Extract common code in string compare loops.

src/x64/code-stubs-x64.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 4505 matching lines @@
   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));
-  __ cmpb(scratch2, Operand(right, index, times_1, 0));
-  __ j(not_equal, &strings_not_equal);
-  __ addq(index, Immediate(1));
-  __ j(not_zero, &loop);
+  NearLabel strings_not_equal;
+  GenerateAsciiCharsCompareLoop(masm, left, right, length, scratch2,
+                                &strings_not_equal);
 
   // 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);
 }
@@ skipping 26 matching lines @@
   __ SmiSub(scratch1, scratch1, length_difference);
   __ bind(&left_shorter);
   // Register scratch1 now holds Min(left.length, right.length).
   const Register min_length = scratch1;
 
   NearLabel compare_lengths;
   // If min-length is zero, go directly to comparing lengths.
   __ SmiTest(min_length);
   __ j(zero, &compare_lengths);
 
-  __ SmiToInteger32(min_length, min_length);
+  // Compare loop.
+  NearLabel result_not_equal;
+  GenerateAsciiCharsCompareLoop(masm, left, right, min_length, scratch2,
+                                &result_not_equal);
 
-  // Registers scratch2 and scratch3 are free.
-  NearLabel result_not_equal;
-  Label loop;
-  {
-    // Check characters 0 .. min_length - 1 in a loop.
-    // Use scratch3 as loop index, min_length as limit and scratch2
-    // for computation.
-    const Register index = scratch3;
-    __ Set(index, 0);  // Index into strings.
-    __ bind(&loop);
-    // Compare characters.
-    // TODO(lrn): Could we load more than one character at a time?
-    __ movb(scratch2, FieldOperand(left,
-                                   index,
-                                   times_1,
-                                   SeqAsciiString::kHeaderSize));
-    // Increment index and use -1 modifier on next load to give
-    // the previous load extra time to complete.
-    __ addl(index, Immediate(1));
-    __ cmpb(scratch2, FieldOperand(right,
-                                   index,
-                                   times_1,
-                                   SeqAsciiString::kHeaderSize - 1));
-    __ j(not_equal, &result_not_equal);
-    __ cmpl(index, min_length);
-    __ j(not_equal, &loop);
-  }
   // Completed loop without finding different characters.
   // Compare lengths (precomputed).
   __ bind(&compare_lengths);
   __ SmiTest(length_difference);
   __ j(not_zero, &result_not_equal);
 
   // Result is EQUAL.
   __ Move(rax, Smi::FromInt(EQUAL));
   __ ret(0);
 
   NearLabel result_greater;
   __ bind(&result_not_equal);
   // Unequal comparison of left to right, either character or length.
   __ j(greater, &result_greater);
 
   // Result is LESS.
   __ Move(rax, Smi::FromInt(LESS));
   __ ret(0);
 
   // Result is GREATER.
   __ bind(&result_greater);
   __ Move(rax, Smi::FromInt(GREATER));
   __ ret(0);
 }
 
 
+void StringCompareStub::GenerateAsciiCharsCompareLoop(
+    MacroAssembler* masm,
+    Register left,
+    Register right,
+    Register length,
+    Register scratch,
+    NearLabel* chars_not_equal) {
+  // 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 loop;
+  __ bind(&loop);
+  __ movb(scratch, Operand(left, index, times_1, 0));
+  __ cmpb(scratch, Operand(right, index, times_1, 0));
+  __ j(not_equal, chars_not_equal);
+  __ addq(index, Immediate(1));
+  __ j(not_zero, &loop);
+}
+
+
 void StringCompareStub::Generate(MacroAssembler* masm) {
   Label runtime;
 
   // Stack frame on entry.
   //  rsp[0]: return address
   //  rsp[8]: right string
   //  rsp[16]: left string
 
   __ movq(rdx, Operand(rsp, 2 * kPointerSize));  // left
   __ movq(rax, Operand(rsp, 1 * kPointerSize));  // right
@@ skipping 217 matching lines @@
   // Do a tail call to the rewritten stub.
   __ jmp(rdi);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64