Faster string equals in generated code.

src/x64/code-stubs-x64.cc

Index: src/x64/code-stubs-x64.cc
diff --git a/src/x64/code-stubs-x64.cc b/src/x64/code-stubs-x64.cc
index cbdc022bf5c62cb3723b6024cc4a85f1d9eb0e8c..1f94870a1ffaaff94b60009089e94bbc5c3cb125 100644
--- a/src/x64/code-stubs-x64.cc
+++ b/src/x64/code-stubs-x64.cc
@@ -2835,13 +2835,21 @@ void CompareStub::Generate(MacroAssembler* masm) {
       rdx, rax, rcx, rbx, &check_unequal_objects);
 
   // Inline comparison of ascii strings.
-  StringCompareStub::GenerateCompareFlatAsciiStrings(masm,
+  if (cc_ == equal) {
+    StringCompareStub::GenerateFlatAsciiStringEquals(masm,
                                                      rdx,
                                                      rax,
                                                      rcx,
-                                                     rbx,
-                                                     rdi,
-                                                     r8);
+                                                     rbx);
+  } else {
+    StringCompareStub::GenerateCompareFlatAsciiStrings(masm,
+                                                       rdx,
+                                                       rax,
+                                                       rcx,
+                                                       rbx,
+                                                       rdi,
+                                                       r8);
+  }
 
 #ifdef DEBUG
   __ Abort("Unexpected fall-through from string comparison");
@@ -4489,6 +4497,64 @@ void SubStringStub::Generate(MacroAssembler* masm) {
 }
 
 
+void StringCompareStub::GenerateFlatAsciiStringEquals(MacroAssembler* masm,
+                                                      Register left,
+                                                      Register right,
+                                                      Register scratch1,
+                                                      Register scratch2) {
+  Register length = scratch1;
+
+  // Compare lengths.
+  NearLabel check_zero_length;
+  __ movq(length, FieldOperand(left, String::kLengthOffset));
+  __ SmiCompare(length, FieldOperand(right, String::kLengthOffset));
+  __ j(equal, &check_zero_length);
+  __ Move(rax, Smi::FromInt(NOT_EQUAL));
+  __ ret(0);
+
+  // Check if the length is zero.
+  NearLabel compare_chars;
+  __ bind(&check_zero_length);
+  STATIC_ASSERT(kSmiTag == 0);
+  __ SmiTest(length);
+  __ j(not_zero, &compare_chars);
+  __ Move(rax, Smi::FromInt(EQUAL));
+  __ ret(0);
+
+  // Compare characters.
+  __ bind(&compare_chars);
+
+  // Change index to run from -length to -1 by adding length to string
+  // start. This means that loop ends when index reaches zero, which
+  // doesn't need an additional compare.
+  __ SmiToInteger32(length, length);
+  __ lea(left,
+         FieldOperand(left, length, times_1, SeqAsciiString::kHeaderSize));
+  __ lea(right,
+         FieldOperand(right, length, times_1, SeqAsciiString::kHeaderSize));
+  __ neg(length);
+  Register index = length;  // index = -length;
+
+  // Compare loop.
+  NearLabel strings_not_equal, loop;
+  __ bind(&loop);
+  __ movb(scratch2, Operand(left, index, times_1, 0));

[Mads Ager (chromium), 2011/05/05 07:42:24]

Do you need "times_1, 0" here?

[Vitaly Repeshko, 2011/05/05 11:40:18]

Same here.

+  __ cmpb(scratch2, Operand(right, index, times_1, 0));
+  __ j(not_equal, &strings_not_equal);
+  __ addq(index, Immediate(1));
+  __ j(not_zero, &loop);
+
+  // Characters are equal.
+  __ Move(rax, Smi::FromInt(EQUAL));
+  __ ret(0);
+
+  // Characters are not equal.
+  __ bind(&strings_not_equal);
+  __ Move(rax, Smi::FromInt(NOT_EQUAL));
+  __ ret(0);
+}
+
+
 void StringCompareStub::GenerateCompareFlatAsciiStrings(MacroAssembler* masm,
                                                         Register left,
                                                         Register right,
@@ -4687,6 +4753,7 @@ void ICCompareStub::GenerateHeapNumbers(MacroAssembler* masm) {
 
 void ICCompareStub::GenerateStrings(MacroAssembler* masm) {
   ASSERT(state_ == CompareIC::STRINGS);
+  ASSERT(GetCondition() == equal);
   Label miss;
 
   // Registers containing left and right operands respectively.
@@ -4695,7 +4762,6 @@ void ICCompareStub::GenerateStrings(MacroAssembler* masm) {
   Register tmp1 = rcx;
   Register tmp2 = rbx;
   Register tmp3 = rdi;
-  Register tmp4 = r8;
 
   // Check that both operands are heap objects.
   Condition cond = masm->CheckEitherSmi(left, right, tmp1);
@@ -4728,7 +4794,6 @@ void ICCompareStub::GenerateStrings(MacroAssembler* masm) {
   // Check that both strings are symbols. If they are, we're done
   // because we already know they are not identical.
   NearLabel do_compare;
-  ASSERT(GetCondition() == equal);
   STATIC_ASSERT(kSymbolTag != 0);
   __ and_(tmp1, tmp2);
   __ testl(tmp1, Immediate(kIsSymbolMask));
@@ -4744,8 +4809,8 @@ void ICCompareStub::GenerateStrings(MacroAssembler* masm) {
   __ JumpIfNotBothSequentialAsciiStrings(left, right, tmp1, tmp2, &runtime);
 
   // Compare flat ASCII strings. Returns when done.
-  StringCompareStub::GenerateCompareFlatAsciiStrings(
-      masm, left, right, tmp1, tmp2, tmp3, tmp4);
+  StringCompareStub::GenerateFlatAsciiStringEquals(
+      masm, left, right, tmp1, tmp2);
 
   // Handle more complex cases in runtime.
   __ bind(&runtime);
@@ -4753,7 +4818,7 @@ void ICCompareStub::GenerateStrings(MacroAssembler* masm) {
   __ push(left);
   __ push(right);
   __ push(tmp1);
-  __ TailCallRuntime(Runtime::kStringCompare, 2, 1);
+  __ TailCallRuntime(Runtime::kStringEquals, 2, 1);
 
   __ bind(&miss);
   GenerateMiss(masm);