Revert #1091 Remove unboxed object code phase 1 + extras.

This should be the last code backout for this. merging this branch
should get us back to the way we were (+ additional code changes for
later changes) as fasr as the unused unboxed code is concerned.

js/src/jit/MIR.h

@@ -30,6 +30,7 @@
 #include "vm/EnvironmentObject.h"
 #include "vm/SharedMem.h"
 #include "vm/TypedArrayCommon.h"
+#include "vm/UnboxedObject.h"
 
 // Undo windows.h damage on Win64
 #undef MemoryBarrier
@@ -9749,6 +9750,59 @@ class MStoreUnboxedString
     ALLOW_CLONE(MStoreUnboxedString)
 };
 
+// Passes through an object, after ensuring it is converted from an unboxed
+// object to a native representation.
+class MConvertUnboxedObjectToNative
+  : public MUnaryInstruction,
+    public SingleObjectPolicy::Data
+{
+    CompilerObjectGroup group_;
+
+    explicit MConvertUnboxedObjectToNative(MDefinition* obj, ObjectGroup* group)
+      : MUnaryInstruction(obj),
+        group_(group)
+    {
+        setGuard();
+        setMovable();
+        setResultType(MIRType::Object);
+    }
+
+  public:
+    INSTRUCTION_HEADER(ConvertUnboxedObjectToNative)
+    NAMED_OPERANDS((0, object))
+
+    static MConvertUnboxedObjectToNative* New(TempAllocator& alloc, MDefinition* obj,
+                                              ObjectGroup* group);
+
+    ObjectGroup* group() const {
+        return group_;
+    }
+    bool congruentTo(const MDefinition* ins) const override {
+        if (!congruentIfOperandsEqual(ins))
+            return false;
+        return ins->toConvertUnboxedObjectToNative()->group() == group();
+    }
+    AliasSet getAliasSet() const override {
+        // This instruction can read and write to all parts of the object, but
+        // is marked as non-effectful so it can be consolidated by LICM and GVN
+        // and avoid inhibiting other optimizations.
+        //
+        // This is valid to do because when unboxed objects might have a native
+        // group they can be converted to, we do not optimize accesses to the
+        // unboxed objects and do not guard on their group or shape (other than
+        // in this opcode).
+        //
+        // Later accesses can assume the object has a native representation
+        // and optimize accordingly. Those accesses cannot be reordered before
+        // this instruction, however. This is prevented by chaining this
+        // instruction with the object itself, in the same way as MBoundsCheck.
+        return AliasSet::None();
+    }
+    bool appendRoots(MRootList& roots) const override {
+        return roots.append(group_);
+    }
+};
+
 // Array.prototype.pop or Array.prototype.shift on a dense array.
 class MArrayPopShift
   : public MUnaryInstruction,
@@ -11128,6 +11182,11 @@ class MGuardShape
         setMovable();
         setResultType(MIRType::Object);
         setResultTypeSet(obj->resultTypeSet());
+
+        // Disallow guarding on unboxed object shapes. The group is better to
+        // guard on, and guarding on the shape can interact badly with
+        // MConvertUnboxedObjectToNative.
+        MOZ_ASSERT(shape->getObjectClass() != &UnboxedPlainObject::class_);
     }
 
   public:
@@ -11222,6 +11281,11 @@ class MGuardObjectGroup
         setGuard();
         setMovable();
         setResultType(MIRType::Object);
+
+        // Unboxed groups which might be converted to natives can't be guarded
+        // on, due to MConvertUnboxedObjectToNative.
+        MOZ_ASSERT_IF(group->maybeUnboxedLayoutDontCheckGeneration(),
+                      !group->unboxedLayoutDontCheckGeneration().nativeGroup());
     }
 
   public:
@@ -11330,6 +11394,73 @@ class MGuardClass
     ALLOW_CLONE(MGuardClass)
 };
 
+// Guard on the presence or absence of an unboxed object's expando.
+class MGuardUnboxedExpando
+  : public MUnaryInstruction,
+    public SingleObjectPolicy::Data
+{
+    bool requireExpando_;
+    BailoutKind bailoutKind_;
+
+    MGuardUnboxedExpando(MDefinition* obj, bool requireExpando, BailoutKind bailoutKind)
+      : MUnaryInstruction(obj),
+        requireExpando_(requireExpando),
+        bailoutKind_(bailoutKind)
+    {
+        setGuard();
+        setMovable();
+        setResultType(MIRType::Object);
+    }
+
+  public:
+    INSTRUCTION_HEADER(GuardUnboxedExpando)
+    TRIVIAL_NEW_WRAPPERS
+    NAMED_OPERANDS((0, object))
+
+    bool requireExpando() const {
+        return requireExpando_;
+    }
+    BailoutKind bailoutKind() const {
+        return bailoutKind_;
+    }
+    bool congruentTo(const MDefinition* ins) const override {
+        if (!congruentIfOperandsEqual(ins))
+            return false;
+        if (requireExpando() != ins->toGuardUnboxedExpando()->requireExpando())
+            return false;
+        return true;
+    }
+    AliasSet getAliasSet() const override {
+        return AliasSet::Load(AliasSet::ObjectFields);
+    }
+};
+
+// Load an unboxed plain object's expando.
+class MLoadUnboxedExpando
+  : public MUnaryInstruction,
+    public SingleObjectPolicy::Data
+{
+  private:
+    explicit MLoadUnboxedExpando(MDefinition* object)
+      : MUnaryInstruction(object)
+    {
+        setResultType(MIRType::Object);
+        setMovable();
+    }
+
+  public:
+    INSTRUCTION_HEADER(LoadUnboxedExpando)
+    TRIVIAL_NEW_WRAPPERS
+    NAMED_OPERANDS((0, object))
+
+    bool congruentTo(const MDefinition* ins) const override {
+        return congruentIfOperandsEqual(ins);
+    }
+    AliasSet getAliasSet() const override {
+        return AliasSet::Load(AliasSet::ObjectFields);
+    }
+};
+
 // Load from vp[slot] (slots that are not inline in an object).
 class MLoadSlot
   : public MUnaryInstruction,