Faster string equals in generated code.

src/ia32/code-stubs-ia32.cc

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 3883 matching lines @@
     // non-zero value, which indicates not equal, so just return.
     __ ret(0);
   }
 
   __ bind(&check_for_strings);
 
   __ JumpIfNotBothSequentialAsciiStrings(edx, eax, ecx, ebx,
                                          &check_unequal_objects);
 
   // Inline comparison of ascii strings.
-  StringCompareStub::GenerateCompareFlatAsciiStrings(masm,
+  if (cc_ == equal) {
+    StringCompareStub::GenerateFlatAsciiStringEquals(masm,
                                                      edx,
                                                      eax,
                                                      ecx,
-                                                     ebx,
-                                                     edi);
+                                                     ebx);
+  } else {
+    StringCompareStub::GenerateCompareFlatAsciiStrings(masm,
+                                                       edx,
+                                                       eax,
+                                                       ecx,
+                                                       ebx,
+                                                       edi);
+  }
 #ifdef DEBUG
   __ Abort("Unexpected fall-through from string comparison");
 #endif
 
   __ bind(&check_unequal_objects);
   if (cc_ == equal && !strict_) {
     // Non-strict equality.  Objects are unequal if
     // they are both JSObjects and not undetectable,
     // and their pointers are different.
     NearLabel not_both_objects;
@@ skipping 1675 matching lines @@
   __ bind(&return_eax);
   __ IncrementCounter(counters->sub_string_native(), 1);
   __ ret(3 * kPointerSize);
 
   // Just jump to runtime to create the sub string.
   __ bind(&runtime);
   __ TailCallRuntime(Runtime::kSubString, 3, 1);
 }
 
 
+void StringCompareStub::GenerateFlatAsciiStringEquals(MacroAssembler* masm,
+                                                      Register left,
+                                                      Register right,
+                                                      Register scratch1,
+                                                      Register scratch2) {
+  Register length = scratch1;
+
+  // Compare lengths.
+  NearLabel strings_not_equal, check_zero_length;
+  __ mov(length, FieldOperand(left, String::kLengthOffset));
+  __ cmp(length, FieldOperand(right, String::kLengthOffset));
+  __ j(equal, &check_zero_length);
+  __ bind(&strings_not_equal);
+  __ Set(eax, Immediate(Smi::FromInt(NOT_EQUAL)));
+  __ ret(0);
+
+  // Check if the length is zero.
+  NearLabel compare_chars;
+  __ bind(&check_zero_length);
+  STATIC_ASSERT(kSmiTag == 0);
+  __ test(length, Operand(length));
+  __ j(not_zero, &compare_chars);
+  __ Set(eax, Immediate(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.
+  __ SmiUntag(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 loop;
+  __ bind(&loop);
+  __ mov_b(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]

There's no Operand(reg, reg), only Operand(reg, imm).

+  __ cmpb(scratch2, Operand(right, index, times_1, 0));
+  __ j(not_equal, &strings_not_equal);
+  __ add(Operand(index), Immediate(1));
+  __ j(not_zero, &loop);
+
+  // Characters are equal.
+  __ Set(eax, Immediate(Smi::FromInt(EQUAL)));
+  __ ret(0);
+}
+
+
 void StringCompareStub::GenerateCompareFlatAsciiStrings(MacroAssembler* masm,
                                                         Register left,
                                                         Register right,
                                                         Register scratch1,
                                                         Register scratch2,
                                                         Register scratch3) {
   Label result_not_equal;
   Label result_greater;
   Label compare_lengths;
 
@@ skipping 186 matching lines @@
   __ bind(&generic_stub);
   __ jmp(stub.GetCode(), RelocInfo::CODE_TARGET);
 
   __ bind(&miss);
   GenerateMiss(masm);
 }
 
 
 void ICCompareStub::GenerateStrings(MacroAssembler* masm) {
   ASSERT(state_ == CompareIC::STRINGS);
+  ASSERT(GetCondition() == equal);
   Label miss;
 
   // Registers containing left and right operands respectively.
   Register left = edx;
   Register right = eax;
   Register tmp1 = ecx;
   Register tmp2 = ebx;
   Register tmp3 = edi;
 
   // Check that both operands are heap objects.
@@ skipping 23 matching lines @@
   STATIC_ASSERT(kSmiTag == 0);
   __ Set(eax, Immediate(Smi::FromInt(EQUAL)));
   __ ret(0);
 
   // Handle not identical strings.
   __ bind(&not_same);
 
   // 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, Operand(tmp2));
   __ test(tmp1, Immediate(kIsSymbolMask));
   __ j(zero, &do_compare);
   // Make sure eax is non-zero. At this point input operands are
   // guaranteed to be non-zero.
   ASSERT(right.is(eax));
   __ ret(0);
 
   // Check that both strings are sequential ASCII.
   Label runtime;
   __ bind(&do_compare);
   __ JumpIfNotBothSequentialAsciiStrings(left, right, tmp1, tmp2, &runtime);
 
   // Compare flat ASCII strings. Returns when done.
-  StringCompareStub::GenerateCompareFlatAsciiStrings(
-      masm, left, right, tmp1, tmp2, tmp3);
+  StringCompareStub::GenerateFlatAsciiStringEquals(
+      masm, left, right, tmp1, tmp2);
 
   // Handle more complex cases in runtime.
   __ bind(&runtime);
   __ pop(tmp1);  // Return address.
   __ push(left);
   __ push(right);
   __ push(tmp1);
-  __ TailCallRuntime(Runtime::kStringCompare, 2, 1);
+  __ TailCallRuntime(Runtime::kStringEquals, 2, 1);
 
   __ bind(&miss);
   GenerateMiss(masm);
 }
 
 
 void ICCompareStub::GenerateObjects(MacroAssembler* masm) {
   ASSERT(state_ == CompareIC::OBJECTS);
   NearLabel miss;
   __ mov(ecx, Operand(edx));
@@ skipping 44 matching lines @@
   // Do a tail call to the rewritten stub.
   __ jmp(Operand(edi));
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_IA32