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reflect: directlyAssignable doesn't need rtype.Name() #32186

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g7r opened this issue May 22, 2019 · 2 comments

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@g7r
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commented May 22, 2019

What version of Go are you using (go version)?

$ go version
go version devel +06b0babf31 Wed May 22 12:54:00 2019 +0000 linux/amd64

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

go env Output
$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/sz/.cache/go-build"
GOENV="/home/sz/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/sz/go"
GOPROXY="direct"
GOROOT="/home/sz/src/go"
GOSUMDB="off"
GOTMPDIR=""
GOTOOLDIR="/home/sz/src/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build870897326=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Benchmark: https://play.golang.org/p/V2BzESPuf2w
Benchmark results:

$ benchstat old_test.txt new_test.txt
name                   old time/op  new time/op  delta
DirectlyAssignable-12  32.7ns ± 6%   6.6ns ± 6%  -79.80%  (p=0.008 n=5+5)

What did you expect to see?

I honestly expect AssignableTo to be as fast as possible.

What did you see instead?

I see that AssignableTo invokes directlyAssignable. And directlyAssignable performs more work than it really needs. directlyAssignable retrieves rtype names and checks whether they are equal to empty string. Retrieving the names isn't required for that. We can just check the flag tflagNamed.

@g7r

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commented May 22, 2019

I'll submit a change to fix the issue.

@gopherbot

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commented May 22, 2019

Change https://golang.org/cl/178518 mentions this issue: reflect: directlyAssignable doesn't need rtype.Name()

@bradfitz bradfitz added this to the Go1.14 milestone May 22, 2019
@bcmills bcmills added the NeedsFix label May 22, 2019
@gopherbot gopherbot closed this in 8057c08 Aug 27, 2019
tomocy added a commit to tomocy/go that referenced this issue Sep 1, 2019
directlyAssignable invoked rtype.Name() just to compare its result
to empty string. We really only need to check whether rtype has
name. It can be done much cheaper, by checking tflagNamed.

Benchmark: https://play.golang.org/p/V2BzESPuf2w
name                   old time/op  new time/op  delta
DirectlyAssignable-12  32.7ns ± 6%   6.6ns ± 6%  -79.80%  (p=0.008 n=5+5)

Fixes golang#32186

Change-Id: I1a2a167dbfddf319fba3015cb6a011bf010f99a8
Reviewed-on: https://go-review.googlesource.com/c/go/+/178518
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
t4n6a1ka added a commit to t4n6a1ka/go that referenced this issue Sep 5, 2019
directlyAssignable invoked rtype.Name() just to compare its result
to empty string. We really only need to check whether rtype has
name. It can be done much cheaper, by checking tflagNamed.

Benchmark: https://play.golang.org/p/V2BzESPuf2w
name                   old time/op  new time/op  delta
DirectlyAssignable-12  32.7ns ± 6%   6.6ns ± 6%  -79.80%  (p=0.008 n=5+5)

Fixes golang#32186

Change-Id: I1a2a167dbfddf319fba3015cb6a011bf010f99a8
Reviewed-on: https://go-review.googlesource.com/c/go/+/178518
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
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