Introduce read buffer for external strings when using charAt (ia32).

src/ia32/macro-assembler-ia32.cc

Index: src/ia32/macro-assembler-ia32.cc
diff --git a/src/ia32/macro-assembler-ia32.cc b/src/ia32/macro-assembler-ia32.cc
index 2d4f4cdb2f551966f55b646299f27c3c014400c9..95a35965a11751938c3daa6652d7733667d2bace 100644
--- a/src/ia32/macro-assembler-ia32.cc
+++ b/src/ia32/macro-assembler-ia32.cc
@@ -2578,9 +2578,9 @@ void MacroAssembler::EnsureNotWhite(
   // Since both black and grey have a 1 in the first position and white does
   // not have a 1 there we only need to check one bit.
   test(mask_scratch, Operand(bitmap_scratch, MemoryChunk::kHeaderSize));
-  j(not_zero, &done, Label::kNear);
 
   if (FLAG_debug_code) {
+    j(not_zero, &done);
     // Check for impossible bit pattern.
     Label ok;
     push(mask_scratch);
@@ -2591,6 +2591,8 @@ void MacroAssembler::EnsureNotWhite(
     int3();
     bind(&ok);
     pop(mask_scratch);
+  } else {
+    j(not_zero, &done, Label::kNear);
   }
 
   // Value is white.  We check whether it is data that doesn't need scanning.
@@ -2621,13 +2623,15 @@ void MacroAssembler::EnsureNotWhite(
   // If it's external, the length is just ExternalString::kSize.
   // Otherwise it's String::kHeaderSize + string->length() * (1 or 2).
   Label not_external;
-  // External strings are the only ones with the kExternalStringTag bit
-  // set.
+  // External strings are the only ones with the kExternalStringTag bit set.
   ASSERT_EQ(0, kSeqStringTag & kExternalStringTag);
   ASSERT_EQ(0, kConsStringTag & kExternalStringTag);
   test_b(instance_type, kExternalStringTag);
   j(zero, &not_external, Label::kNear);
   mov(length, Immediate(ExternalString::kSize));
+  test_b(instance_type, kBufferedStringTag);
+  j(zero, &is_data_object, Label::kNear);
+  mov(length, Immediate(ExternalString::kExtendedSize));
   jmp(&is_data_object, Label::kNear);
 
   bind(&not_external);