JavaScriptCore/jit/ExecutableAllocatorFixedVMPool.cpp - platform/external/webkit - Git at Google

 /*
  * Copyright (C) 2009 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. 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.
  *
  * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``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 APPLE INC. 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 "config.h"

 #include "ExecutableAllocator.h"

 #if ENABLE(ASSEMBLER) && OS(DARWIN) && CPU(X86_64)

 #include <errno.h>

 #include "TCSpinLock.h"
 #include <mach/mach_init.h>
 #include <mach/vm_map.h>
 #include <sys/mman.h>
 #include <unistd.h>
 #include <wtf/AVLTree.h>
 #include <wtf/VMTags.h>

 using namespace WTF;

 namespace JSC {

 #define TWO_GB (2u * 1024u * 1024u * 1024u)
 #define SIXTEEN_MB (16u * 1024u * 1024u)

 // FreeListEntry describes a free chunk of memory, stored in the freeList.
 struct FreeListEntry {
     FreeListEntry(void* pointer, size_t size)
         : pointer(pointer)
         , size(size)
         , nextEntry(0)
         , less(0)
         , greater(0)
         , balanceFactor(0)
     {
     }

     // All entries of the same size share a single entry
     // in the AVLTree, and are linked together in a linked
     // list, using nextEntry.
     void* pointer;
     size_t size;
     FreeListEntry* nextEntry;

     // These fields are used by AVLTree.
     FreeListEntry* less;
     FreeListEntry* greater;
     int balanceFactor;
 };

 // Abstractor class for use in AVLTree.
 // Nodes in the AVLTree are of type FreeListEntry, keyed on
 // (and thus sorted by) their size.
 struct AVLTreeAbstractorForFreeList {
     typedef FreeListEntry* handle;
     typedef int32_t size;
     typedef size_t key;

     handle get_less(handle h) { return h->less; }
     void set_less(handle h, handle lh) { h->less = lh; }
     handle get_greater(handle h) { return h->greater; }
     void set_greater(handle h, handle gh) { h->greater = gh; }
     int get_balance_factor(handle h) { return h->balanceFactor; }
     void set_balance_factor(handle h, int bf) { h->balanceFactor = bf; }

     static handle null() { return 0; }

     int compare_key_key(key va, key vb) { return va - vb; }
     int compare_key_node(key k, handle h) { return compare_key_key(k, h->size); }
     int compare_node_node(handle h1, handle h2) { return compare_key_key(h1->size, h2->size); }
 };

 // Used to reverse sort an array of FreeListEntry pointers.
 static int reverseSortFreeListEntriesByPointer(const void* leftPtr, const void* rightPtr)
 {
     FreeListEntry* left = *(FreeListEntry**)leftPtr;
     FreeListEntry* right = *(FreeListEntry**)rightPtr;

     return (intptr_t)(right->pointer) - (intptr_t)(left->pointer);
 }

 // Used to reverse sort an array of pointers.
 static int reverseSortCommonSizedAllocations(const void* leftPtr, const void* rightPtr)
 {
     void* left = *(void**)leftPtr;
     void* right = *(void**)rightPtr;

     return (intptr_t)right - (intptr_t)left;
 }

 class FixedVMPoolAllocator
 {
     // The free list is stored in a sorted tree.
     typedef AVLTree<AVLTreeAbstractorForFreeList, 40> SizeSortedFreeTree;

     // Use madvise as apropriate to prevent freed pages from being spilled,
     // and to attempt to ensure that used memory is reported correctly.
 #if HAVE(MADV_FREE_REUSE)
     void release(void* position, size_t size)
     {
         while (madvise(position, size, MADV_FREE_REUSABLE) == -1 && errno == EAGAIN) { }
     }

     void reuse(void* position, size_t size)
     {
         while (madvise(position, size, MADV_FREE_REUSE) == -1 && errno == EAGAIN) { }
     }
 #elif HAVE(MADV_DONTNEED)
     void release(void* position, size_t size)
     {
         while (madvise(position, size, MADV_DONTNEED) == -1 && errno == EAGAIN) { }
     }

     void reuse(void*, size_t) {}
 #else
     void release(void*, size_t) {}
     void reuse(void*, size_t) {}
 #endif

