Optimize one byte string comparisons.

runtime/vm/intermediate_language_ia32.cc

Index: runtime/vm/intermediate_language_ia32.cc
===================================================================
--- runtime/vm/intermediate_language_ia32.cc	(revision 31282)
+++ runtime/vm/intermediate_language_ia32.cc	(working copy)
@@ -721,7 +721,7 @@
 }
 
 
-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,
@@ -730,15 +730,32 @@
 }
 
 
-void StringFromCharCodeInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
-  Register char_code = locs()->in(0).reg();
-  Register result = locs()->out().reg();
-  __ movl(result,
-          Immediate(reinterpret_cast<uword>(Symbols::PredefinedAddress())));
-  __ movl(result, Address(result,
-                          char_code,
-                          TIMES_HALF_WORD_SIZE,  // Char code is a smi.
-                          Symbols::kNullCharCodeSymbolOffset * kWordSize));
+void StringCharCodeInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
+  if (kind() == StringCharCodeInstr::kFromCharCode) {
+    Register char_code = locs()->in(0).reg();
+    Register result = locs()->out().reg();
+    __ movl(result,
+            Immediate(reinterpret_cast<uword>(Symbols::PredefinedAddress())));
+    __ movl(result, Address(result,
+                            char_code,
+                            TIMES_HALF_WORD_SIZE,  // Char code is a smi.
+                            Symbols::kNullCharCodeSymbolOffset * kWordSize));
+  } else {
+    ASSERT(kind() == StringCharCodeInstr::kToCharCode);
+    ASSERT(cid_ == kOneByteStringCid);
+    Register str = locs()->in(0).reg();
+    Register result = locs()->out().reg();
+    Label is_one, done;
+    __ movl(result, FieldAddress(str, String::length_offset()));
+    __ cmpl(result, Immediate(Smi::RawValue(1)));
+    __ j(EQUAL, &is_one, Assembler::kNearJump);
+    __ movl(result, Immediate(Smi::RawValue(-1)));
+    __ jmp(&done);
+    __ Bind(&is_one);
+    __ movzxb(result, FieldAddress(str, OneByteString::data_offset()));
+    __ SmiTag(result);
+    __ Bind(&done);
+  }
 }