cmd/gc: address 1½ liveness bottlenecks
As much as 7x speedup on some programs, cuts all.bash time by 20%.
Change splicebefore function from O(n) to O(1).
The approach was suggested by Carl during the code's review
but apparently did not make it into the tree.
It makes a huge difference on huge programs.
Make twobitwalktype1 slightly faster by using & instead of %.
Really it needs to be cached; left a note to that effect.
(Not a complete fix, hence the ½.)
big.go (output of test/chan/select5.go)
47.53u 0.50s 48.14r before this CL
7.09u 0.47s 7.59r with splicebefore change (6.7x speedup)
6.15u 0.42s 6.59r with twobitwalktype1 change (1.15x speedup; total 7.7x)
slow.go (variant of program in go.text, by mpvl)
77.75u 2.11s 80.03r before this CL
24.40u 1.97s 26.44r with splicebefore change (3.2x speedup)
18.12u 2.19s 20.38r with twobitwalktype1 change (1.35x speedup; total 4.3x)
test/run
150.63u 49.57s 81.08r before this CL
88.01u 45.60s 46.65r after this CL (1.7x speedup)
all.bash
369.70u 115.64s 256.21r before this CL
298.52u 110.35s 214.67r after this CL (1.24x speedup)
The test programs are at
https://rsc.googlecode.com/hg/testdata/big.go (36k lines, 276kB)
https://rsc.googlecode.com/hg/testdata/slow.go (7k lines, 352kB)
R=golang-codereviews, gobot, r
CC=cshapiro, golang-codereviews
https://golang.org/cl/
43210045