math: faster Lgamma
Converting from polynomial constants to counted array speeds up Lgamma from 51.3 to 37.7 ns/op. Variables renamed in Gamma to avoid overlap in Lgamma.
R=rsc, golang-dev
CC=golang-dev
https://golang.org/cl/5359045
diff --git a/src/pkg/math/gamma.go b/src/pkg/math/gamma.go
index e117158..ae2c0c4 100644
--- a/src/pkg/math/gamma.go
+++ b/src/pkg/math/gamma.go
@@ -63,7 +63,7 @@
// Stephen L. Moshier
// moshier@na-net.ornl.gov
-var _P = [...]float64{
+var _gamP = [...]float64{
1.60119522476751861407e-04,
1.19135147006586384913e-03,
1.04213797561761569935e-02,
@@ -72,7 +72,7 @@
4.94214826801497100753e-01,
9.99999999999999996796e-01,
}
-var _Q = [...]float64{
+var _gamQ = [...]float64{
-2.31581873324120129819e-05,
5.39605580493303397842e-04,
-4.45641913851797240494e-03,
@@ -82,7 +82,7 @@
7.14304917030273074085e-02,
1.00000000000000000320e+00,
}
-var _S = [...]float64{
+var _gamS = [...]float64{
7.87311395793093628397e-04,
-2.29549961613378126380e-04,
-2.68132617805781232825e-03,
@@ -98,7 +98,7 @@
MaxStirling = 143.01608
)
w := 1 / x
- w = 1 + w*((((_S[0]*w+_S[1])*w+_S[2])*w+_S[3])*w+_S[4])
+ w = 1 + w*((((_gamS[0]*w+_gamS[1])*w+_gamS[2])*w+_gamS[3])*w+_gamS[4])
y := Exp(x)
if x > MaxStirling { // avoid Pow() overflow
v := Pow(x, 0.5*x-0.25)
@@ -176,8 +176,8 @@
}
x = x - 2
- p = (((((x*_P[0]+_P[1])*x+_P[2])*x+_P[3])*x+_P[4])*x+_P[5])*x + _P[6]
- q = ((((((x*_Q[0]+_Q[1])*x+_Q[2])*x+_Q[3])*x+_Q[4])*x+_Q[5])*x+_Q[6])*x + _Q[7]
+ p = (((((x*_gamP[0]+_gamP[1])*x+_gamP[2])*x+_gamP[3])*x+_gamP[4])*x+_gamP[5])*x + _gamP[6]
+ q = ((((((x*_gamQ[0]+_gamQ[1])*x+_gamQ[2])*x+_gamQ[3])*x+_gamQ[4])*x+_gamQ[5])*x+_gamQ[6])*x + _gamQ[7]
return z * p / q
small:
