cmd/compile: internal compiler error: Value live at entry. It shouldn't be.

[tonyghita]

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

$ go version
go version go1.16 darwin/amd64

Does this issue reproduce with the latest release?

Yes. I've git bisect the error to commit f2c0c2b

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/tonyghita/Library/Caches/go-build"
GOENV="/Users/tonyghita/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/tonyghita/go/pkg/mod"
GONOPROXY="*"
GONOSUMDB="*"
GOOS="darwin"
GOPATH="/Users/tonyghita/go"
GOPRIVATE="*"
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/Users/tonyghita/sdk/go1.16"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/Users/tonyghita/sdk/go1.16/pkg/tool/darwin_amd64"
GOVCS=""
GOVERSION="go1.16"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/tonyghita/go/src/github.com/tonyghita/repro/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/gv/vdmq3_l541b_0lq_ds37xslnzgdpkw/T/go-build3906706978=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

I made a small reproducer repository to demonstrate the issue https://github.com/tonyghita/repro and run go test against it.

What did you expect to see?

Test pass without issue, as in Go 1.15.

What did you see instead?

# github.com/tonyghita/repro_test [github.com/tonyghita/repro.test]
./repro_test.go:10:13: internal compiler error: 'TestF.func1': Value live at entry. It shouldn't be. func TestF.func1, node ~R0, value v19

[ianlancetaylor]

CC @mdempsky @dr2chase

If this is a real bug it probably needs a backport to the 1.16 branch.

[tonyghita]

I forgot to describe the setup to reproduce the issue.

package repro

type A struct{}

func (*A) F() (_ *B) { return }

type B struct{}

and then in the repro_test package (or some other package)

package repro_test

import (
	"testing"

	"github.com/tonyghita/repro"
)

func TestF(t *testing.T) {
	var a *repro.A

	if a.F() != nil {
		t.Fail()
	}
}

[mdempsky]

Thanks for the minimized test case, @tonyghita . I'm able to reproduce the issue locally.

[mdempsky]

The issue is due to inlining an imported function with a blank result parameter and only one return statement, which also doesn't have any result arguments.

[mdempsky]

@tonyghita Can I ask what the original function looks like and why it was written that way? It's definitely a compiler issue that it's mishandling this case that I expect to have a fix for shortly, but I'm surprised there's real world code that exhibits this behavior, so I'm curious.

FWIW, if you're in need of a workaround until Go 1.16.1 is available, you can should be able to either change the bare return statement to explicitly return zero values, or just add a second (unreachable) return statement. Either of these would suppress the optimization that's crashing here.

[josharian]

Aside: From all my years working on the compiler, there is no internal compiler error I hate more. (Of course, that means I also love it in a way, because it is catching bugs.)

[gopherbot]

Change https://golang.org/cl/293293 mentions this issue: cmd/compile: declare inlined result params early for empty returns

[mdempsky]

@gopherbot please backport to Go 1.16