Simplify VirtualMemory by removing unused ReserveAligned method.

runtime/vm/virtual_memory.h

 // 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.
 
 #ifndef VM_VIRTUAL_MEMORY_H_
 #define VM_VIRTUAL_MEMORY_H_
 
 #include "platform/utils.h"
 #include "vm/globals.h"
 #include "vm/memory_region.h"
@@ skipping 30 matching lines @@
   }
 
   // Changes the protection of the virtual memory area.
   static bool Protect(void* address, intptr_t size, Protection mode);
   bool Protect(Protection mode) {
     return Protect(address(), size(), mode);
   }
 
   // Reserves a virtual memory segment with size. If a segment of the requested
   // size cannot be allocated NULL is returned.
-  static VirtualMemory* Reserve(intptr_t size);
-
-  // Reserves a virtual memory segment with the start address being aligned to
-  // the requested power of two.
-  static VirtualMemory* ReserveAligned(intptr_t size, intptr_t alignment);
+  static VirtualMemory* Reserve(intptr_t size) {
+    return ReserveInternal(size);

[Ivan Posva, 2014/10/09 07:01:14]

Why this extra indirection?

[koda, 2014/10/09 15:58:52]

To allow for inserting a bit of OS-independent logic in the future without
having to update all the OS-specific files (e.g., the write barrier
verification).

+  }
 
   static intptr_t PageSize() {
     ASSERT(page_size_ != 0);
     ASSERT(Utils::IsPowerOfTwo(page_size_));
     return page_size_;
   }
 
   static bool InSamePage(uword address0, uword address1);
 
   // Truncate this virtual memory segment.
-  void Truncate(uword new_start, intptr_t size);
+  void Truncate(intptr_t new_size, bool try_unmap = true);

[Ivan Posva, 2014/10/09 07:01:13]

When would it be beneficial to truncate without unmapping?

[koda, 2014/10/09 15:58:52]

I will use it to implement the writer barrier verification mode with minimal
changes to the code using VirtualMemory. Each VirtualMemory will have a trailing
"shadow" area, and pointer writes will be duplicated at a fixed offset.

 
  private:
+  static VirtualMemory* ReserveInternal(intptr_t size);
+
   // Free a sub segment. On operating systems that support it this
-  // can give back the virtual memory to the system.
-  void FreeSubSegment(void* address, intptr_t size);
+  // can give back the virtual memory to the system. Returns true on success.
+  static bool FreeSubSegment(void* address, intptr_t size);
 
   // This constructor is only used internally when reserving new virtual spaces.
   // It does not reserve any virtual address space on its own.
-  VirtualMemory(const MemoryRegion& region, void* reserved_pointer) :
+  explicit VirtualMemory(const MemoryRegion& region) :
       region_(region.pointer(), region.size()),
-      reserved_pointer_(reserved_pointer) { }
+      reserved_size_(region.size()) { }
 
   // Commit a reserved memory area, so that the memory can be accessed.
   bool Commit(uword addr, intptr_t size, bool is_executable);
 
   MemoryRegion region_;
 
-  // The original pointer returned by the OS for this virtual memory
-  // allocation or NULL. reserved_pointer_ is non-NULL only for
-  // platforms where virtual memory subregions cannot be given back to
-  // the OS. When non-null it might not coincide with
-  // region_.pointer() if the virtual memory region has been
-  // truncated.
-  void* reserved_pointer_;
+  // The size of the underlying reservation not yet given back to the OS.
+  // Its start coincides with region_, but its size might not, due to Truncate.
+  intptr_t reserved_size_;
 
   static uword page_size_;
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(VirtualMemory);
 };
 
 }  // namespace dart
 
 #endif  // VM_VIRTUAL_MEMORY_H_