Fixes a bug in that caused IceAssembler to use Allocator before it was initialized.

src/IceAssembler.h

 //===- subzero/src/IceAssembler.h - Integrated assembler --------*- C++ -*-===//
 // Copyright (c) 2012, 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.
 //
 // Modified by the Subzero authors.
 //
 //===----------------------------------------------------------------------===//
 //
 //                        The Subzero Code Generator
@@ skipping 138 matching lines @@
 
   void extendCapacity();
 };
 
 class Assembler {
   Assembler(const Assembler &) = delete;
   Assembler &operator=(const Assembler &) = delete;
 
 public:
   Assembler()
-      : Buffer(*this), FunctionName(""), IsInternal(false),
-        Preliminary(false) {}
+      : Allocator(), FunctionName(""), IsInternal(false), Preliminary(false),
+        Buffer(*this) {}
   virtual ~Assembler() = default;
 
   // Allocate a chunk of bytes using the per-Assembler allocator.
   uintptr_t allocateBytes(size_t bytes) {
     // For now, alignment is not related to NaCl bundle alignment, since
     // the buffer's GetPosition is relative to the base. So NaCl bundle
     // alignment checks can be relative to that base. Later, the buffer
     // will be copied out to a ".text" section (or an in memory-buffer
     // that can be mprotect'ed with executable permission), and that
     // second buffer should be aligned for NaCl.
@@ skipping 34 matching lines @@
   bool getInternal() const { return IsInternal; }
   void setInternal(bool Internal) { IsInternal = Internal; }
   const IceString &getFunctionName() { return FunctionName; }
   void setFunctionName(const IceString &NewName) { FunctionName = NewName; }
   intptr_t getBufferSize() const { return Buffer.size(); }
   // Roll back to a (smaller) size.
   void setBufferSize(intptr_t NewSize) { Buffer.setSize(NewSize); }
   void setPreliminary(bool Value) { Preliminary = Value; }
   bool getPreliminary() const { return Preliminary; }
 
-protected:
-  AssemblerBuffer Buffer;
-
 private:
   ArenaAllocator<32 * 1024> Allocator;
   // FunctionName and IsInternal are transferred from the original Cfg
   // object, since the Cfg object may be deleted by the time the
   // assembler buffer is emitted.
   IceString FunctionName;
   bool IsInternal;
   // Preliminary indicates whether a preliminary pass is being made
   // for calculating bundle padding (Preliminary=true), versus the
   // final pass where all changes to label bindings, label links, and
   // relocation fixups are fully committed (Preliminary=false).
   bool Preliminary;
+
+protected:
+  // Buffer's constructor uses the Allocator, so it needs to appear after it.
+  // TODO(jpp): dependencies on construction order are a nice way of shooting
+  // yourself in the foot. Fix this.
+  AssemblerBuffer Buffer;
 };
 
 } // end of namespace Ice
 
 #endif // SUBZERO_SRC_ICEASSEMBLER_H_