Currently, LLInt uses d8 register as scratch register as followings: ARM_SCRATCH_FPR = SpecialRegister.new("d8") in arm.rb C_LOOP_SCRATCH_FPR = SpecialRegister.new("d8") in cloop.rb However, AAPCS §5.1.2.1 says Registers s16-s31 (d8-d15, q4-q7) must be preserved across subroutine calls; registers s0-s15 (d0-d7, q0-q3) do not need to be preserved (and can be used for passing arguments or returning results in standard procedure-call variants). Registers d16-d31 (q8-q15), if present, do not need to be preserved. Therefore it should not use d8 for that purpose. I think it would be safe to use d6. (Register d7 is already in use for other purpose. ) ARM_SCRATCH_FPR = SpecialRegister.new("d6") in arm.rb C_LOOP_SCRATCH_FPR = SpecialRegister.new("d6") in cloop.rb