cmd/compile: Optimize strings.Join with finite number of elements

[dlsniper]

1. What version of Go are you using (go version)?
   go version go1.7beta2 linux/amd64
2. What operating system and processor architecture are you using (go env)?
   GOHOSTARCH="amd64"
   GOHOSTOS="linux"

Intel(R) Core(TM) i7-4870HQ CPU @ 2.50GHz

Given the following example: https://play.golang.org/p/a_ow6e7II7

I would expect the compiler to transform the calls to strings.Join with a finite number of elements into simple string concatenations. The reason is that these calls might be used on hot-paths for code, for example: aws/aws-sdk-go#729

The results might be small, see below for my machine, but any speed improvement is a good one imho

go test -bench=.
BenchmarkStringsJoin-8          10000000               140 ns/op
BenchmarkStringsConcat-8        20000000                92.6 ns/op
PASS
ok      some/package        3.505s


go test -bench=. -benchmem
BenchmarkStringsJoin-8          10000000               136 ns/op             128 B/op          2 allocs/op
BenchmarkStringsConcat-8        20000000                96.6 ns/op            64 B/op          1 allocs/op
PASS
ok      some/package        3.565s

Thank you.

[randall77]

Could be done. The simple fix is to make strings.Join work similarly to runtime.concatstrings, in particular to use rawstring instead of doing the concat in a byte slice and then converting to a string.

A bigger change would be to do semantic inlining of strings.Join in the compiler.

[bradfitz]

A bigger change would be to do semantic inlining of strings.Join in the compiler.

That seems too big. Most of the standard library should not be special. Maybe parts of sync, runtime, and time can be recognized by the compiler, but not high-level packages likes "strings".

[bradfitz]

FWIW, 3ddc9ad was already submitted to optimize the common cases of strings.Join.

@randall77, I'm of the opinion that this bug should be closed and the compiler shouldn't special case things in the standard library, at least not in regular (unprivileged) packages like "strings".

[randall77]

Sure, I'll agree with that. Note that #6714 will fix this issue as a side effect.