Transform STRUCT-typed uses of primitives in local morph (#78131)

* Add fwd-sub workaround

* Fold STRUCT-based access to primitives in local morph

* Solve a tiny regression

* Adapt args morhing

* Adapt block morphing

* Adapt return morphing

* Delete dead code

Author: SingleAccretion

src/coreclr/jit/compiler.h

@@ -5712,7 +5712,7 @@ class Compiler
     GenTree* fgMorphExpandStackArgForVarArgs(GenTreeLclVarCommon* lclNode);
 #endif // TARGET_X86
     GenTree* fgMorphExpandImplicitByRefArg(GenTreeLclVarCommon* lclNode);
-    GenTree* fgMorphLocalVar(GenTree* tree, bool forceRemorph);
+    GenTree* fgMorphLocalVar(GenTree* tree);
 
 public:
     bool fgAddrCouldBeNull(GenTree* addr);

src/coreclr/jit/flowgraph.cpp

@@ -944,6 +944,8 @@ bool Compiler::fgAddrCouldBeNull(GenTree* addr)
         case GT_CNS_STR:
         case GT_ADDR:
         case GT_FIELD_ADDR:
+        case GT_LCL_VAR_ADDR:
+        case GT_LCL_FLD_ADDR:
         case GT_CLS_VAR_ADDR:
             // A GT_ADDR node, by itself, never requires null checking.  The expression whose address is being
             // taken is either a local or static variable, whose address is necessarily non-null, or else it is

src/coreclr/jit/forwardsub.cpp

@@ -641,6 +641,15 @@ bool Compiler::fgForwardSubStatement(Statement* stmt)
         return false;
     }
 
+    // A "CanBeReplacedWithItsField" SDSU can serve as a sort of "BITCAST<primitive>(struct)"
+    // device, forwarding it risks forcing things to memory.
+    //
+    if (fwdSubNode->IsCall() && varDsc->CanBeReplacedWithItsField(this))
+    {
+        JITDUMP(" fwd sub local is 'CanBeReplacedWithItsField'\n");
+        return false;
+    }
+
     // There are implicit assumptions downstream on where/how multi-reg ops
     // can appear.
     //

src/coreclr/jit/lclmorph.cpp

@@ -992,7 +992,13 @@ class LocalAddressVisitor final : public GenTreeVisitor<LocalAddressVisitor>
                 indir->AsLclFld()->SetLayout(indirLayout);
                 lclNode = indir->AsLclVarCommon();
 
-                m_compiler->lvaSetVarDoNotEnregister(lclNum DEBUGARG(DoNotEnregisterReason::LocalField));
+                if (!indir->TypeIs(TYP_STRUCT))
+                {
+                    // The general invariant in the compiler is that whoever creates a LCL_FLD node after local morph
+                    // must mark the associated local DNER. We break this invariant here, for STRUCT fields, to allow
+                    // global morph to transform these into enregisterable LCL_VARs, applying DNER otherwise.
+                    m_compiler->lvaSetVarDoNotEnregister(val.LclNum() DEBUGARG(DoNotEnregisterReason::LocalField));
+                }
                 break;
 
             default:
@@ -1097,14 +1103,6 @@ class LocalAddressVisitor final : public GenTreeVisitor<LocalAddressVisitor>
             return IndirTransform::LclFld;
         }
 
-        if (varDsc->TypeGet() != TYP_STRUCT)
-        {
-            // TODO-ADDR: STRUCT uses of primitives require more work: "fgMorphOneAsgBlockOp"
-            // and init block morphing need to be updated to recognize them. Alternatively,
-            // we could consider moving some of their functionality here.
-            return IndirTransform::None;
-        }
-
         ClassLayout* indirLayout = nullptr;
 
         if (indir->OperIs(GT_FIELD))
@@ -1122,8 +1120,10 @@ class LocalAddressVisitor final : public GenTreeVisitor<LocalAddressVisitor>
 
         *pStructLayout = indirLayout;
 
-        // We're only processing TYP_STRUCT uses and variables now.
-        assert(indir->TypeIs(TYP_STRUCT) && (varDsc->GetLayout() != nullptr));
+        if (varDsc->TypeGet() != TYP_STRUCT)
+        {
+            return IndirTransform::LclFld;
+        }
 
         if ((val.Offset() == 0) && ClassLayout::AreCompatible(indirLayout, varDsc->GetLayout()))
         {