     // All addition to the free list should go through this method, rather than
     // calling insert directly, to avoid multiple entries beging added with the
     // same key.  All nodes being added should be singletons, they should not
     // already be a part of a chain.
     void addToFreeList(FreeListEntry* entry)
     {
         ASSERT(!entry->nextEntry);

         if (entry->size == m_commonSize) {
             m_commonSizedAllocations.append(entry->pointer);
             delete entry;
         } else if (FreeListEntry* entryInFreeList = m_freeList.search(entry->size, m_freeList.EQUAL)) {
             // m_freeList already contain an entry for this size - insert this node into the chain.
             entry->nextEntry = entryInFreeList->nextEntry;
             entryInFreeList->nextEntry = entry;
         } else
             m_freeList.insert(entry);
     }

     // We do not attempt to coalesce addition, which may lead to fragmentation;
     // instead we periodically perform a sweep to try to coalesce neigboring
     // entries in m_freeList.  Presently this is triggered at the point 16MB
     // of memory has been released.
     void coalesceFreeSpace()
     {
         Vector<FreeListEntry*> freeListEntries;
         SizeSortedFreeTree::Iterator iter;
         iter.start_iter_least(m_freeList);

         // Empty m_freeList into a Vector.
         for (FreeListEntry* entry; (entry = *iter); ++iter) {
             // Each entry in m_freeList might correspond to multiple
             // free chunks of memory (of the same size).  Walk the chain
             // (this is likely of couse only be one entry long!) adding
             // each entry to the Vector (at reseting the next in chain
             // pointer to separate each node out).
             FreeListEntry* next;
             do {
                 next = entry->nextEntry;
                 entry->nextEntry = 0;
                 freeListEntries.append(entry);
             } while ((entry = next));
         }
         // All entries are now in the Vector; purge the tree.
         m_freeList.purge();

         // Reverse-sort the freeListEntries and m_commonSizedAllocations Vectors.
         // We reverse-sort so that we can logically work forwards through memory,
         // whilst popping items off the end of the Vectors using last() and removeLast().
         qsort(freeListEntries.begin(), freeListEntries.size(), sizeof(FreeListEntry*), reverseSortFreeListEntriesByPointer);
         qsort(m_commonSizedAllocations.begin(), m_commonSizedAllocations.size(), sizeof(void*), reverseSortCommonSizedAllocations);

         // The entries from m_commonSizedAllocations that cannot be
         // coalesced into larger chunks will be temporarily stored here.
         Vector<void*> newCommonSizedAllocations;

         // Keep processing so long as entries remain in either of the vectors.
         while (freeListEntries.size() || m_commonSizedAllocations.size()) {
             // We're going to try to find a FreeListEntry node that we can coalesce onto.
             FreeListEntry* coalescionEntry = 0;

             // Is the lowest addressed chunk of free memory of common-size, or is it in the free list?
             if (m_commonSizedAllocations.size() && (!freeListEntries.size() || (m_commonSizedAllocations.last() < freeListEntries.last()->pointer))) {
                 // Pop an item from the m_commonSizedAllocations vector - this is the lowest
                 // addressed free chunk.  Find out the begin and end addresses of the memory chunk.
                 void* begin = m_commonSizedAllocations.last();
                 void* end = (void*)((intptr_t)begin + m_commonSize);
                 m_commonSizedAllocations.removeLast();

                 // Try to find another free chunk abutting onto the end of the one we have already found.
                 if (freeListEntries.size() && (freeListEntries.last()->pointer == end)) {
                     // There is an existing FreeListEntry for the next chunk of memory!
                     // we can reuse this.  Pop it off the end of m_freeList.
                     coalescionEntry = freeListEntries.last();
                     freeListEntries.removeLast();
                     // Update the existing node to include the common-sized chunk that we also found.
                     coalescionEntry->pointer = (void*)((intptr_t)coalescionEntry->pointer - m_commonSize);
                     coalescionEntry->size += m_commonSize;
                 } else if (m_commonSizedAllocations.size() && (m_commonSizedAllocations.last() == end)) {
                     // There is a second common-sized chunk that can be coalesced.
                     // Allocate a new node.
                     m_commonSizedAllocations.removeLast();
                     coalescionEntry = new FreeListEntry(begin, 2 * m_commonSize);
                 } else {
                     // Nope - this poor little guy is all on his own. :-(
                     // Add him into the newCommonSizedAllocations vector for now, we're
                     // going to end up adding him back into the m_commonSizedAllocations
                     // list when we're done.
                     newCommonSizedAllocations.append(begin);
                     continue;
                 }
             } else {
                 ASSERT(freeListEntries.size());
                 ASSERT(!m_commonSizedAllocations.size() || (freeListEntries.last()->pointer < m_commonSizedAllocations.last()));
                 // The lowest addressed item is from m_freeList; pop it from the Vector.
                 coalescionEntry = freeListEntries.last();
                 freeListEntries.removeLast();
             }

