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: Cmp/1same-8 4.78ns ± 6% 0.94ns ± 9% -80.26% (p=0.000 n=20+20) Cmp/1diffbytes-8 6.43ns ± 6% 0.96ns ±11% -85.13% (p=0.000 n=20+20) Cmp/3same-8 4.71ns ± 5% 1.28ns ± 5% -72.90% (p=0.000 n=20+20) Cmp/3difffirstbyte-8 6.33ns ± 7% 1.27ns ± 7% -79.90% (p=0.000 n=20+20) Cmp/3difflastbyte-8 6.34ns ± 8% 1.26ns ± 9% -80.13% (p=0.000 n=20+20) 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. Change-Id: I593ac303b0d11f275672090c5c786ea0c6b8da13 Reviewed-on: https://go-review.googlesource.com/26758 Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by:Brad Fitzpatrick <bradfitz@golang.org>
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