math: faster Gamma
math: faster Gamma

Having the compiler count the number of array elements speeds up Gamma from 63.7 to 56.6 ns/op.

R=rsc, golang-dev, r
CC=golang-dev
http://codereview.appspot.com/5362043

Committer: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

apparentlymart/golang,apparentlymart/golang,gnanderson/go,gnanderson/go,gnanderson/go,tv42/old-go,gnanderson/go,dsriram/golang,tv42/old-go,dsriram/golang,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,gnanderson/go,tv42/old-go,tv42/old-go,apparentlymart/golang,gnanderson/go,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,dsriram/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,tv42/old-go