[interpreter] Reduce size of OSR nesting level field.

R=rmcilroy@chromium.org
BUG=v8:4764

Review-Url: https://codereview.chromium.org/2185293004
Cr-Commit-Position: refs/heads/master@{#38147}

src/interpreter/interpreter-assembler.cc

@@ -848,7 +848,7 @@ Node* InterpreterAssembler::StackCheckTriggeredInterrupt() {
 Node* InterpreterAssembler::LoadOSRNestingLevel() {
   Node* offset =
       IntPtrConstant(BytecodeArray::kOSRNestingLevelOffset - kHeapObjectTag);
-  return Load(MachineType::Int32(), BytecodeArrayTaggedPointer(), offset);
+  return Load(MachineType::Int8(), BytecodeArrayTaggedPointer(), offset);
 }
 
 void InterpreterAssembler::Abort(BailoutReason bailout_reason) {

src/objects-inl.h

@@ -4099,12 +4099,13 @@ void BytecodeArray::set_interrupt_budget(int interrupt_budget) {
 }
 
 int BytecodeArray::osr_loop_nesting_level() const {
-  return READ_INT_FIELD(this, kOSRNestingLevelOffset);
+  return READ_INT8_FIELD(this, kOSRNestingLevelOffset);
 }
 
 void BytecodeArray::set_osr_loop_nesting_level(int depth) {
   DCHECK(0 <= depth && depth <= AbstractCode::kMaxLoopNestingMarker);
-  WRITE_INT_FIELD(this, kOSRNestingLevelOffset, depth);
+  STATIC_ASSERT(AbstractCode::kMaxLoopNestingMarker < kMaxInt8);
+  WRITE_INT8_FIELD(this, kOSRNestingLevelOffset, depth);
 }
 
 int BytecodeArray::parameter_count() const {

src/objects.h

@@ -4575,10 +4575,8 @@ class BytecodeArray : public FixedArrayBase {
   static const int kFrameSizeOffset = kSourcePositionTableOffset + kPointerSize;
   static const int kParameterSizeOffset = kFrameSizeOffset + kIntSize;
   static const int kInterruptBudgetOffset = kParameterSizeOffset + kIntSize;
-  // TODO(4764): The OSR nesting level is guaranteed to be in [0;6] bounds and
-  // could potentially be merged with another field (e.g. parameter_size).
   static const int kOSRNestingLevelOffset = kInterruptBudgetOffset + kIntSize;
-  static const int kHeaderSize = kOSRNestingLevelOffset + kIntSize;
+  static const int kHeaderSize = kOSRNestingLevelOffset + kCharSize;
 
   // Maximal memory consumption for a single BytecodeArray.
   static const int kMaxSize = 512 * MB;