diff --git a/src/x64/regexp-macro-assembler-x64.cc b/src/x64/regexp-macro-assembler-x64.cc
index 13a4caa3fb..cb1e029317 100644
--- a/src/x64/regexp-macro-assembler-x64.cc
+++ b/src/x64/regexp-macro-assembler-x64.cc
@@ -803,7 +803,7 @@ Handle<HeapObject> RegExpMacroAssemblerX64::GetCode(Handle<String> source) {
 #endif
 
   __ push(Immediate(0));  // Number of successful matches in a global regexp.
-  __ push(Immediate(0));  // Make room for "at start" constant.
+  __ push(Immediate(0));  // Make room for "input start - 1" constant.
 
   // Check if we have space on the stack for registers.
   Label stack_limit_hit;
@@ -872,7 +872,7 @@ Handle<HeapObject> RegExpMacroAssemblerX64::GetCode(Handle<String> source) {
 
   Label load_char_start_regexp, start_regexp;
   // Load newline if index is at start, previous character otherwise.
-  __ cmpb(Operand(rbp, kStartIndex), Immediate(0));
+  __ cmpl(Operand(rbp, kStartIndex), Immediate(0));
   __ j(not_equal, &load_char_start_regexp, Label::kNear);
   __ Set(current_character(), '\n');
   __ jmp(&start_regexp, Label::kNear);
@@ -944,7 +944,7 @@ Handle<HeapObject> RegExpMacroAssemblerX64::GetCode(Handle<String> source) {
       __ incq(Operand(rbp, kSuccessfulCaptures));
       // Capture results have been stored, so the number of remaining global
       // output registers is reduced by the number of stored captures.
-      __ movq(rcx, Operand(rbp, kNumOutputRegisters));
+      __ movsxlq(rcx, Operand(rbp, kNumOutputRegisters));
       __ subq(rcx, Immediate(num_saved_registers_));
       // Check whether we have enough room for another set of capture results.
       __ cmpq(rcx, Immediate(num_saved_registers_));
diff --git a/src/x64/regexp-macro-assembler-x64.h b/src/x64/regexp-macro-assembler-x64.h
index 5dfd288353..31fc8efd93 100644
--- a/src/x64/regexp-macro-assembler-x64.h
+++ b/src/x64/regexp-macro-assembler-x64.h
@@ -156,7 +156,8 @@ class RegExpMacroAssemblerX64: public NativeRegExpMacroAssembler {
   static const int kRegisterOutput = kInputEnd + kPointerSize;
   // For the case of global regular expression, we have room to store at least
   // one set of capture results.  For the case of non-global regexp, we ignore
-  // this value.
+  // this value. NumOutputRegisters is passed as 32-bit value.  The upper
+  // 32 bit of this 64-bit stack slot may contain garbage.
   static const int kNumOutputRegisters = kRegisterOutput + kPointerSize;
   static const int kStackHighEnd = kNumOutputRegisters + kPointerSize;
   // DirectCall is passed as 32 bit int (values 0 or 1).
@@ -195,7 +196,7 @@ class RegExpMacroAssemblerX64: public NativeRegExpMacroAssembler {
   static const int kLastCalleeSaveRegister = kBackup_rbx;
 #endif
 
-  static const int kSuccessfulCaptures = kBackup_rbx - kPointerSize;
+  static const int kSuccessfulCaptures = kLastCalleeSaveRegister - kPointerSize;
   // When adding local variables remember to push space for them in
   // the frame in GetCode.
   static const int kInputStartMinusOne = kSuccessfulCaptures - kPointerSize;