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cmd/compile: unnecessary bounds checking using result of intrinsic function #58334

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ericlagergren opened this issue Feb 5, 2023 · 3 comments
Closed

cmd/compile: unnecessary bounds checking using result of intrinsic function #58334

ericlagergren opened this issue Feb 5, 2023 · 3 comments
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compiler/runtime Issues related to the Go compiler and/or runtime. FrozenDueToAge

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

$ go version
go version go1.20 darwin/arm64

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="arm64"
GOBIN="/Users/eric/gopath/bin"
GOCACHE="/Users/eric/Library/Caches/go-build"
GOENV="/Users/eric/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="arm64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/eric/gopath/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/eric/gopath"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_arm64"
GOVCS=""
GOVERSION="go1.20"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/tmp/prove/go.mod"
GOWORK=""
CGO_CFLAGS="-O2 -g"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-O2 -g"
CGO_FFLAGS="-O2 -g"
CGO_LDFLAGS="-O2 -g"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/2f/2_yz21b92h94_pvr2m00gzh80000gn/T/go-build1716388533=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

https://go.dev/play/p/OZrfW7G5A6i

What did you expect to see?

No bounds checking for the array lookup in B.

What did you see instead?

Bounds checking because the index variable is signed.

While the result of LeadingZeros64 is signed, the compiler should know that the possible range is [0, 64] and deduce that n (and therefore i) will always be positive. (Even better would be using the result of LeadingZeros64 directly since the the n == 0 case reduces the range to [0, 63].)
@gopherbot gopherbot added the compiler/runtime Issues related to the Go compiler and/or runtime. label Feb 5, 2023
@cuonglm
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cuonglm commented Feb 5, 2023

Seems duplicated of #40084.

@ericlagergren
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@cuonglm yes, I think you're right. Actually, it looks like there are a handful of duplicates.

@dr2chase
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dr2chase commented Feb 6, 2023

Duplicate of #40084.

@dr2chase dr2chase closed this as completed Feb 6, 2023
@golang golang locked and limited conversation to collaborators Feb 6, 2024
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@dr2chase @cuonglm @ericlagergren @gopherbot