             // Right, we have a FreeListEntry, we just need check if there is anything else
             // to coalesce onto the end.
             ASSERT(coalescionEntry);
             while (true) {
                 // Calculate the end address of the chunk we have found so far.
                 void* end = (void*)((intptr_t)coalescionEntry->pointer - coalescionEntry->size);

                 // Is there another chunk adjacent to the one we already have?
                 if (freeListEntries.size() && (freeListEntries.last()->pointer == end)) {
                     // Yes - another FreeListEntry -pop it from the list.
                     FreeListEntry* coalescee = freeListEntries.last();
                     freeListEntries.removeLast();
                     // Add it's size onto our existing node.
                     coalescionEntry->size += coalescee->size;
                     delete coalescee;
                 } else if (m_commonSizedAllocations.size() && (m_commonSizedAllocations.last() == end)) {
                     // We can coalesce the next common-sized chunk.
                     m_commonSizedAllocations.removeLast();
                     coalescionEntry->size += m_commonSize;
                 } else
                     break; // Nope, nothing to be added - stop here.
             }

             // We've coalesced everything we can onto the current chunk.
             // Add it back into m_freeList.
             addToFreeList(coalescionEntry);
         }

         // All chunks of free memory larger than m_commonSize should be
         // back in m_freeList by now.  All that remains to be done is to
         // copy the contents on the newCommonSizedAllocations back into
         // the m_commonSizedAllocations Vector.
         ASSERT(m_commonSizedAllocations.size() == 0);
         m_commonSizedAllocations.append(newCommonSizedAllocations);
     }

 public:

     FixedVMPoolAllocator(size_t commonSize, size_t totalHeapSize)
         : m_commonSize(commonSize)
         , m_countFreedSinceLastCoalesce(0)
         , m_totalHeapSize(totalHeapSize)
     {
         // Cook up an address to allocate at, using the following recipe:
         //   17 bits of zero, stay in userspace kids.
         //   26 bits of randomness for ASLR.
         //   21 bits of zero, at least stay aligned within one level of the pagetables.
         //
         // But! - as a temporary workaround for some plugin problems (rdar://problem/6812854),
         // for now instead of 2^26 bits of ASLR lets stick with 25 bits of randomization plus
         // 2^24, which should put up somewhere in the middle of usespace (in the address range
         // 0x200000000000 .. 0x5fffffffffff).
         intptr_t randomLocation = arc4random() & ((1 << 25) - 1);
         randomLocation += (1 << 24);
         randomLocation <<= 21;
         m_base = mmap(reinterpret_cast<void*>(randomLocation), m_totalHeapSize, INITIAL_PROTECTION_FLAGS, MAP_PRIVATE | MAP_ANON, VM_TAG_FOR_EXECUTABLEALLOCATOR_MEMORY, 0);
         if (!m_base)
             CRASH();

         // For simplicity, we keep all memory in m_freeList in a 'released' state.
         // This means that we can simply reuse all memory when allocating, without
         // worrying about it's previous state, and also makes coalescing m_freeList
         // simpler since we need not worry about the possibility of coalescing released
         // chunks with non-released ones.
         release(m_base, m_totalHeapSize);
         m_freeList.insert(new FreeListEntry(m_base, m_totalHeapSize));
     }

     void* alloc(size_t size)
     {
         void* result;

         // Freed allocations of the common size are not stored back into the main
         // m_freeList, but are instead stored in a separate vector.  If the request
         // is for a common sized allocation, check this list.
         if ((size == m_commonSize) && m_commonSizedAllocations.size()) {
             result = m_commonSizedAllocations.last();
             m_commonSizedAllocations.removeLast();
         } else {
             // Serach m_freeList for a suitable sized chunk to allocate memory from.
             FreeListEntry* entry = m_freeList.search(size, m_freeList.GREATER_EQUAL);

