Improve implementation of HSeqStringSetChar.

src/arm/lithium-codegen-arm.cc

Index: src/arm/lithium-codegen-arm.cc
diff --git a/src/arm/lithium-codegen-arm.cc b/src/arm/lithium-codegen-arm.cc
index fbe8e171fa596967a65aff143823863b4570dca8..8eca27d77d9427f5c4fa58ae0d58e8cf4956b28c 100644
--- a/src/arm/lithium-codegen-arm.cc
+++ b/src/arm/lithium-codegen-arm.cc
@@ -1924,14 +1924,37 @@ void LCodeGen::DoDateField(LDateField* instr) {
 }
 
 
+MemOperand LCodeGen::BuildSeqStringOperand(Register string,
+                                           LOperand* index,
+                                           String::Encoding encoding) {
+  if (index->IsConstantOperand()) {
+    int offset = ToInteger32(LConstantOperand::cast(index));
+    if (encoding == String::TWO_BYTE_ENCODING) {
+      offset *= kUC16Size;
+    }
+    STATIC_ASSERT(kCharSize == 1);
+    return FieldMemOperand(string, SeqString::kHeaderSize + offset);
+  }
+  Register scratch = scratch0();
+  ASSERT(!scratch.is(string));
+  ASSERT(!scratch.is(ToRegister(index)));
+  if (encoding == String::ONE_BYTE_ENCODING) {
+    __ add(scratch, string, Operand(ToRegister(index)));
+  } else {
+    STATIC_ASSERT(kUC16Size == 2);
+    __ add(scratch, string, Operand(ToRegister(index), LSL, 1));
+  }
+  return FieldMemOperand(scratch, SeqString::kHeaderSize);
+}
+
+
 void LCodeGen::DoSeqStringSetChar(LSeqStringSetChar* instr) {
+  String::Encoding encoding = instr->hydrogen()->encoding();
   Register string = ToRegister(instr->string());
-  LOperand* index_op = instr->index();
   Register value = ToRegister(instr->value());
-  Register scratch = scratch0();
-  String::Encoding encoding = instr->encoding();
 
   if (FLAG_debug_code) {
+    Register scratch = scratch0();
     __ ldr(scratch, FieldMemOperand(string, HeapObject::kMapOffset));
     __ ldrb(scratch, FieldMemOperand(scratch, Map::kInstanceTypeOffset));
 
@@ -1944,24 +1967,11 @@ void LCodeGen::DoSeqStringSetChar(LSeqStringSetChar* instr) {
     __ Check(eq, kUnexpectedStringType);
   }
 
-  if (index_op->IsConstantOperand()) {
-    int constant_index = ToInteger32(LConstantOperand::cast(index_op));
-    if (encoding == String::ONE_BYTE_ENCODING) {
-      __ strb(value,
-              FieldMemOperand(string, SeqString::kHeaderSize + constant_index));
-    } else {
-      __ strh(value,
-          FieldMemOperand(string, SeqString::kHeaderSize + constant_index * 2));
-    }
+  MemOperand operand = BuildSeqStringOperand(string, instr->index(), encoding);
+  if (encoding == String::ONE_BYTE_ENCODING) {
+    __ strb(value, operand);
   } else {
-    Register index = ToRegister(index_op);
-    if (encoding == String::ONE_BYTE_ENCODING) {
-      __ add(scratch, string, Operand(index));
-      __ strb(value, FieldMemOperand(scratch, SeqString::kHeaderSize));
-    } else {
-      __ add(scratch, string, Operand(index, LSL, 1));
-      __ strh(value, FieldMemOperand(scratch, SeqString::kHeaderSize));
-    }
+    __ strh(value, operand);
   }
 }