cmd/compile: panic during sccp while compiling main runtime error: index out of range [-5]

[tchung1118]

Go version

go version go1.22rc1 X:nocoverageredesign darwin/arm64

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

GO111MODULE=''
GOARCH='arm64'
GOBIN=''
GOCACHE='/Users/taiwon/Library/Caches/go-build'
GOENV='/Users/taiwon/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT='nocoverageredesign'
GOFLAGS=''
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/taiwon/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/taiwon/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/Users/taiwon/go/src/github.com/golang/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/Users/taiwon/go/src/github.com/golang/go/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.22rc1 X:nocoverageredesign'
GCCGO='gccgo'
AR='ar'
CC='clang'
CXX='clang++'
CGO_ENABLED='1'
GOMOD='/Users/taiwon/go/src/github.com/tchung1118/test-app/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 -ffile-prefix-map=/var/folders/t0/jhcv48q51ydbyzqn85y8rsgm0000gn/T/go-build3057666933=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

https://go.dev/play/p/tuaTn9Ke1VG?v=gotip

What did you expect to see?

I expect the program to compile

What did you see instead?

./prog.go:123:2: internal compiler error: 'main': panic during sccp while compiling main:

runtime error: index out of range [-5]

goroutine 9 [running]:
cmd/compile/internal/ssa.Compile.func1()
	/usr/local/go/src/cmd/compile/internal/ssa/compile.go:49 +0x6c
panic({0xdb62e0?, 0xc00002b638?})
	/usr/local/go/src/runtime/panic.go:770 +0x132
cmd/compile/internal/ssa.(*Block).swapSuccessorsByIdx(...)
	/usr/local/go/src/cmd/compile/internal/ssa/block.go:334
cmd/compile/internal/ssa.rewireSuccessor(0xc00062aac0, 0xc0006630b0?)
	/usr/local/go/src/cmd/compile/internal/ssa/sccp.go:538 +0x20d
cmd/compile/internal/ssa.(*worklist).replaceConst(...)
	/usr/local/go/src/cmd/compile/internal/ssa/sccp.go:568
cmd/compile/internal/ssa.sccp(0xc00048e9c0)
	/usr/local/go/src/cmd/compile/internal/ssa/sccp.go:118 +0xc95
cmd/compile/internal/ssa.Compile(0xc00048e9c0)
	/usr/local/go/src/cmd/compile/internal/ssa/compile.go:97 +0xa44
cmd/compile/internal/ssagen.buildssa(0xc000448480, 0x3)
	/usr/local/go/src/cmd/compile/internal/ssagen/ssa.go:575 +0x2869
cmd/compile/internal/ssagen.Compile(0xc000448480, 0x3)
	/usr/local/go/src/cmd/compile/internal/ssagen/pgen.go:216 +0x3e
cmd/compile/internal/gc.compileFunctions.func5.1(0x45215d?)
	/usr/local/go/src/cmd/compile/internal/gc/compile.go:182 +0x34
cmd/compile/internal/gc.compileFunctions.func3.1()
	/usr/local/go/src/cmd/compile/internal/gc/compile.go:164 +0x30
created by cmd/compile/internal/gc.compileFunctions.func3 in goroutine 8
	/usr/local/go/src/cmd/compile/internal/gc/compile.go:163 +0x247



goroutine 9 [running]:
runtime/debug.Stack()
	/usr/local/go/src/runtime/debug/stack.go:24 +0x5e
cmd/compile/internal/base.FatalfAt({0x667690?, 0xc0?}, {0xc0004cf680, 0x32}, {0xc0006431d0, 0x5, 0x5})
	/usr/local/go/src/cmd/compile/internal/base/print.go:225 +0x1d7
cmd/compile/internal/base.Fatalf(...)
	/usr/local/go/src/cmd/compile/internal/base/print.go:194
cmd/compile/internal/ssagen.(*ssafn).Fatalf(0x0?, {0x0?, 0x0?}, {0xe155e5, 0x2c}, {0xc000647880, 0x4, 0x4000?})
	/usr/local/go/src/cmd/compile/internal/ssagen/ssa.go:8225 +0x16a
cmd/compile/internal/ssa.(*Func).Fatalf(0xc00048e9c0, {0xe155e5, 0x2c}, {0xc000647880, 0x4, 0x4})
	/usr/local/go/src/cmd/compile/internal/ssa/func.go:741 +0x279
cmd/compile/internal/ssa.Compile.func1()
	/usr/local/go/src/cmd/compile/internal/ssa/compile.go:54 +0x1a7
panic({0xdb62e0?, 0xc00002b638?})
	/usr/local/go/src/runtime/panic.go:770 +0x132
cmd/compile/internal/ssa.(*Block).swapSuccessorsByIdx(...)
	/usr/local/go/src/cmd/compile/internal/ssa/block.go:334
cmd/compile/internal/ssa.rewireSuccessor(0xc00062aac0, 0xc0006630b0?)
	/usr/local/go/src/cmd/compile/internal/ssa/sccp.go:538 +0x20d
cmd/compile/internal/ssa.(*worklist).replaceConst(...)
	/usr/local/go/src/cmd/compile/internal/ssa/sccp.go:568
cmd/compile/internal/ssa.sccp(0xc00048e9c0)
	/usr/local/go/src/cmd/compile/internal/ssa/sccp.go:118 +0xc95
cmd/compile/internal/ssa.Compile(0xc00048e9c0)
	/usr/local/go/src/cmd/compile/internal/ssa/compile.go:97 +0xa44
cmd/compile/internal/ssagen.buildssa(0xc000448480, 0x3)
	/usr/local/go/src/cmd/compile/internal/ssagen/ssa.go:575 +0x2869
cmd/compile/internal/ssagen.Compile(0xc000448480, 0x3)
	/usr/local/go/src/cmd/compile/internal/ssagen/pgen.go:216 +0x3e
cmd/compile/internal/gc.compileFunctions.func5.1(0x45215d?)
	/usr/local/go/src/cmd/compile/internal/gc/compile.go:182 +0x34
cmd/compile/internal/gc.compileFunctions.func3.1()
	/usr/local/go/src/cmd/compile/internal/gc/compile.go:164 +0x30
created by cmd/compile/internal/gc.compileFunctions.func3 in goroutine 8
	/usr/local/go/src/cmd/compile/internal/gc/compile.go:163 +0x247


Go build failed.

[randall77]

This looks kinda like #64606, but I don't think it is directly related.

I think what is going on here is that sccp is trying to optimize unreachable code. That unreachable code fails the invariant that jump table indexes are in range. I'm not entirely sure whether some previous phase is generating bad unreachable jump tables, or whether we just need to defend against unreachable code. I'll look some more.

[randall77]

(The unreachable code is the jump table in the body of NewNotificationFromString inlined into main at the last callsite in main. It is unreachable because we compare the string length and it is shorter than any string in the case statement.)

[randall77]

Here's a simpler reproducer:

package main

func main() {
	f(g(false))
}
func g(b bool) string {
	if b {
		return "z"
	}
	return "q"
}
func f(x string) int {
	switch len(x) {
	case 4:
		return 4
	case 5:
		return 5
	case 6:
		return 6
	case 7:
		return 7
	case 8:
		return 8
	case 9:
		return 9
	case 10:
		return 10
	case 11:
		return 11
	}
	return 0
}

[gopherbot]

Change https://go.dev/cl/552055 mentions this issue: cmd/compile: handle constant-folding of an out-of-range jump table index