https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122871
--- Comment #16 from Roger Sayle <roger at nextmovesoftware dot com> --- The slightly unusual results from synth_mult on ARM with -mthumb are due to incorrect values being returned by rtx_costs. DImode addition and subtraction should both be COSTS_N_INSNS(2), i.e. adds+adcs and subs+sbcs respectively, but currently return COSTS_N_INSNS(1).