             // This would be bad news.
             if (!entry) {
                 // Errk!  Lets take a last-ditch desparation attempt at defragmentation...
                 coalesceFreeSpace();
                 // Did that free up a large enough chunk?
                 entry = m_freeList.search(size, m_freeList.GREATER_EQUAL);
                 // No?...  *BOOM!*
                 if (!entry)
                     CRASH();
             }
             ASSERT(entry->size != m_commonSize);

             // Remove the entry from m_freeList.  But! -
             // Each entry in the tree may represent a chain of multiple chunks of the
             // same size, and we only want to remove one on them.  So, if this entry
             // does have a chain, just remove the first-but-one item from the chain.
             if (FreeListEntry* next = entry->nextEntry) {
                 // We're going to leave 'entry' in the tree; remove 'next' from its chain.
                 entry->nextEntry = next->nextEntry;
                 next->nextEntry = 0;
                 entry = next;
             } else
                 m_freeList.remove(entry->size);

             // Whoo!, we have a result!
             ASSERT(entry->size >= size);
             result = entry->pointer;

             // If the allocation exactly fits the chunk we found in the,
             // m_freeList then the FreeListEntry node is no longer needed.
             if (entry->size == size)
                 delete entry;
             else {
                 // There is memory left over, and it is not of the common size.
                 // We can reuse the existing FreeListEntry node to add this back
                 // into m_freeList.
                 entry->pointer = (void*)((intptr_t)entry->pointer + size);
                 entry->size -= size;
                 addToFreeList(entry);
             }
         }

         // Call reuse to report to the operating system that this memory is in use.
         ASSERT(isWithinVMPool(result, size));
         reuse(result, size);
         return result;
     }

     void free(void* pointer, size_t size)
     {
         // Call release to report to the operating system that this
         // memory is no longer in use, and need not be paged out.
         ASSERT(isWithinVMPool(pointer, size));
         release(pointer, size);

         // Common-sized allocations are stored in the m_commonSizedAllocations
         // vector; all other freed chunks are added to m_freeList.
         if (size == m_commonSize)
             m_commonSizedAllocations.append(pointer);
         else
             addToFreeList(new FreeListEntry(pointer, size));

         // Do some housekeeping.  Every time we reach a point that
         // 16MB of allocations have been freed, sweep m_freeList
         // coalescing any neighboring fragments.
         m_countFreedSinceLastCoalesce += size;
         if (m_countFreedSinceLastCoalesce >= SIXTEEN_MB) {
             m_countFreedSinceLastCoalesce = 0;
             coalesceFreeSpace();
         }
     }

 private:

 #ifndef NDEBUG
     bool isWithinVMPool(void* pointer, size_t size)
     {
         return pointer >= m_base && (reinterpret_cast<char*>(pointer) + size <= reinterpret_cast<char*>(m_base) + m_totalHeapSize);
     }
 #endif

     // Freed space from the most common sized allocations will be held in this list, ...
     const size_t m_commonSize;
     Vector<void*> m_commonSizedAllocations;

     // ... and all other freed allocations are held in m_freeList.
     SizeSortedFreeTree m_freeList;

     // This is used for housekeeping, to trigger defragmentation of the freed lists.
     size_t m_countFreedSinceLastCoalesce;

     void* m_base;
     size_t m_totalHeapSize;
 };

 void ExecutableAllocator::intializePageSize()
 {
     ExecutableAllocator::pageSize = getpagesize();
 }

 static FixedVMPoolAllocator* allocator = 0;
 static SpinLock spinlock = SPINLOCK_INITIALIZER;

 ExecutablePool::Allocation ExecutablePool::systemAlloc(size_t size)
 {
   SpinLockHolder lock_holder(&spinlock);

     if (!allocator)
         allocator = new FixedVMPoolAllocator(JIT_ALLOCATOR_LARGE_ALLOC_SIZE, TWO_GB);
     ExecutablePool::Allocation alloc = {reinterpret_cast<char*>(allocator->alloc(size)), size};
     return alloc;
 }

 void ExecutablePool::systemRelease(const ExecutablePool::Allocation& allocation)
 {
   SpinLockHolder lock_holder(&spinlock);

