cmd/compile: panic while running compiler: index out of range during lowered cse

[philhofer]

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

go version devel +9e04097 Wed Sep 7 19:07:38 2016 +0000 linux/amd64
(sha 794fb71)

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

linux/amd64

What did you do?

Run 'go install'

What did you expect to see?

Nothing (this code compiles correctly under 1.7.3)

What did you see instead?

<REDACTED>: internal compiler error: panic during lowered cse while compiling <REDACTED>: 
runtime error: index out of range

goroutine 1 [running]:
cmd/compile/internal/ssa.Compile.func1(0xc420e28b20, 0xc421378c60)
        /home/phil/gopath/go/src/cmd/compile/internal/ssa/compile.go:36 +0xc8
panic(0xa513c0, 0xd47720)
        /home/phil/gopath/go/src/runtime/panic.go:489 +0x2cf
cmd/compile/internal/ssa.cse(0xc421378c60)
        /home/phil/gopath/go/src/cmd/compile/internal/ssa/cse.go:191 +0x21c6
cmd/compile/internal/ssa.Compile(0xc421378c60)
        /home/phil/gopath/go/src/cmd/compile/internal/ssa/compile.go:68 +0x304
cmd/compile/internal/gc.buildssa(0xc42088c900, 0x0)
        /home/phil/gopath/go/src/cmd/compile/internal/gc/ssa.go:177 +0x8fe
cmd/compile/internal/gc.compile(0xc42088c900)
        /home/phil/gopath/go/src/cmd/compile/internal/gc/pgen.go:362 +0x204
cmd/compile/internal/gc.funccompile(0xc42088c900)
        /home/phil/gopath/go/src/cmd/compile/internal/gc/dcl.go:1292 +0xdc
cmd/compile/internal/gc.Main()
        /home/phil/gopath/go/src/cmd/compile/internal/gc/main.go:460 +0x1f0a
main.main()
        /home/phil/gopath/go/src/cmd/compile/main.go:50 +0xfe

goroutine 1 [running]:
runtime/debug.Stack(0x0, 0x0, 0x0)
        /home/phil/gopath/go/src/runtime/debug/stack.go:24 +0x79
cmd/compile/internal/gc.Fatalf(0xaae85e, 0x2c, 0xc421371440, 0x4, 0x4)
        /home/phil/gopath/go/src/cmd/compile/internal/gc/subr.go:165 +0x226
cmd/compile/internal/gc.(*ssaExport).Fatalf(0xdbccd5, 0x27c, 0xaae85e, 0x2c, 0xc421371440, 0x4, 0x4)
        /home/phil/gopath/go/src/cmd/compile/internal/gc/ssa.go:4976 +0x5d
cmd/compile/internal/ssa.(*Config).Fatalf(0xc420f1e000, 0xc40000027c, 0xaae85e, 0x2c, 0xc421371440, 0x4, 0x4)
        /home/phil/gopath/go/src/cmd/compile/internal/ssa/config.go:343 +0x6e
cmd/compile/internal/ssa.(*Func).Fatalf(0xc421378c60, 0xaae85e, 0x2c, 0xc421371440, 0x4, 0x4)
        /home/phil/gopath/go/src/cmd/compile/internal/ssa/func.go:413 +0x6b
cmd/compile/internal/ssa.Compile.func1(0xc420e28b20, 0xc421378c60)
        /home/phil/gopath/go/src/cmd/compile/internal/ssa/compile.go:38 +0x250
panic(0xa513c0, 0xd47720)
        /home/phil/gopath/go/src/runtime/panic.go:489 +0x2cf
cmd/compile/internal/ssa.cse(0xc421378c60)
        /home/phil/gopath/go/src/cmd/compile/internal/ssa/cse.go:191 +0x21c6
cmd/compile/internal/ssa.Compile(0xc421378c60)
        /home/phil/gopath/go/src/cmd/compile/internal/ssa/compile.go:68 +0x304
cmd/compile/internal/gc.buildssa(0xc42088c900, 0x0)
        /home/phil/gopath/go/src/cmd/compile/internal/gc/ssa.go:177 +0x8fe
cmd/compile/internal/gc.compile(0xc42088c900)
        /home/phil/gopath/go/src/cmd/compile/internal/gc/pgen.go:362 +0x204
cmd/compile/internal/gc.funccompile(0xc42088c900)
        /home/phil/gopath/go/src/cmd/compile/internal/gc/dcl.go:1292 +0xdc
cmd/compile/internal/gc.Main()
        /home/phil/gopath/go/src/cmd/compile/internal/gc/main.go:460 +0x1f0a
main.main()
        /home/phil/gopath/go/src/cmd/compile/main.go:50 +0xfe

I'll try to isolate a test case; this is all I have at the moment.

[ianlancetaylor]

CC @randall77 @josharian

[philhofer]

Isolated test case:

package main

import (
        "fmt"
        "time"
)

var (
        iqlTimeUnits = []struct {
                divisor time.Duration
                unit    rune
        }{
                {1000000, 's'},                                                                                       
                {60, 'm'},                                                                                            
                {60, 'h'},                                                                                            
                {24, 'd'},                                                                                            
                {7, 'w'},
        }
)

func foobar(d time.Duration) string {
        d /= time.Microsecond
        unit := 'u'

        for _, f := range iqlTimeUnits {
                if d%f.divisor != 0 {
                        break
                }
                d /= f.divisor
                unit = f.unit
        }
        return fmt.Sprintf("%d%c", d, unit)
}

func main() {
        println(foobar(time.Second))
}

[philhofer]

Confirmed this is still an issue at the current head commit (bb00a8d)

[philhofer]

Based on my superficial reading of the code in question (lines 180-200 in ssa/cse.go), it looks like rewrite[v.ID] goes out of bounds because v.copyInto(t.Block) (line 210) can create new ssa IDs (via (*Value).copyInto() -> (*Block).NewValue0() -> (*Func).newValue() -> (*idAlloc).get())

[gopherbot]

CL https://golang.org/cl/33240 mentions this issue.