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cmd/compile: stringtoslicebytetmp optimization broken #14973

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mdempsky opened this issue Mar 26, 2016 · 4 comments
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cmd/compile: stringtoslicebytetmp optimization broken #14973

mdempsky opened this issue Mar 26, 2016 · 4 comments
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Go1.7

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@mdempsky
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cmd/compile is emitting calls to runtime.stringtoslicebyte when it could/should be using runtime.stringtoslicebytetmp instead:

$ cat sum.go
package f

func sum(s string) byte {
        var x byte
        for _, c := range []byte(s) {
                x += c
        }
        return x
}

$ go tool compile -S sum.go | grep stringtoslice
        0x0030 00048 (sum.go:5) CALL    runtime.stringtoslicebyte(SB)
        rel 49+4 t=6 runtime.stringtoslicebyte+0

Noticed during review of CL 21173.
@mdempsky mdempsky added this to the Go1.7 milestone Mar 26, 2016
@mdempsky
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Seems broken in Go 1.6 too.

@gopherbot
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CL https://golang.org/cl/21175 mentions this issue.

@mdempsky
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Hrm, there's actually already a regress test for this in runtime_test.go: https://github.com/golang/go/blob/master/src/runtime/string_test.go#L223

Why isn't it failing?

/cc @dvyukov @randall77

@mdempsky
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To answer my own question, TestCompareTempString and TestRangeStringCast were both using test strings short enough to fit into the 32-byte tmpBuf stack storage, so they weren't testing the intended compiler optimizations.

On the upside, TestCompareTempString's optimization was already working correctly.

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