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

src/write-buffer.cc

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+
+#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