Some important and hot data is stored in Ps, such as write barrier buffers and defer pools. Getting from a G to a P currently requires evaluating g.m.p. We should consider adding a direct link, g.p, to avoid an extra memory lookup. This should reduce memory lookups and cache pressure / cache misses on hot code, particularly write barrier writes.

This might also enable us to eliminate the mcache.

This requires updating g.p every time a G gets (un)scheduled.

I took a stab at this but never quite got it working. I hope someone else might.

cc @aclements @dvyukov