cmd/compile: arm64 build fails with: constrained unpredictable behavior

[stapelberg]

Go version

go version go1.26-devel_77f911e31c Fri Aug 15 18:10:28 2025 -0700 linux/amd64

Output of go env in your module/workspace:

AR='ar'
CC='gcc'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='g++'
GCCGO='gccgo'
GO111MODULE=''
GOAMD64='v1'
GOARCH='amd64'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/usr/local/google/home/stapelberg/.cache/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/usr/local/google/home/stapelberg/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build3818096045=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/usr/local/google/home/stapelberg/src/arm-unpredictable/go.mod'
GOMODCACHE='/usr/local/google/home/stapelberg/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/usr/local/google/home/stapelberg/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/google/home/stapelberg/upstream-go'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/usr/local/google/home/stapelberg/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/google/home/stapelberg/upstream-go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.26-devel_77f911e31c Fri Aug 15 18:10:28 2025 -0700'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

1. go mod init repro
2. Write the following repro.go file:

package reorder

type Element struct {
	A     string
	B     string
	C     string
	D     string
	E     string
	Text  []string
	List  []string
	Child Elements
	F     string
	G     bool
	H     bool
	I     string
}

type Elements []Element

func DoesNotCompile(ve Elements) Elements {
	aa := Elements{}
	bb := Elements{}
	cc := Elements{}
	dd := Elements{}
	ee := Elements{}
	ff := Elements{}
	gg := Elements{}
	hh := Elements{}
	ii := Elements{}

	if len(ve) != 1 {
		return ve
	}
	for _, e := range ve[0].Child {
		if len(e.Text) == 1 && (e.Text[0] == "xx") {
			ee = append(ee, e)
		} else if len(e.Text) == 1 && e.Text[0] == "yy" {
			for _, c := range e.Child {
				if len(c.Text) == 1 && c.Text[0] == "zz" {
					ii = append(ii, c)
				} else {
					hh = append(hh, c)
				}
			}
			ii = append(ii, hh...)
			e.Child = ii
			gg = append(gg, e)
		} else if len(e.Text) == 1 && e.Text[0] == "tt" {
			for _, entry := range e.Child {
				for _, c := range entry.Child {
					if len(c.Text) == 1 && c.Text[0] == "ee" {
						cc = append(cc, c)
					} else {
						dd = append(dd, c)
					}
				}
				cc = append(cc, dd...)
				entry.Child = cc
				bb = append(bb, entry)
				cc, dd = Elements{}, Elements{}
			}
			e.Child = bb
			aa = append(aa, e)
		} else {
			ff = append(ff, e)
		}
	}
	return ve
}

3. Compile this package using: GOARCH=arm64 go build

What did you see happen?

# repro
./repro.go:68:2: constrained unpredictable behavior: 01460 (/usr/local/google/home/stapelberg/src/arm-unpredictable/repro.go:49)	LDP.P	16(R24), (R24, R25)
./repro.go:68:2: constrained unpredictable behavior: 01468 (/usr/local/google/home/stapelberg/src/arm-unpredictable/repro.go:49)	LDP.P	16(R24), (R24, R25)
./repro.go:68:2: constrained unpredictable behavior: 01476 (/usr/local/google/home/stapelberg/src/arm-unpredictable/repro.go:49)	LDP.P	16(R24), (R24, R25)
./repro.go:68:2: constrained unpredictable behavior: 01484 (/usr/local/google/home/stapelberg/src/arm-unpredictable/repro.go:49)	LDP.P	16(R24), (R24, R25)
./repro.go:68:2: constrained unpredictable behavior: 03220 (/usr/local/google/home/stapelberg/src/arm-unpredictable/repro.go:38)	LDP.P	16(R24), (R24, R25)
./repro.go:68:2: constrained unpredictable behavior: 03228 (/usr/local/google/home/stapelberg/src/arm-unpredictable/repro.go:38)	LDP.P	16(R24), (R24, R25)
./repro.go:68:2: constrained unpredictable behavior: 03236 (/usr/local/google/home/stapelberg/src/arm-unpredictable/repro.go:38)	LDP.P	16(R24), (R24, R25)
./repro.go:68:2: constrained unpredictable behavior: 03244 (/usr/local/google/home/stapelberg/src/arm-unpredictable/repro.go:38)	LDP.P	16(R24), (R24, R25)

What did you expect to see?

Commit bca3e98 is the last working commit.

Commit 15d6dbc (“cmd/compile: use generated loops instead of DUFFCOPY on arm64”) is the first broken commit.

[gabyhelp]

Related Issues

• cmd/buildid: failures in cmd/go introduced in commit afd090c on multiple platforms including ppc64le, s390, and arm64 #23339 (closed)
• cmd/compile: random compile error running tests #35760 (closed)
• cmd/go: internal/test.test build faild on linux/loong64 #67538 (closed)
• cmd/compile: unreachable panic with GODEBUG=gotypesalias=1 #65778 (closed)
• cmd/compile: range over rune literal causes internal compiler error #64471 (closed)
• cmd/compile: stack overflow on invalid map key type #21657 (closed)
• cmd/compile: internal compiler error: assertion failed #53762 (closed)
• strconv: "invalid array index" compiler error was reported after changing reloc type #31955 (closed)
• cmd/compile: panic: branch too far on arm64 #23889 (closed)
• cmd/compile: bad code generation for ARM (bounds test fails) #41780 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[randall77]

Strange. Must be something wrong with regalloc. This instruction specifically asks for the input not to be in R24.

After regalloc:

v353 (49) = Copy <unsafe.Pointer> v602 : R24
v607 (49) = LoweredMoveLoop <mem> [192] v562 v353 v605

But LoweredMoveLoop's spec is:

				inputs:       []regMask{gp &^ r23to25, gp &^ r23to25},

I will investigate.

[gopherbot]

Change https://go.dev/cl/697015 mentions this issue: cmd/compile: ensure we use allowed registers for input-clobbering instructions