     ASSERT(allocator);
     allocator->free(allocation.pages, allocation.size);
 }

 }

 #endif // HAVE(ASSEMBLER)
	/*
	* Copyright (C) 2009 Apple Inc. All rights reserved.
	*
	* Redistribution and use in source and binary forms, with or without
	* modification, are permitted provided that the following conditions
	* are met:
	* 1. Redistributions of source code must retain the above copyright
	* notice, this list of conditions and the following disclaimer.
	* 2. 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.
	*
	* THIS SOFTWARE IS PROVIDED BY APPLE INC. ``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 APPLE INC. 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 "config.h"

	#include "ExecutableAllocator.h"

	#if ENABLE(ASSEMBLER) && OS(DARWIN) && CPU(X86_64)

	#include <errno.h>

	#include "TCSpinLock.h"
	#include <mach/mach_init.h>
	#include <mach/vm_map.h>
	#include <sys/mman.h>
	#include <unistd.h>
	#include <wtf/AVLTree.h>
	#include <wtf/VMTags.h>

	using namespace WTF;

	namespace JSC {

	#define TWO_GB (2u * 1024u * 1024u * 1024u)
	#define SIXTEEN_MB (16u * 1024u * 1024u)

	// FreeListEntry describes a free chunk of memory, stored in the freeList.
	struct FreeListEntry {
	FreeListEntry(void* pointer, size_t size)
	: pointer(pointer)
	, size(size)
	, nextEntry(0)
	, less(0)
	, greater(0)
	, balanceFactor(0)
	{
	}

	// All entries of the same size share a single entry
	// in the AVLTree, and are linked together in a linked
	// list, using nextEntry.
	void* pointer;
	size_t size;
	FreeListEntry* nextEntry;

	// These fields are used by AVLTree.
	FreeListEntry* less;
	FreeListEntry* greater;
	int balanceFactor;
	};

	// Abstractor class for use in AVLTree.
	// Nodes in the AVLTree are of type FreeListEntry, keyed on
	// (and thus sorted by) their size.
	struct AVLTreeAbstractorForFreeList {
	typedef FreeListEntry* handle;
	typedef int32_t size;
	typedef size_t key;

	handle get_less(handle h) { return h->less; }
	void set_less(handle h, handle lh) { h->less = lh; }
	handle get_greater(handle h) { return h->greater; }
	void set_greater(handle h, handle gh) { h->greater = gh; }
	int get_balance_factor(handle h) { return h->balanceFactor; }
	void set_balance_factor(handle h, int bf) { h->balanceFactor = bf; }

	static handle null() { return 0; }

	int compare_key_key(key va, key vb) { return va - vb; }
	int compare_key_node(key k, handle h) { return compare_key_key(k, h->size); }
	int compare_node_node(handle h1, handle h2) { return compare_key_key(h1->size, h2->size); }
	};

	// Used to reverse sort an array of FreeListEntry pointers.
	static int reverseSortFreeListEntriesByPointer(const void* leftPtr, const void* rightPtr)
	{
	FreeListEntry* left = (FreeListEntry*)leftPtr;
	FreeListEntry* right = (FreeListEntry*)rightPtr;

	return (intptr_t)(right->pointer) - (intptr_t)(left->pointer);
	}

	// Used to reverse sort an array of pointers.
	static int reverseSortCommonSizedAllocations(const void* leftPtr, const void* rightPtr)
	{
	void* left = (void*)leftPtr;
	void* right = (void*)rightPtr;

	return (intptr_t)right - (intptr_t)left;
	}

	class FixedVMPoolAllocator
	{
	// The free list is stored in a sorted tree.
	typedef AVLTree<AVLTreeAbstractorForFreeList, 40> SizeSortedFreeTree;

	// Use madvise as apropriate to prevent freed pages from being spilled,
	// and to attempt to ensure that used memory is reported correctly.
	#if HAVE(MADV_FREE_REUSE)
	void release(void* position, size_t size)
	{
	while (madvise(position, size, MADV_FREE_REUSABLE) == -1 && errno == EAGAIN) { }
	}

	void reuse(void* position, size_t size)
	{
	while (madvise(position, size, MADV_FREE_REUSE) == -1 && errno == EAGAIN) { }
	}
	#elif HAVE(MADV_DONTNEED)
	void release(void* position, size_t size)
	{
	while (madvise(position, size, MADV_DONTNEED) == -1 && errno == EAGAIN) { }
	}

	void reuse(void*, size_t) {}
	#else
	void release(void*, size_t) {}
	void reuse(void*, size_t) {}
	#endif

