[9fans] cheep improvement for rendezvous

[9fans] cheep improvement for rendezvous

[erik quanstrom]

i noticed this today by inspection.  the tag passed to rendezvous is usually
a naturally-aligned address.  for the same size rendhash, we can get shorter
hash chains for the usual case by dropping the low-order bits.

the "2" assumes 32 bits, and should really be replaced by log2ciel(sizeof(uintptr)).
but that's not constant.

/n/dump/2015/0207/sys/src/nix/port/portdat.h:489,495 - ../port/portdat.h:489,496
  	PGHLOG  =	9,
  	PGHSIZE	=	1<<PGHLOG,	/* Page hash for image lookup */
  };
- #define REND(p,s)	((p)->rendhash[(s)&((1<<RENDLOG)-1)])
+ /*
+  * assume natural alignment for minimum sized pointer
+  * 2 is a stand in for log2ciel(sizeof(uintptr))
+  */
+ #define REND(p,s)	((p)->rendhash[((s)>>2)&((1<<RENDLOG)-1)])
  #define MOUNTH(p,qid)	((p)->mnthash[(qid).path&((1<<MNTLOG)-1)])

  struct Pgrp


- erik