Limit end-anchored regexps to testing end of string where possible.

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

Index: src/x64/regexp-macro-assembler-x64.cc
diff --git a/src/x64/regexp-macro-assembler-x64.cc b/src/x64/regexp-macro-assembler-x64.cc
index 91e2b449e015fe0ec42ad2c113a2c95c44864dcc..47c19c7e1f6fcdb04b547da39521ef1f7ad56c5b 100644
--- a/src/x64/regexp-macro-assembler-x64.cc
+++ b/src/x64/regexp-macro-assembler-x64.cc
@@ -145,7 +145,6 @@ int RegExpMacroAssemblerX64::stack_limit_slack()  {
 
 void RegExpMacroAssemblerX64::AdvanceCurrentPosition(int by) {
   if (by != 0) {
-    Label inside_string;
     __ addq(rdi, Immediate(by * char_size()));
   }
 }
@@ -1053,6 +1052,19 @@ void RegExpMacroAssemblerX64::ReadStackPointerFromRegister(int reg) {
 }
 
 
+void RegExpMacroAssemblerX64::SetCurrentPositionFromEnd(int by) {
+  NearLabel after_position;
+  __ cmpq(rdi, Immediate(-by * char_size()));
+  __ j(greater_equal, &after_position);
+  __ movq(rdi, Immediate(-by * char_size()));
+  // On RegExp code entry (where this operation is used), the character before
+  // the current position is expected to be already loaded.
+  // We have advanced the position, so it's safe to read backwards.
+  LoadCurrentCharacterUnchecked(-1, 1);
+  __ bind(&after_position);
+}
+
+
 void RegExpMacroAssemblerX64::SetRegister(int register_index, int to) {
   ASSERT(register_index >= num_saved_registers_);  // Reserved for positions!
   __ movq(register_location(register_index), Immediate(to));