[interpreter] Add OSR nesting level to bytecode header.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 82155dafc2820a8d5f9827f1351ebf4de998f727..02d2c914e10113794dc499bb0870f5ebe9d8f93d 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -4515,6 +4515,10 @@ class BytecodeArray : public FixedArrayBase {
   inline int interrupt_budget() const;
   inline void set_interrupt_budget(int interrupt_budget);
 
+  // Accessors for OSR loop nesting level.
+  inline int osr_loop_nesting_level() const;
+  inline void set_osr_loop_nesting_level(int depth);
+
   // Accessors for the constant pool.
   DECL_ACCESSORS(constant_pool, FixedArray)
 
@@ -4554,7 +4558,8 @@ class BytecodeArray : public FixedArrayBase {
   static const int kFrameSizeOffset = kSourcePositionTableOffset + kPointerSize;
   static const int kParameterSizeOffset = kFrameSizeOffset + kIntSize;
   static const int kInterruptBudgetOffset = kParameterSizeOffset + kIntSize;
-  static const int kHeaderSize = kInterruptBudgetOffset + kIntSize;
+  static const int kOSRNestingLevelOffset = kInterruptBudgetOffset + kIntSize;
+  static const int kHeaderSize = kOSRNestingLevelOffset + kIntSize;

[rmcilroy, 2016/07/22 09:46:01]

The OSR nesting level is always less than a byte (<=6), and the interrupt budget
is set to 0x1800 by flags, so less than two bytes. Could we pack these to avoid
increasing the size of the bytecode array? 

[Michael Starzinger, 2016/07/25 10:24:42]

As discussed offline: Yes, the OSR nesting level is fully bounded by
kMaxNestingLevel (i.e. <= 6). But the interrupt budget is not necessarily
bounded by the initial value, because forward branches will end up increasing
the value temporarily. The budget computation would need to truncate it to the
correct size. The following is an example of how I can imagine the value running
out of bounds (i.e. 32 * 0x1800 + 1000 * n) ...

function f() {
  if (x == 0) {
    // very large block containing n bytes of bytecode.
  }
  f(x - 1)
}
f(1000);

[rmcilroy, 2016/07/25 11:14:47]

Yeah this is an issue I hadn't thought of.  We could also potentially merge it
with parameter_count (since we seem to have a limit of 2^16 arguments in V8) but
it might not be worthwhile. Could you add a TODO(rmcilroy) in case we decide we
want to trim the size of bytecode arrays.

[Michael Starzinger, 2016/07/25 11:21:28]

Done.

 
   // Maximal memory consumption for a single BytecodeArray.
   static const int kMaxSize = 512 * MB;