	// All addition to the free list should go through this method, rather than
	// calling insert directly, to avoid multiple entries beging added with the
	// same key. All nodes being added should be singletons, they should not
	// already be a part of a chain.
	void addToFreeList(FreeListEntry* entry)
	{
	ASSERT(!entry->nextEntry);

	if (entry->size == m_commonSize) {
	m_commonSizedAllocations.append(entry->pointer);
	delete entry;
	} else if (FreeListEntry* entryInFreeList = m_freeList.search(entry->size, m_freeList.EQUAL)) {
	// m_freeList already contain an entry for this size - insert this node into the chain.
	entry->nextEntry = entryInFreeList->nextEntry;
	entryInFreeList->nextEntry = entry;
	} else
	m_freeList.insert(entry);
	}

	// We do not attempt to coalesce addition, which may lead to fragmentation;
	// instead we periodically perform a sweep to try to coalesce neigboring
	// entries in m_freeList. Presently this is triggered at the point 16MB
	// of memory has been released.
	void coalesceFreeSpace()
	{
	Vector<FreeListEntry*> freeListEntries;
	SizeSortedFreeTree::Iterator iter;
	iter.start_iter_least(m_freeList);

	// Empty m_freeList into a Vector.
	for (FreeListEntry* entry; (entry = *iter); ++iter) {
	// Each entry in m_freeList might correspond to multiple
	// free chunks of memory (of the same size). Walk the chain
	// (this is likely of couse only be one entry long!) adding
	// each entry to the Vector (at reseting the next in chain
	// pointer to separate each node out).
	FreeListEntry* next;
	do {
	next = entry->nextEntry;
	entry->nextEntry = 0;
	freeListEntries.append(entry);
	} while ((entry = next));
	}
	// All entries are now in the Vector; purge the tree.
	m_freeList.purge();

	// Reverse-sort the freeListEntries and m_commonSizedAllocations Vectors.
	// We reverse-sort so that we can logically work forwards through memory,
	// whilst popping items off the end of the Vectors using last() and removeLast().
	qsort(freeListEntries.begin(), freeListEntries.size(), sizeof(FreeListEntry*), reverseSortFreeListEntriesByPointer);
	qsort(m_commonSizedAllocations.begin(), m_commonSizedAllocations.size(), sizeof(void*), reverseSortCommonSizedAllocations);

	// The entries from m_commonSizedAllocations that cannot be
	// coalesced into larger chunks will be temporarily stored here.
	Vector<void*> newCommonSizedAllocations;

	// Keep processing so long as entries remain in either of the vectors.
	while (freeListEntries.size() \|\| m_commonSizedAllocations.size()) {
	// We're going to try to find a FreeListEntry node that we can coalesce onto.
	FreeListEntry* coalescionEntry = 0;

	// Is the lowest addressed chunk of free memory of common-size, or is it in the free list?
	if (m_commonSizedAllocations.size() && (!freeListEntries.size() \|\| (m_commonSizedAllocations.last() < freeListEntries.last()->pointer))) {
	// Pop an item from the m_commonSizedAllocations vector - this is the lowest
	// addressed free chunk. Find out the begin and end addresses of the memory chunk.
	void* begin = m_commonSizedAllocations.last();
	void* end = (void*)((intptr_t)begin + m_commonSize);
	m_commonSizedAllocations.removeLast();

	// Try to find another free chunk abutting onto the end of the one we have already found.
	if (freeListEntries.size() && (freeListEntries.last()->pointer == end)) {
	// There is an existing FreeListEntry for the next chunk of memory!
	// we can reuse this. Pop it off the end of m_freeList.
	coalescionEntry = freeListEntries.last();
	freeListEntries.removeLast();
	// Update the existing node to include the common-sized chunk that we also found.
	coalescionEntry->pointer = (void*)((intptr_t)coalescionEntry->pointer - m_commonSize);
	coalescionEntry->size += m_commonSize;
	} else if (m_commonSizedAllocations.size() && (m_commonSizedAllocations.last() == end)) {
	// There is a second common-sized chunk that can be coalesced.
	// Allocate a new node.
	m_commonSizedAllocations.removeLast();
	coalescionEntry = new FreeListEntry(begin, 2 * m_commonSize);
	} else {
	// Nope - this poor little guy is all on his own. :-(
	// Add him into the newCommonSizedAllocations vector for now, we're
	// going to end up adding him back into the m_commonSizedAllocations
	// list when we're done.
	newCommonSizedAllocations.append(begin);
	continue;
	}
	} else {
	ASSERT(freeListEntries.size());
	ASSERT(!m_commonSizedAllocations.size() \|\| (freeListEntries.last()->pointer < m_commonSizedAllocations.last()));
	// The lowest addressed item is from m_freeList; pop it from the Vector.
	coalescionEntry = freeListEntries.last();
	freeListEntries.removeLast();
	}

