not
                     // actually passed in registers.
                     if (argEntry->isPassedInRegisters())
                     {
                         assert(argEntry->structDesc.passedInRegisters);
-                        if (lclVar != nullptr)
-                        {
-                            if (lvaGetPromotionType(lclVar->AsLclVarCommon()->GetLclNum()) ==
-                                PROMOTION_TYPE_INDEPENDENT)
-                            {
-                                copyBlkClass = objClass;
-                            }
-                        }
-                        else if (argObj->OperIs(GT_OBJ))
+                        if (argObj->OperIs(GT_OBJ))
                         {
                             if (passingSize != structSize)
                             {
                                 copyBlkClass = objClass;
                             }
                         }
-                        else
+                        else if (lclVar == nullptr)
                         {
                             // This should only be the case of a value directly producing a known struct type.
                             assert(argObj->TypeGet() != TYP_STRUCT);
@@ -9720,7 +9710,9 @@ GenTree* Compiler::fgMorphInitBlock(GenTree* tree)
             }
 #endif // LOCAL_ASSERTION_PROP
 
-            if (destLclVar->lvPromoted)
+            // If we have already determined that a promoted TYP_STRUCT lclVar will not be enregistered,
+            // we are better off doing a block init.
+            if (destLclVar->lvPromoted && (!destLclVar->lvDoNotEnregister || !destLclNode->TypeIs(TYP_STRUCT)))
             {
                 GenTree* newTree = fgMorphPromoteLocalInitBlock(destLclNode->AsLclVar(), initVal, blockSize);
 
@@ -10604,15 +10596,30 @@ GenTree* Compiler::fgMorphCopyBlock(GenTree* tree)
             // Are both dest and src promoted structs?
             if (destDoFldAsg && srcDoFldAsg)
             {
-                // Both structs should be of the same type, or each have a single field of the same type.
+                // Both structs should be of the same type, or have the same number of fields of the same type.
                 // If not we will use a copy block.
-                if (lvaTable[destLclNum].lvVerTypeInfo.GetClassHandle() !=
-                    lvaTable[srcLclNum].lvVerTypeInfo.GetClassHandle())
+                bool misMatchedTypes = false;
+                if (destLclVar->lvVerTypeInfo.GetClassHandle() != srcLclVar->lvVerTypeInfo.GetClassHandle())
                 {
-                    unsigned destFieldNum = lvaTable[destLclNum].lvFieldLclStart;
-                    unsigned srcFieldNum  = lvaTable[srcLclNum].lvFieldLclStart;
-                    if ((lvaTable[destLclNum].lvFieldCnt != 1) || (lvaTable[srcLclNum].lvFieldCnt != 1) ||
-                        (lvaTable[destFieldNum].lvType != lvaTable[srcFieldNum].lvType))
+                    if (destLclVar->lvFieldCnt != srcLclVar->lvFieldCnt)
+                    {
+                        misMatchedTypes = true;
+                    }
+                    else
+                    {
+                        for (int i = 0; i < destLclVar->lvFieldCnt; i++)
+                        {
+                            LclVarDsc* destFieldVarDsc = lvaGetDesc(destLclVar->lvFieldLclStart + i);
+                            LclVarDsc* srcFieldVarDsc  = lvaGetDesc(srcLclVar->lvFieldLclStart + i);
+                            if ((destFieldVarDsc->lvType != srcFieldVarDsc->lvType) ||
+                                (destFieldVarDsc->lvFldOffset != srcFieldVarDsc->lvFldOffset))
+                            {
+                                misMatchedTypes = true;
+                                break;
+                            }
+                        }
+                    }
+                    if (misMatchedTypes)
                     {
                         requiresCopyBlock = true; // Mismatched types, leave as a CopyBlock
                         JITDUMP(" with mismatched types");