Optimize one byte string comparisons.

runtime/vm/intermediate_language_arm.cc

Index: runtime/vm/intermediate_language_arm.cc
===================================================================
--- runtime/vm/intermediate_language_arm.cc	(revision 31282)
+++ runtime/vm/intermediate_language_arm.cc	(working copy)
@@ -720,7 +720,7 @@
 }
 
 
-LocationSummary* StringFromCharCodeInstr::MakeLocationSummary(bool opt) const {
+LocationSummary* StringCharCodeInstr::MakeLocationSummary(bool opt) const {
   const intptr_t kNumInputs = 1;
   // TODO(fschneider): Allow immediate operands for the char code.
   return LocationSummary::Make(kNumInputs,
@@ -729,13 +729,25 @@
 }
 
 
-void StringFromCharCodeInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
-  Register char_code = locs()->in(0).reg();
-  Register result = locs()->out().reg();
-  __ LoadImmediate(result,
-                   reinterpret_cast<uword>(Symbols::PredefinedAddress()));
-  __ AddImmediate(result, Symbols::kNullCharCodeSymbolOffset * kWordSize);
-  __ ldr(result, Address(result, char_code, LSL, 1));  // Char code is a smi.
+void StringCharCodeInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
+  if (kind() == StringCharCodeInstr::kFromCharCode) {
+    Register char_code = locs()->in(0).reg();
+    Register result = locs()->out().reg();
+    __ LoadImmediate(result,
+                     reinterpret_cast<uword>(Symbols::PredefinedAddress()));
+    __ AddImmediate(result, Symbols::kNullCharCodeSymbolOffset * kWordSize);
+    __ ldr(result, Address(result, char_code, LSL, 1));  // Char code is a smi.
+  } else {
+    ASSERT(kind() == StringCharCodeInstr::kToCharCode);
+    ASSERT(cid_ == kOneByteStringCid);
+    Register str = locs()->in(0).reg();
+    Register result = locs()->out().reg();
+    __ ldr(result, FieldAddress(str, String::length_offset()));
+    __ cmp(result, ShifterOperand(Smi::RawValue(1)));
+    __ LoadImmediate(result, Smi::RawValue(-1), NE);
+    __ ldrb(result, FieldAddress(str, OneByteString::data_offset()), EQ);
+    __ SmiTag(result);
+  }
 }