	// Right, we have a FreeListEntry, we just need check if there is anything else
	// to coalesce onto the end.
	ASSERT(coalescionEntry);
	while (true) {
	// Calculate the end address of the chunk we have found so far.
	void* end = (void*)((intptr_t)coalescionEntry->pointer - coalescionEntry->size);

	// Is there another chunk adjacent to the one we already have?
	if (freeListEntries.size() && (freeListEntries.last()->pointer == end)) {
	// Yes - another FreeListEntry -pop it from the list.
	FreeListEntry* coalescee = freeListEntries.last();
	freeListEntries.removeLast();
	// Add it's size onto our existing node.
	coalescionEntry->size += coalescee->size;
	delete coalescee;
	} else if (m_commonSizedAllocations.size() && (m_commonSizedAllocations.last() == end)) {
	// We can coalesce the next common-sized chunk.
	m_commonSizedAllocations.removeLast();
	coalescionEntry->size += m_commonSize;
	} else
	break; // Nope, nothing to be added - stop here.
	}

	// We've coalesced everything we can onto the current chunk.
	// Add it back into m_freeList.
	addToFreeList(coalescionEntry);
	}

	// All chunks of free memory larger than m_commonSize should be
	// back in m_freeList by now. All that remains to be done is to
	// copy the contents on the newCommonSizedAllocations back into
	// the m_commonSizedAllocations Vector.
	ASSERT(m_commonSizedAllocations.size() == 0);
	m_commonSizedAllocations.append(newCommonSizedAllocations);
	}

	public:

	FixedVMPoolAllocator(size_t commonSize, size_t totalHeapSize)
	: m_commonSize(commonSize)
	, m_countFreedSinceLastCoalesce(0)
	, m_totalHeapSize(totalHeapSize)
	{
	// Cook up an address to allocate at, using the following recipe:
	// 17 bits of zero, stay in userspace kids.
	// 26 bits of randomness for ASLR.
	// 21 bits of zero, at least stay aligned within one level of the pagetables.
	//
	// But! - as a temporary workaround for some plugin problems (rdar://problem/6812854),
	// for now instead of 2^26 bits of ASLR lets stick with 25 bits of randomization plus
	// 2^24, which should put up somewhere in the middle of usespace (in the address range
	// 0x200000000000 .. 0x5fffffffffff).
	intptr_t randomLocation = arc4random() & ((1 << 25) - 1);
	randomLocation += (1 << 24);
	randomLocation <<= 21;
	m_base = mmap(reinterpret_cast<void*>(randomLocation), m_totalHeapSize, INITIAL_PROTECTION_FLAGS, MAP_PRIVATE \| MAP_ANON, VM_TAG_FOR_EXECUTABLEALLOCATOR_MEMORY, 0);
	if (!m_base)
	CRASH();

	// For simplicity, we keep all memory in m_freeList in a 'released' state.
	// This means that we can simply reuse all memory when allocating, without
	// worrying about it's previous state, and also makes coalescing m_freeList
	// simpler since we need not worry about the possibility of coalescing released
	// chunks with non-released ones.
	release(m_base, m_totalHeapSize);
	m_freeList.insert(new FreeListEntry(m_base, m_totalHeapSize));
	}

	void* alloc(size_t size)
	{
	void* result;

	// Freed allocations of the common size are not stored back into the main
	// m_freeList, but are instead stored in a separate vector. If the request
	// is for a common sized allocation, check this list.
	if ((size == m_commonSize) && m_commonSizedAllocations.size()) {
	result = m_commonSizedAllocations.last();
	m_commonSizedAllocations.removeLast();
	} else {
	// Serach m_freeList for a suitable sized chunk to allocate memory from.
	FreeListEntry* entry = m_freeList.search(size, m_freeList.GREATER_EQUAL);

