[release/3.1] Port fix for JIT silent bad code (#27972)

* [release/3.1] Port fix for JIT silent bad code

Release/3.1 port of https://github.com/dotnet/runtime/pull/797.
Fixes https://github.com/dotnet/runtime/issues/764

The jit might incorrectly order a read from a struct field with an operation
that modifies the field, so that the read returns the wrong value.

Silent bad code; program behaves incorrectly.

Yes, introduced in the 3.0 cycle.

Verified the user's test case now passes; no diffs seen in any existing framework
or test code.

**Low**: the jit is now spilling the eval stack entries to temps in cases where it
did not before; this should be conservatively safe.

cc: @brucefo

____

If we're appending an assignment whose LHS is is a location within a local
struct, we need to spill all references to that struct from the eval stack.

Update the existing logic for this to handle the case where the LHS is a field
of a local struct, and the field is updated by unusual means (here, `initobj`).

Fixes dotnet/runtime#764.

* Fix test

1 files changed, 16 insertions, 5 deletions

diff --git a/src/jit/importer.cpp b/src/jit/importer.cpp
index bbb0d14e97..50ddc58d89 100644
--- a/src/jit/importer.cpp
+++ b/src/jit/importer.cpp
@@ -10837,11 +10837,22 @@ void Compiler::impImportBlockCode(BasicBlock* block)
                     }
                     else if (lhs->OperIsBlk())
                     {
-                        // Check for ADDR(LCL_VAR), or ADD(ADDR(LCL_VAR),CNS_INT))
-                        // (the latter may appear explicitly in the IL).
-                        // Local field stores will cause the stack to be spilled when
-                        // they are encountered.
-                        lclVar = lhs->AsBlk()->Addr()->IsLocalAddrExpr();
+                        // Check if LHS address is within some struct local, to catch
+                        // cases where we're updating the struct by something other than a stfld
+                        GenTree* addr = lhs->AsBlk()->Addr();
+
+                        // Catches ADDR(LCL_VAR), or ADD(ADDR(LCL_VAR),CNS_INT))
+                        lclVar = addr->IsLocalAddrExpr();
+
+                        // Catches ADDR(FIELD(... ADDR(LCL_VAR)))
+                        if (lclVar == nullptr)
+                        {
+                            GenTree* lclTree = nullptr;
+                            if (impIsAddressInLocal(addr, &lclTree))
+                            {
+                                lclVar = lclTree->AsLclVarCommon();
+                            }
+                        }
                     }
                     if (lclVar != nullptr)
                     {