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runtime: convT2E called for function value #19335

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cstockton opened this issue Mar 1, 2017 · 3 comments

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@cstockton
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commented Mar 1, 2017

Go 1.8 amd64 linux when running the below program convT2E is called for BenchmarkEfaceFnVal but not for BenchmarkEfaceFn.

Program:

package main

import "testing"

func efaceFn(v interface{}) {
	return
}

func BenchmarkEfaceFnVal(b *testing.B) {
	b.ReportAllocs()

	efaceFnVal := efaceFn
	for i := 0; i < b.N; i++ {
		efaceFnVal(0x1)
	}
}

func BenchmarkEfaceFn(b *testing.B) {
	b.ReportAllocs()

	for i := 0; i < b.N; i++ {
		efaceFn(0x1)
	}
}

Result:

Name Iterations ns/op B/op allocs/op
BenchmarkEfaceFnVal-24 20000000 60.8 ns/op 8 B/op 1 allocs/op
BenchmarkEfaceFn-24 2000000000 0.48 ns/op 0 B/op 0 allocs/op
@bradfitz

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commented Mar 1, 2017

@bradfitz bradfitz added the Performance label Mar 1, 2017

@bradfitz bradfitz added this to the Go1.9Maybe milestone Mar 1, 2017

@randall77

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commented Mar 1, 2017

This is fixed at tip.

In BenchmarkEfaceFn, the call gets inlined and the conversion get compiled completely away.

In BenchmarkEfaceFnVal, the call still has to happen (we have no devirtualization of calls at the moment), but the conversion to interface now happens statically at compile time.

BenchmarkEfaceFnVal-8   	1000000000	         2.33 ns/op	       0 B/op	       0 allocs/op
BenchmarkEfaceFn-8      	2000000000	         0.52 ns/op	       0 B/op	       0 allocs/op

@randall77 randall77 closed this Mar 1, 2017

@cstockton

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commented Mar 1, 2017

@randall77 Sorry I didn't check tip first, should have, thanks for the quick response

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