diff --git a/lib/mjit/codegen.rb b/lib/mjit/codegen.rb
index 6ebefde860..757e08c1a0 100644
--- a/lib/mjit/codegen.rb
+++ b/lib/mjit/codegen.rb
@@ -7,7 +7,7 @@ module RubyVM::MJIT
     # @param ctx [RubyVM::MJIT::Context]
     # @param asm [RubyVM::MJIT::X86Assembler]
     def putnil(ctx, asm)
-      asm.mov([:rbx], Qnil)
+      asm.mov([SP], Qnil)
       ctx.stack_size += 1
       KeepCompiling
     end
@@ -19,14 +19,14 @@ module RubyVM::MJIT
       # TODO: Check interrupts
 
       # Pop the current frame (ec->cfp++)
-      asm.add(:rsi, C.rb_control_frame_t.size) # rsi = cfp + 1
-      asm.mov([:rdi, C.rb_execution_context_t.offsetof(:cfp)], :rsi) # ec->cfp = rsi
+      asm.add(CFP, C.rb_control_frame_t.size) # cfp = cfp + 1
+      asm.mov([EC, C.rb_execution_context_t.offsetof(:cfp)], CFP) # ec->cfp = cfp
 
       # Return a value
-      asm.mov(:rax, [:rbx])
+      asm.mov(:rax, [SP])
 
       # Restore callee-saved registers
-      asm.pop(:rbx)
+      asm.pop(SP)
 
       asm.ret
       EndBlock
diff --git a/lib/ruby_vm/mjit/compiler.rb b/lib/ruby_vm/mjit/compiler.rb
index 8a5614c081..652feb9a34 100644
--- a/lib/ruby_vm/mjit/compiler.rb
+++ b/lib/ruby_vm/mjit/compiler.rb
@@ -13,6 +13,11 @@ module RubyVM::MJIT
   Qnil = Fiddle::Qnil
   Qundef = Fiddle::Qundef
 
+  # Fixed registers
+  EC  = :rdi # TODO: change this
+  CFP = :rsi # TODO: change this
+  SP  = :rbx
+
   class Compiler
     attr_accessor :write_pos
 
@@ -65,10 +70,10 @@ module RubyVM::MJIT
     # @param asm [RubyVM::MJIT::X86Assembler]
     def compile_prologue(asm)
       # Save callee-saved registers used by JITed code
-      asm.push(:rbx)
+      asm.push(SP)
 
       # Load sp to a register
-      asm.mov(:rbx, [:rsi, C.rb_control_frame_t.offsetof(:sp)]) # rbx = cfp->sp
+      asm.mov(SP, [CFP, C.rb_control_frame_t.offsetof(:sp)]) # rbx = cfp->sp
     end
 
     # @param asm [RubyVM::MJIT::X86Assembler]
@@ -103,16 +108,16 @@ module RubyVM::MJIT
     def compile_exit(ctx, asm, exit_pc)
       # update pc
       asm.mov(:rax, exit_pc) # rax = exit_pc
-      asm.mov([:rsi, C.rb_control_frame_t.offsetof(:pc)], :rax) # cfp->pc = rax
+      asm.mov([CFP, C.rb_control_frame_t.offsetof(:pc)], :rax) # cfp->pc = rax
 
       # update sp
       if ctx.stack_size > 0
-        asm.add(:rbx, C.VALUE.size * ctx.stack_size) # rbx += stack_size
-        asm.mov([:rsi, C.rb_control_frame_t.offsetof(:sp)], :rbx) # cfp->sp = rbx
+        asm.add(SP, C.VALUE.size * ctx.stack_size) # rbx += stack_size
+        asm.mov([CFP, C.rb_control_frame_t.offsetof(:sp)], SP) # cfp->sp = rbx
       end
 
       # Restore callee-saved registers
-      asm.pop(:rbx)
+      asm.pop(SP)
 
       asm.mov(:rax, Qundef)
       asm.ret