Support use of external strings as inputs to LoadCodeUnitsInstr.

runtime/vm/regexp_assembler.cc

Index: runtime/vm/regexp_assembler.cc
diff --git a/runtime/vm/regexp_assembler.cc b/runtime/vm/regexp_assembler.cc
index fa666ae3e55f77a89a0fefa3d1117b628693ded0..5784506aa7d52aabb89011b4fc28a0a7ff7db189 100644
--- a/runtime/vm/regexp_assembler.cc
+++ b/runtime/vm/regexp_assembler.cc
@@ -613,6 +613,32 @@ Value* IRRegExpMacroAssembler::BindLoadLocal(const LocalVariable& local) {
 }
 
 
+Value* IRRegExpMacroAssembler::BindUntaggedExternalString(
+    const LocalVariable& str) {
+  Value* str_val = BindLoadLocal(str);
+  if (!RawObject::IsExternalStringClassId(specialization_cid_)) {
+    return str_val;
+  }
+  // The data of an external string is stored through two indirections.
+  intptr_t external_offset;
+  intptr_t data_offset;
+  if (specialization_cid_ == kExternalOneByteStringCid) {
+    external_offset = RawExternalOneByteString::data_offset();
+    data_offset = RawExternalOneByteString::ExternalData::data_offset();
+  } else if (specialization_cid_ == kExternalTwoByteStringCid) {
+    external_offset = RawExternalTwoByteString::data_offset();
+    data_offset = RawExternalTwoByteString::ExternalData::data_offset();
+  } else {
+    UNREACHABLE();
+  }
+  Value* external_val =
+      Bind(new(I) LoadUntaggedInstr(str_val, external_offset));
+  Value* data_val =
+      Bind(new(I) LoadUntaggedInstr(external_val, data_offset));
+  return data_val;
+}
+
+
 // In some cases, the V8 irregexp engine generates unreachable code by emitting
 // a jmp not followed by a bind. We cannot do the same, since it is impossible
 // to append to a block following a jmp. In such cases, assume that we are doing
@@ -1766,7 +1792,7 @@ void IRRegExpMacroAssembler::LoadCurrentCharacterUnchecked(
   }
 
   // Bind the pattern as the load receiver.
-  Value* pattern = BindLoadLocal(*string_param_);
+  Value* pattern = BindUntaggedExternalString(*string_param_);
 
   // Calculate the addressed string index as:
   //    cp_offset + current_position_ + string_param_length_
@@ -1789,7 +1815,7 @@ void IRRegExpMacroAssembler::LoadCurrentCharacterUnchecked(
 
 
 Value* IRRegExpMacroAssembler::CharacterAt(Definition* index) {
-  Value* pattern_val = BindLoadLocal(*string_param_);
+  Value* pattern_val = BindUntaggedExternalString(*string_param_);
   Value* index_val = Bind(index);
   return LoadCodeUnitsAt(pattern_val, index_val, 1);
 }