Don't pass the "at start" parameter to native RegExp...

src/x64/regexp-macro-assembler-x64.cc

 // Copyright 2009 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 53 matching lines @@
  * If changed to use r12+, they should be saved as callee-save registers.
  *
  * Each call to a C++ method should retain these registers.
  *
  * The stack will have the following content, in some order, indexable from the
  * frame pointer (see, e.g., kStackHighEnd):
  *    - direct_call          (if 1, direct call from JavaScript code, if 0 call
  *                            through the runtime system)
  *    - stack_area_base      (High end of the memory area to use as
  *                            backtracking stack)
- *    - at_start             (if 1, we are starting at the start of the
- *                            string, otherwise 0)
  *    - int* capture_array   (int[num_saved_registers_], for output).
  *    - end of input         (Address of end of string)
  *    - start of input       (Address of first character in string)
  *    - start index          (character index of start)
  *    - String* input_string (input string)
  *    - return address
  *    - backup of callee save registers (rbx, possibly rsi and rdi).
  *    - Offset of location before start of input (effectively character
  *      position -1). Used to initialize capture registers to a non-position.
+ *    - At start of string (if 1, we are starting at the start of the
+ *      string, otherwise 0)
  *    - register 0  rbp[-n]   (Only positions must be stored in the first
  *    - register 1  rbp[-n-8]  num_saved_registers_ registers)
  *    - ...
  *
  * The first num_saved_registers_ registers are initialized to point to
  * "character -1" in the string (i.e., char_size() bytes before the first
  * character of the string). The remaining registers starts out uninitialized.
  *
  * The first seven values must be provided by the calling code by
  * calling the code's entry address cast to a function pointer with the
@@ skipping 559 matching lines @@
   __ push(rdi);
   __ push(rbx);
 #else
   // GCC passes arguments in rdi, rsi, rdx, rcx, r8, r9 (and then on stack).
   // Push register parameters on stack for reference.
   ASSERT_EQ(kInputString, -1 * kPointerSize);
   ASSERT_EQ(kStartIndex, -2 * kPointerSize);
   ASSERT_EQ(kInputStart, -3 * kPointerSize);
   ASSERT_EQ(kInputEnd, -4 * kPointerSize);
   ASSERT_EQ(kRegisterOutput, -5 * kPointerSize);
-  ASSERT_EQ(kAtStart, -6 * kPointerSize);
+  ASSERT_EQ(kStackHighEnd, -6 * kPointerSize);
   __ push(rdi);
   __ push(rsi);
   __ push(rdx);
   __ push(rcx);
   __ push(r8);
   __ push(r9);
 
   __ push(rbx);  // Callee-save
 #endif
   __ push(Immediate(0));  // Make room for "input start - 1" constant.
+  __ push(Immediate(0));  // Make room for "at start" constant.
 
   // Check if we have space on the stack for registers.
   Label stack_limit_hit;
   Label stack_ok;
 
   ExternalReference stack_limit =
       ExternalReference::address_of_stack_limit();
   __ movq(rcx, rsp);
   __ movq(kScratchRegister, stack_limit);
   __ subq(rcx, Operand(kScratchRegister, 0));
@@ skipping 24 matching lines @@
   // Load input position.
   __ movq(rdi, Operand(rbp, kInputStart));
   // Set up rdi to be negative offset from string end.
   __ subq(rdi, rsi);
   // Set rax to address of char before start of input
   // (effectively string position -1).
   __ lea(rax, Operand(rdi, -char_size()));
   // Store this value in a local variable, for use when clearing
   // position registers.
   __ movq(Operand(rbp, kInputStartMinusOne), rax);
+
+  // Determine whether the start index is zero, that is at the start of the
+  // string, and store that value in a local variable.
+  __ movq(rbx, Operand(rbp, kStartIndex));
+  __ xor_(rcx, rcx);  // setcc only operates on cl (lower byte of rcx).
+  __ testq(rbx, rbx);
+  __ setcc(zero, rcx);  // 1 if 0 (start of string), 0 if positive.
+  __ movq(Operand(rbp, kAtStart), rcx);
+
   if (num_saved_registers_ > 0) {
     // Fill saved registers with initial value = start offset - 1
     // Fill in stack push order, to avoid accessing across an unwritten
     // page (a problem on Windows).
     __ movq(rcx, Immediate(kRegisterZero));
     Label init_loop;
     __ bind(&init_loop);
     __ movq(Operand(rbp, rcx, times_1, 0), rax);
     __ subq(rcx, Immediate(kPointerSize));
     __ cmpq(rcx,
@@ skipping 588 matching lines @@
                  Operand(rsi, rdi, times_1, cp_offset * sizeof(uc16)));
     }
   }
 }
 
 #undef __
 
 #endif  // V8_NATIVE_REGEXP
 
 }}  // namespace v8::internal