JIT: Add ordering side effects in fgMorphExpandInstanceField (#92998)

In #92710 I removed an ordering side effect set on a LCL_VAR node in
fgMorphExpandInstanceField, since the side effect is meaningless on a
local. However, I did not realize that setting the ordering effect was
important because propagating it to parent nodes was actually necessary.

This PR explicitly sets the ordering effect on the ADD<byref> nodes
created, since forming these and reporting them to GC is illegal if the
base is invalid.

Additionally, it sets the ordering effect on the consuming indirection;
if that indirection made use of the fact that the FIELD_ADDR is non-null
to become non-faulting, then it has an ordering dependency once we
expand the FIELD_ADDR to a control-flow based null-check. This is not
actually necessary since we always set end up with an ADD<byref> that
propagates it, but that seems more like a coincidence than anything
else.

Fix #92990

Author: jakobbotsch

src/coreclr/jit/compiler.h

@@ -5917,8 +5917,8 @@ class Compiler
     // small; hence the other fields of MorphAddrContext.
     struct MorphAddrContext
     {
-        size_t m_totalOffset = 0;     // Sum of offsets between the top-level indirection and here (current context).
-        bool   m_used        = false; // Whether this context was used to elide a null check.
+        GenTreeIndir* m_user = nullptr;  // Indirection using this address.
+        size_t        m_totalOffset = 0; // Sum of offsets between the top-level indirection and here (current context).
     };
 
 #ifdef FEATURE_SIMD

src/coreclr/jit/morph.cpp

@@ -5088,9 +5088,24 @@ GenTree* Compiler::fgMorphExpandInstanceField(GenTree* tree, MorphAddrContext* m
         // will implicitly null-check the produced address.
         addExplicitNullCheck = (mac == nullptr) || fgIsBigOffset(mac->m_totalOffset + fieldOffset);
 
-        if (!addExplicitNullCheck)
+        // The transformation here turns a value dependency (FIELD_ADDR being a
+        // known non-null operand) into a control-flow dependency (introducing
+        // explicit COMMA(NULLCHECK, ...)). This effectively "disconnects" the
+        // null check from the parent of the FIELD_ADDR node. For the cases
+        // where we made use of non-nullness we need to make the dependency
+        // explicit now.
+        if (addExplicitNullCheck)
         {
-            mac->m_used = true;
+            if (mac != nullptr)
+            {
+                mac->m_user->SetHasOrderingSideEffect();
+            }
+        }
+        else
+        {
+            // We can elide the null check only by letting it happen as part of
+            // the consuming indirection, so it is no longer non-faulting.
+            mac->m_user->gtFlags &= ~GTF_IND_NONFAULTING;
         }
     }
 
@@ -5153,6 +5168,13 @@ GenTree* Compiler::fgMorphExpandInstanceField(GenTree* tree, MorphAddrContext* m
         }
 
         addr = gtNewOperNode(GT_ADD, (objRefType == TYP_I_IMPL) ? TYP_I_IMPL : TYP_BYREF, addr, offsetNode);
+
+        // We cannot form and GC report an invalid byref, so this must preserve
+        // its ordering with the null check.
+        if (addExplicitNullCheck && addr->TypeIs(TYP_BYREF))
+        {
+            addr->SetHasOrderingSideEffect();
+        }
     }
 #endif
 
@@ -5168,6 +5190,13 @@ GenTree* Compiler::fgMorphExpandInstanceField(GenTree* tree, MorphAddrContext* m
     {
         addr = gtNewOperNode(GT_ADD, (objRefType == TYP_I_IMPL) ? TYP_I_IMPL : TYP_BYREF, addr,
                              gtNewIconNode(fieldOffset, fieldSeq));
+
+        // We cannot form and GC report an invalid byref, so this must preserve
+        // its ordering with the null check.
+        if (addExplicitNullCheck && addr->TypeIs(TYP_BYREF))
+        {
+            addr->SetHasOrderingSideEffect();
+        }
     }
 
     if (addExplicitNullCheck)
@@ -8937,7 +8966,8 @@ GenTree* Compiler::fgMorphSmpOp(GenTree* tree, MorphAddrContext* mac, bool* optA
         if (tree->OperIsIndir()) // TODO-CQ: add more operators here (e. g. atomics).
         {
             // Communicate to FIELD_ADDR morphing that the parent is an indirection.
-            mac = &indMac;
+            indMac.m_user = tree->AsIndir();
+            mac           = &indMac;
         }
         // For additions, if we already have a context, keep track of whether all offsets added
         // to the address are constant, and their sum does not overflow.
@@ -8961,13 +8991,6 @@ GenTree* Compiler::fgMorphSmpOp(GenTree* tree, MorphAddrContext* mac, bool* optA
 
         tree->AsOp()->gtOp1 = op1 = fgMorphTree(op1, mac);
 
-        if ((mac != nullptr) && mac->m_used && tree->OperIsIndir())
-        {
-            // This context was used to elide an explicit null check on a FIELD_ADDR, with the expectation that
-            // this indirection will now be responsible for null-checking (implicitly).
-            tree->gtFlags &= ~GTF_IND_NONFAULTING;
-        }
-
         // If we are exiting the "then" part of a Qmark-Colon we must
         // save the state of the current copy assignment table
         // so that we can merge this state with the "else" part exit