Function pmus_for 

pub fn pmus_for(words: usize, depth: usize, t: &PlasticineTarget) -> usize

PMUs a depth-deep N-buffer of words needs on t: a PMU’s address space is partitioned depth ways (Spatial’s N-buffering), so one PMU holds words_per_pmu / depth words per buffer; an empty tile still occupies one PMU (the buffer exists).