	// This would be bad news.
	if (!entry) {
	// Errk! Lets take a last-ditch desparation attempt at defragmentation...
	coalesceFreeSpace();
	// Did that free up a large enough chunk?
	entry = m_freeList.search(size, m_freeList.GREATER_EQUAL);
	// No?... BOOM!
	if (!entry)
	CRASH();
	}
	ASSERT(entry->size != m_commonSize);

	// Remove the entry from m_freeList. But! -
	// Each entry in the tree may represent a chain of multiple chunks of the
	// same size, and we only want to remove one on them. So, if this entry
	// does have a chain, just remove the first-but-one item from the chain.
	if (FreeListEntry* next = entry->nextEntry) {
	// We're going to leave 'entry' in the tree; remove 'next' from its chain.
	entry->nextEntry = next->nextEntry;
	next->nextEntry = 0;
	entry = next;
	} else
	m_freeList.remove(entry->size);

	// Whoo!, we have a result!
	ASSERT(entry->size >= size);
	result = entry->pointer;

	// If the allocation exactly fits the chunk we found in the,
	// m_freeList then the FreeListEntry node is no longer needed.
	if (entry->size == size)
	delete entry;
	else {
	// There is memory left over, and it is not of the common size.
	// We can reuse the existing FreeListEntry node to add this back
	// into m_freeList.
	entry->pointer = (void*)((intptr_t)entry->pointer + size);
	entry->size -= size;
	addToFreeList(entry);
	}
	}

	// Call reuse to report to the operating system that this memory is in use.
	ASSERT(isWithinVMPool(result, size));
	reuse(result, size);
	return result;
	}

	void free(void* pointer, size_t size)
	{
	// Call release to report to the operating system that this
	// memory is no longer in use, and need not be paged out.
	ASSERT(isWithinVMPool(pointer, size));
	release(pointer, size);

	// Common-sized allocations are stored in the m_commonSizedAllocations
	// vector; all other freed chunks are added to m_freeList.
	if (size == m_commonSize)
	m_commonSizedAllocations.append(pointer);
	else
	addToFreeList(new FreeListEntry(pointer, size));

	// Do some housekeeping. Every time we reach a point that
	// 16MB of allocations have been freed, sweep m_freeList
	// coalescing any neighboring fragments.
	m_countFreedSinceLastCoalesce += size;
	if (m_countFreedSinceLastCoalesce >= SIXTEEN_MB) {
	m_countFreedSinceLastCoalesce = 0;
	coalesceFreeSpace();
	}
	}

	private:

	#ifndef NDEBUG
	bool isWithinVMPool(void* pointer, size_t size)
	{
	return pointer >= m_base && (reinterpret_cast<char>(pointer) + size <= reinterpret_cast<char>(m_base) + m_totalHeapSize);
	}
	#endif

	// Freed space from the most common sized allocations will be held in this list, ...
	const size_t m_commonSize;
	Vector<void*> m_commonSizedAllocations;

	// ... and all other freed allocations are held in m_freeList.
	SizeSortedFreeTree m_freeList;

	// This is used for housekeeping, to trigger defragmentation of the freed lists.
	size_t m_countFreedSinceLastCoalesce;

	void* m_base;
	size_t m_totalHeapSize;
	};

	void ExecutableAllocator::intializePageSize()
	{
	ExecutableAllocator::pageSize = getpagesize();
	}

	static FixedVMPoolAllocator* allocator = 0;
	static SpinLock spinlock = SPINLOCK_INITIALIZER;

	ExecutablePool::Allocation ExecutablePool::systemAlloc(size_t size)
	{
	SpinLockHolder lock_holder(&spinlock);

	if (!allocator)
	allocator = new FixedVMPoolAllocator(JIT_ALLOCATOR_LARGE_ALLOC_SIZE, TWO_GB);
	ExecutablePool::Allocation alloc = {reinterpret_cast<char*>(allocator->alloc(size)), size};
	return alloc;
	}

	void ExecutablePool::systemRelease(const ExecutablePool::Allocation& allocation)
	{
	SpinLockHolder lock_holder(&spinlock);

	ASSERT(allocator);
	allocator->free(allocation.pages, allocation.size);
	}

	}

	#endif // HAVE(ASSEMBLER)