Move aligned allocation to the platform files.

src/platform.h

Index: src/platform.h
diff --git a/src/platform.h b/src/platform.h
index d95ee62b1275274b9cde422de42b01fc103fdaa5..85901240f3d0740e86d1f6569710906e8754da1c 100644
--- a/src/platform.h
+++ b/src/platform.h
@@ -301,23 +301,46 @@ class OS {
   DISALLOW_IMPLICIT_CONSTRUCTORS(OS);
 };
 
-
+// Represents and controls an area of reserved memory.
+// Control of the reserved memory can be assigned to another VirtualMemory
+// object by assignment or copy-contructing. This removes the reserved memory
+// from the original object.
 class VirtualMemory {
  public:
+  // Empty VirtualMemory object, controlling no reserved memory.
+  VirtualMemory();
+
   // Reserves virtual memory with size.
   explicit VirtualMemory(size_t size);
+
+  // Reserves virtual memory containing an area of the given size that
+  // is aligned per alignment. This may not be at the position returned
+  // by address().
+  VirtualMemory(size_t size, size_t alignment);
+
+  // Releases the reserved memory, if any, controlled by this VirtualMemory
+  // object.
   ~VirtualMemory();
 
   // Returns whether the memory has been reserved.
   bool IsReserved();
 
+  // Initialize or resets an embedded VirtualMemory object.
+  void Reset();
+
   // Returns the start address of the reserved memory.
+  // If the memory was reserved with an alignment, this address is not
+  // necessarily aligned. The user might need to round it up to a multiple of
+  // the alignment to get the start of the aligned block.
   void* address() {
     ASSERT(IsReserved());
     return address_;
   }
 
-  // Returns the size of the reserved memory.
+  // Returns the size of the reserved memory. The returned value is only
+  // meaningful when IsReserved() returns true.
+  // If the memory was reserved with an alignment, this size may be larger
+  // than the requested size.
   size_t size() { return size_; }
 
   // Commits real memory. Returns whether the operation succeeded.
@@ -326,12 +349,33 @@ class VirtualMemory {
   // Uncommit real memory.  Returns whether the operation succeeded.
   bool Uncommit(void* address, size_t size);
 
+  void Release() {
+    ASSERT(IsReserved());
+    // Notice: Order is somportant here. The VirtualMemory object might live
+    // inside the allocated region.
+    void* address = address_;
+    size_t size = size_;
+    Reset();
+    ReleaseRegion(address, size);
+  }
+
+  // Assign control of the reserved region to a different VirtualMemory object.
+  // The old object is no longer functional (IsReserved() returns false).
+  void TakeControl(VirtualMemory* from) {
+    ASSERT(!IsReserved());
+    address_ = from->address_;
+    size_ = from->size_;
+    from->Reset();
+  }
+
   static void* ReserveRegion(size_t size);
 
   static bool CommitRegion(void* base, size_t size, bool is_executable);
 
   static bool UncommitRegion(void* base, size_t size);
 
+  // Must be called with a base pointer that has been returned by ReserveRegion
+  // and the same size it was reserved with.
   static bool ReleaseRegion(void* base, size_t size);
 
  private:
@@ -339,6 +383,7 @@ class VirtualMemory {
   size_t size_;  // Size of the virtual memory.
 };
 
+
 // ----------------------------------------------------------------------------
 // Thread
 //