Add a write buffer, aligned so that a single bit tells us whether it has...

src/write-buffer.cc

Index: src/write-buffer.cc
===================================================================
--- src/write-buffer.cc	(revision 0)
+++ src/write-buffer.cc	(revision 0)
@@ -0,0 +1,61 @@
+// Copyright 2010 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include "write-buffer.h"
+
+namespace v8 {
+namespace internal {
+
+Address WriteBuffer::top_;
+Address WriteBuffer::start_;
+Address WriteBuffer::limit_;
+VirtualMemory* WriteBuffer::virtual_memory_;
+
+void WriteBuffer::Setup() {
+  virtual_memory_ = new VirtualMemory(kWriteBufferSize * 3);

[Vyacheslav Egorov (Chromium), 2010/12/14 22:40:14]

This allocation looks OK for single buffer but I am not sure that it is OK for
multiple buffers (if we decide to have many of them).

On Windows which has allocation granularity 64k this is going to waste 98k of
address space.

Even on Linux it might be beneficial to return unused parts of this memory back
to OS.

I see no obvious benefit for single bit vs. several bits on x86 (though benefit
is obvious on ARM): mask = (kWriteBufferOverflowBit - 1), alignent =
kWriteBufferOverflowBit.

[Erik Corry, 2010/12/15 08:00:13]

I'll burn that bridge when I come to it.
I think that 0.0025% of the address space is worth spending on my awsum idea. 
The new space will use 160 times as much.
It's definitely more compact on ARM and I like that.

+  uintptr_t start_as_int = reinterpret_cast<uintptr_t>(virtual_memory_->address());
+  start_ = reinterpret_cast<Address>(RoundUp(start_as_int, kWriteBufferSize * 2));
+  limit_ = start_ + kWriteBufferSize;
+
+  ASSERT(start_ >= virtual_memory_->address());
+  ASSERT(limit_ >= virtual_memory_->address());
+  ASSERT(start <= virtual_memory_->address() + virtual_memory_->size());
+  ASSERT(limit <= virtual_memory_->address() + virtual_memory_->size());
+  ASSERT((reinterpret_cast<uintptr_t>(limit) & kWriteBufferOverflowBit) != 0);
+  ASSERT((reinterpret_cast<uintptr_t>(limit - 1) & kWriteBufferOverflowBit) == 0);
+
+  virtual_memory_->Commit(start_, kWriteBufferSize, false);  // Not executable.
+  top_ = start_;
+}
+
+
+void WriteBuffer::TearDown() {
+  delete virtual_memory_;
+  top_ = start_ = limit_ = NULL;
+}
+
+} }  // namespace v8::internal