Optimize one byte string comparisons.

runtime/vm/intermediate_language_mips.cc

Index: runtime/vm/intermediate_language_mips.cc
===================================================================
--- runtime/vm/intermediate_language_mips.cc	(revision 31282)
+++ runtime/vm/intermediate_language_mips.cc	(working copy)
@@ -780,7 +780,7 @@
 }
 
 
-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,
@@ -789,18 +789,36 @@
 }
 
 
-void StringFromCharCodeInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
-  Register char_code = locs()->in(0).reg();
-  Register result = locs()->out().reg();
+void StringCharCodeInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
+  if (kind() == StringCharCodeInstr::kFromCharCode) {
+    Register char_code = locs()->in(0).reg();
+    Register result = locs()->out().reg();
 
-  __ TraceSimMsg("StringFromCharCodeInstr");
+    __ TraceSimMsg("StringCharCodeInstr From");
 
-  __ LoadImmediate(result,
-                   reinterpret_cast<uword>(Symbols::PredefinedAddress()));
-  __ AddImmediate(result, Symbols::kNullCharCodeSymbolOffset * kWordSize);
-  __ sll(TMP, char_code, 1);  // Char code is a smi.
-  __ addu(TMP, TMP, result);
-  __ lw(result, Address(TMP));
+    __ LoadImmediate(result,
+                     reinterpret_cast<uword>(Symbols::PredefinedAddress()));
+    __ AddImmediate(result, Symbols::kNullCharCodeSymbolOffset * kWordSize);
+    __ sll(TMP, char_code, 1);  // Char code is a smi.
+    __ addu(TMP, TMP, result);
+    __ lw(result, Address(TMP));
+  } else {
+    __ TraceSimMsg("StringCharCodeInstr To");
+
+    ASSERT(kind() == StringCharCodeInstr::kToCharCode);
+    ASSERT(cid_ == kOneByteStringCid);
+    Register str = locs()->in(0).reg();
+    Register result = locs()->out().reg();
+    Label done, is_one;
+    __ lw(result, FieldAddress(str, String::length_offset()));
+    __ BranchEqual(result, Smi::RawValue(1), &is_one);
+    __ LoadImmediate(result, Smi::RawValue(-1));
+    __ b(&done);
+    __ Bind(&is_one);
+    __ lbu(result, FieldAddress(str, OneByteString::data_offset()));
+    __ SmiTag(result);
+    __ Bind(&done);
+  }
 }