Serendipitously arrange the tags so that String.length() becomes a branch-fre...

src/objects-inl.h

 // Copyright 2006-2008 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * 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
@@ skipping 1253 matching lines @@
 bool String::Equals(String* other) {
   if (other == this) return true;
   if (IsSymbol() && other->IsSymbol()) return false;
   return SlowEquals(other);
 }
 
 
 int String::length() {
   uint32_t len = READ_INT_FIELD(this, kLengthOffset);
 
-  switch (size_tag()) {
-    case kShortStringTag:
-      return  len >> kShortLengthShift;
-    case kMediumStringTag:
-      return len >> kMediumLengthShift;
-    case kLongStringTag:
-      return len >> kLongLengthShift;
-    default:
-      break;
-  }
-  UNREACHABLE();
-  return 0;
+  ASSERT(kShortStringTag + kLongLengthShift == kShortLengthShift);
+  ASSERT(kMediumStringTag + kLongLengthShift == kMediumLengthShift);
+  ASSERT(kLongStringTag == 0);
+
+  return len >> (size_tag() + kLongLengthShift);
 }
 
 
 void String::set_length(int value) {
-  switch (size_tag()) {
-    case kShortStringTag:
-      WRITE_INT_FIELD(this, kLengthOffset, value << kShortLengthShift);
-      break;
-    case kMediumStringTag:
-      WRITE_INT_FIELD(this, kLengthOffset, value << kMediumLengthShift);
-      break;
-    case kLongStringTag:
-      WRITE_INT_FIELD(this, kLengthOffset, value << kLongLengthShift);
-      break;
-    default:
-      UNREACHABLE();
-      break;
-  }
+  ASSERT(kShortStringTag + kLongLengthShift == kShortLengthShift);
+  ASSERT(kMediumStringTag + kLongLengthShift == kMediumLengthShift);
+  ASSERT(kLongStringTag == 0);
+
+  WRITE_INT_FIELD(this, kLengthOffset, value << (size_tag() + kLongLengthShift));
 }
 
 
 uint32_t String::length_field() {
   return READ_UINT32_FIELD(this, kLengthOffset);
 }
 
 
 void String::set_length_field(uint32_t value) {
   WRITE_UINT32_FIELD(this, kLengthOffset, value);
@@ skipping 163 matching lines @@
 
 void SeqTwoByteString::SeqTwoByteStringSet(int index, uint16_t value) {
   ASSERT(index >= 0 && index < length());
   WRITE_SHORT_FIELD(this, kHeaderSize + index * kShortSize, value);
 }
 
 
 int SeqTwoByteString::SeqTwoByteStringSize(Map* map) {
   uint32_t length = READ_INT_FIELD(this, kLengthOffset);
 
+  ASSERT(kShortStringTag + kLongLengthShift == kShortLengthShift);
+  ASSERT(kMediumStringTag + kLongLengthShift == kMediumLengthShift);
+  ASSERT(kLongStringTag == 0);
+
   // Use the map (and not 'this') to compute the size tag, since
   // TwoByteStringSize is called during GC when maps are encoded.
-  switch (map_size_tag(map)) {
-    case kShortStringTag:
-      length = length >> kShortLengthShift;
-      break;
-    case kMediumStringTag:
-      length = length >> kMediumLengthShift;
-      break;
-    case kLongStringTag:
-      length = length >> kLongLengthShift;
-      break;
-    default:
-      break;
-  }
+  length >>= map_size_tag(map) + kLongLengthShift;
+
   return SizeFor(length);
 }
 
 
 int SeqAsciiString::SeqAsciiStringSize(Map* map) {
   uint32_t length = READ_INT_FIELD(this, kLengthOffset);
 
+  ASSERT(kShortStringTag + kLongLengthShift == kShortLengthShift);
+  ASSERT(kMediumStringTag + kLongLengthShift == kMediumLengthShift);
+  ASSERT(kLongStringTag == 0);
+
   // Use the map (and not 'this') to compute the size tag, since
   // AsciiStringSize is called during GC when maps are encoded.
-  switch (map_size_tag(map)) {
-    case kShortStringTag:
-      length = length >> kShortLengthShift;
-      break;
-    case kMediumStringTag:
-      length = length >> kMediumLengthShift;
-      break;
-    case kLongStringTag:
-      length = length >> kLongLengthShift;
-      break;
-    default:
-      break;
-  }
+  length >>= map_size_tag(map) + kLongLengthShift;
 
   return SizeFor(length);
 }
 
 
 Object* ConsString::first() {
   return READ_FIELD(this, kFirstOffset);
 }
 
 
@@ skipping 850 matching lines @@
 #undef WRITE_INT_FIELD
 #undef READ_SHORT_FIELD
 #undef WRITE_SHORT_FIELD
 #undef READ_BYTE_FIELD
 #undef WRITE_BYTE_FIELD
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_INL_H_