Reserve code range block for evacuation.

src/heap/spaces.h

Index: src/heap/spaces.h
diff --git a/src/heap/spaces.h b/src/heap/spaces.h
index ef294b243948b3cd4bbdc5101385baf295cb8a73..dcd336437e9cdc161ff5b157a3f1de512a4e051e 100644
--- a/src/heap/spaces.h
+++ b/src/heap/spaces.h
@@ -900,6 +900,9 @@ class CodeRange {
   bool UncommitRawMemory(Address start, size_t length);
   void FreeRawMemory(Address buf, size_t length);
 
+  void ReserveEmergencyBlock();
+  void ReleaseEmergencyBlock();
+
  private:
   Isolate* isolate_;
 
@@ -908,6 +911,7 @@ class CodeRange {
   // Plain old data class, just a struct plus a constructor.
   class FreeBlock {
    public:
+    FreeBlock() : start(0), size(0) {}
     FreeBlock(Address start_arg, size_t size_arg)
         : start(start_arg), size(size_arg) {
       DCHECK(IsAddressAligned(start, MemoryChunk::kAlignment));
@@ -932,6 +936,12 @@ class CodeRange {
   List<FreeBlock> allocation_list_;
   int current_allocation_block_index_;
 
+  // Emergency block guarantees that we can always allocate a page for
+  // evacuation candidates when code space is compacted. Emergency block is
+  // reserved immediately after GC and is released immedietely before
+  // allocating a page for evacuation.
+  FreeBlock emergency_block_;
+
   // Finds a block on the allocation list that contains at least the
   // requested amount of memory.  If none is found, sorts and merges
   // the existing free memory blocks, and searches again.
@@ -940,6 +950,8 @@ class CodeRange {
   // Compares the start addresses of two free blocks.
   static int CompareFreeBlockAddress(const FreeBlock* left,
                                      const FreeBlock* right);
+  bool ReserveBlock(const size_t requested_size, FreeBlock* block);
+  void ReleaseBlock(const FreeBlock* block);
 
   DISALLOW_COPY_AND_ASSIGN(CodeRange);
 };