DBC: Fix reading locals out of a deopt frame.

runtime/vm/stack_frame_dbc.h

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_STACK_FRAME_DBC_H_
 #define VM_STACK_FRAME_DBC_H_
 
 namespace dart {
 
 /* DBC Frame Layout
 
 IMPORTANT: On DBC stack is growing upwards which is different from all other
-architectures. This enables effecient addressing for locals via unsigned index.
+architectures. This enables efficient addressing for locals via unsigned index.
 
                |                    | <- TOS
 Callee frame   | ...                |
                | saved FP           |    (FP of current frame)
                | saved PC           |    (PC of current frame)
                | code object        |
                | function object    |
                +--------------------+
 Current frame  | ...               T| <- SP of current frame
                | ...               T|
@@ skipping 25 matching lines @@
 
 // Note: These constants don't match actual DBC behavior. This is done because
 // setting kFirstLocalSlotFromFp to 0 breaks assumptions spread across the code.
 // Instead for the purposes of local variable allocation we pretend that DBC
 // behaves as other architectures (stack growing downwards) and later fix
 // these indices during code generation in the backend.
 static const int kParamEndSlotFromFp = 4;  // One slot past last parameter.
 static const int kFirstLocalSlotFromFp = -1;
 
 
-DART_FORCE_INLINE static uword LocalVarAddress(uword fp, intptr_t index) {
-  ASSERT(index != 0);
-  if (index > 0) {
-     return fp - index * kWordSize;
+DART_FORCE_INLINE static intptr_t LocalVarIndex(intptr_t fp_offset,
+                                                intptr_t var_index) {
+  ASSERT(var_index != 0);
+  if (var_index > 0) {
+    return fp_offset - var_index;
   } else {
-     return fp - (index + 1) * kWordSize;
+    return fp_offset - (var_index + 1);
   }
 }
 
+
 DART_FORCE_INLINE static uword ParamAddress(uword fp, intptr_t reverse_index) {
   return fp - (kDartFrameFixedSize + reverse_index) * kWordSize;
 }
 
 DART_FORCE_INLINE static bool IsCalleeFrameOf(uword fp, uword other_fp) {
   return other_fp > fp;
 }
 
 static const int kExitLinkSlotFromEntryFp = 0;
 
 // Value for stack limit that is used to cause an interrupt.
 // Note that on DBC stack is growing upwards so interrupt limit is 0 unlike
 // on all other architectures.
 static const uword kInterruptStackLimit = 0;
 
 }  // namespace dart
 
 #endif  // VM_STACK_FRAME_DBC_H_