It is a GC issue?

[Walnux]

Meet a weird issue, someone can have a look at it. Thanks!

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

$ go version
go version go1.16 linux/amd64

Does this issue reproduce with the latest release?

go1.16 is the latest stable version I can get now

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

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/martin/.cache/go-build"
GOENV="/home/martin/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/martin/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/martin/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.16"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
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-build901155952=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Please see the code below

package main
import "fmt"

func getSubset(nums, combine []int, re *[][]int){
    *re = append(*re, combine)
    fmt.Println(*re)
    if len(combine) > 0 {
            fmt.Println(&combine[len(combine) - 1])
    }

    if len(nums) == 0 {
        return
    }

    for i, _ := range nums {
            getSubset(nums[i + 1:], append(combine, nums[i]), re)
    }

    return
}

func subsets(nums []int) [][]int {
    var re [][]int
    getSubset(nums, []int{}, &re)
    return re
}

func main() {
        re := subsets([]int{1,2,3,4,5})
        fmt.Println(re)
}

What did you expect to see?

[[]]
[[] [1]]
0xc0000140d0
[[] [1] [1 2]]
0xc0000140f8
[[] [1] [1 2] [1 2 3]]
0xc00001c130
[[] [1] [1 2] [1 2 3] [1 2 3 4]]
0xc00001c138
[[] [1] [1 2] [1 2 3] [1 2 3 4] [1 2 3 4 5]]
0xc00001e0e0
[[] [1] [1 2] [1 2 3] [1 2 3 4] [1 2 3 4 5] [1 2 3 5]]
0xc0000????  any memory space which is not used
...

What did you see instead?

[[]]
[[] [1]]
0xc0000140d0
[[] [1] [1 2]]
0xc0000140f8
[[] [1] [1 2] [1 2 3]]
0xc00001c130
[[] [1] [1 2] [1 2 3] [1 2 3 4]]
0xc00001c138
this address is used to save the last item of slice []int{1,2,3,4}. Since this slice is appended to *re which will be accessed later, according to my understanding address 0xc00001c138 should not be released and used by others before *re is released.

[[] [1] [1 2] [1 2 3] [1 2 3 4] [1 2 3 4 5]]
0xc00001e0e0
[[] [1] [1 2] [1 2 3] [1 2 3 5] [1 2 3 4 5] [1 2 3 5]]
0xc00001c138

but looks like here 0xc00001c138 is released and used to save the last items of slice[]int{1,2,3,5}.
it can be prvoed that the last item of slice []int{1,2,3,4} is changed from 4 to 5

[ianlancetaylor]

Please see https://blog.golang.org/slices.