frac32 in1
frac32 tail
frac32 in2
frac32 psub
frac32 ptail
frac32 out
int32_t base = inlet_psub + ___SMMUL(inlet_ptail - inlet_psub << 2,
(1 << 27) - inlet_tail << 3);
int32_t vol = base + ___SMMUL(inlet_phit - base << 2, inlet_hit << 3);
outlet_out = vol;