cmd/compile: unroll comparisons to short constant strings
Unroll s == "ab" to
len(s) == 2 && s[0] == 'a' && s[1] == 'b'
This generates faster and shorter code
by avoiding a runtime call.
Do something similar for !=.
The cutoff length is 6. This was chosen empirically
by examining binary sizes on arm, arm64, 386, and amd64
using the SSA backend.
For all architectures examined, 4, 5, and 6 were
the ideal cutoff, with identical binary sizes.
The distribution of constant string equality sizes
during 'go build -a std' is:
40.81% 622 len 0
14.11% 215 len 4
9.45% 144 len 1
7.81% 119 len 3
7.48% 114 len 5
5.12% 78 len 7
4.13% 63 len 2
3.54% 54 len 8
2.69% 41 len 6
1.18% 18 len 10
0.85% 13 len 9
0.66% 10 len 14
0.59% 9 len 17
0.46% 7 len 11
0.26% 4 len 12
0.20% 3 len 19
0.13% 2 len 13
0.13% 2 len 15
0.13% 2 len 16
0.07% 1 len 20
0.07% 1 len 23
0.07% 1 len 33
0.07% 1 len 36
A cutoff of length 6 covers most of the cases.
Benchmarks on amd64 comparing a string to a constant of length 3:
The change to the prove test preserves the
existing intent of the test. When the string was
short, there was a new "proved in bounds" report
that referred to individual byte comparisons.