cmd/compile: internal compiler error with GOEXPERIMENT=cgocheck2 on github.com/leodido/go-urn

[trofkm]

Go version

go version go1.25.0 darwin/arm64

Output of go env in your module/workspace:

AR='ar'
CC='cc'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='c++'
GCCGO='gccgo'
GO111MODULE=''
GOARCH='arm64'
GOARM64='v8.0'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/Users/trofkm/Library/Caches/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/Users/trofkm/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT='cgocheck2'
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/2t/lsktd7d90qq6r8l_3hqstmvc0000gn/T/go-build2504032626=/tmp/go-build -gno-record-gcc-switches -fno-common'
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMOD='/Users/trofkm/Programs/goprojects/bug_sandbox/go.mod'
GOMODCACHE='/Users/trofkm/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/trofkm/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/opt/homebrew/Cellar/go/1.25.0/libexec'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/Users/trofkm/Library/Application Support/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/opt/homebrew/Cellar/go/1.25.0/libexec/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.25.0'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

Set GOEXPERIMENT=cgocheck2 and tried to build a minimal program importing github.com/leodido/go-urn@v1.4.0.

GOEXPERIMENT=cgocheck2  go build -a

What did you see happen?

# github.com/leodido/go-urn
.../machine.go:4378:17: internal compiler error: bad live variable at entry of (*machine).Parse: *SCIM value

What did you expect to see?

Successful compilation.

Additional info:

• This ICE only occurs with GOEXPERIMENT=cgocheck2.
• Also appears on go version go1.26-devel_7f6ff5e 2025-09-19 17:06:23 -0700 linux/amd64
• Minimal repro: https://github.com/trofkm/cgocheck2_ice
• Since cgocheck2 is experimental, this may be expected, but reporting for tracking.

[gabyhelp]

Related Issues

• test #71713 (closed)
• Test #67906 (closed)
• cmd/compile: random compile error running tests #35760 (closed)
• test #74008 (closed)
• Test #70447 (closed)
• go build #52634 (closed)
• cmd/compile: internal compiler error: Value live at entry. It shouldn't be. #44355 (closed)
• cmd/compile: internal compiler error: found illegal assignment #49875 (closed)
• Test #73656 (closed)
• cmd/link: go test -c on darwin/arm64 produces invalid code signature in combination with cgo #63576 (closed)

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

[trofkm]

I’ve done some research into the compiler internals. As far as I understand, the internal compiler error is not caused by the go-urn package itself, but by how the experimental cgocheck2 feature interacts with the liveness analysis pass — specifically, by injecting pointer-checking calls too early in the SSA, which corrupts the liveness state at function entry.

Here’s a brief summary of my findings with code locations:

• The ICE triggers in src/cmd/compile/internal/liveness/plive.go, where the compiler asserts that only input parameters should be live at function entry. The variable *SCIM value (from go-urn) is incorrectly marked as live at entry.

• The root cause lies in the writebarrier pass (src/cmd/compile/internal/ssa/writebarrier.go). When GOEXPERIMENT=cgocheck2 is enabled, this pass injects calls to cgoCheckPtrWrite and cgoCheckMemmove (defined in src/runtime/cgocheck.go) into the SSA.

• These calls are injected in the bEnd block (the “after” block), but this happens before liveness analysis expects the referenced variables to be alive. As a result, liveness analysis sees *SCIM value as live at function entry — violating the invariant and triggering the fatal error.

• The function addresses for cgoCheckPtrWrite and cgoCheckMemmove are looked up in src/cmd/compile/internal/ssagen/ssa.go via f.fe.Syslook.

• The problematic injection occurs around this code in writebarrier.go:

  if buildcfg.Experiment.CgoCheck2 {
      mem = wbcall(pos, bEnd, cgoCheckPtrWrite, sp, mem, dst, val)
  }

  (or probably the same issue with cgoCheckMemmove)

  if buildcfg.Experiment.CgoCheck2 {
    // Issue cgo checking code.
    typ := reflectdata.TypeLinksym(w.Aux.(*types.Type))
    taddr := b.NewValue1A(pos, OpAddr, b.Func.Config.Types.Uintptr, typ, sb)
    mem = wbcall(pos, bEnd, cgoCheckMemmove, sp, mem, taddr, dst, src)
  }

  This likely needs to be moved to bThen or delayed until after the variable’s first actual assignment, to avoid corrupting entry-block liveness.

This appears to be a phase-ordering or liveness-tracking bug in the interaction between cgocheck2 and the compiler’s SSA passes.

It's just my thoughts, I’m not a compiler expert, so I welcome any corrections or guidance.

[trofkm]

I tried to implement the simple working example, but find another ICE, the root of the problem might be the same, so I put it here.

example:

https://github.com/trofkm/cgocheck2_ice/blob/main/cmd/simple/main.go

ICE:

cmd/simple/main.go:48:9: internal compiler error: '(*Machine).Parse': can't find source for b87->b2: v897 = Phi <*Machine> v349 v45 : R3 (m[*Machine])

[JunyangShao]

@golang/compiler

[randall77]

GOEXPERIMENT=cgocheck2 should work.

It looks broken back to at least 1.23.

This looks like a bug in the tighten pass. It is making an SSA graph where a value has an argument which doesn't dominate the value. You can see this by adding -gcflags=-d=ssa/check/on to the build.
Something is off about the loop logic, here:

go/src/cmd/compile/internal/ssa/tighten.go

Line 124 in 7bc1935

// If the target location is inside a loop,

The loop information may be wrong or inaccurate.

Not sure what that has to do with cgocheck2. Maybe it just makes more complex control flow that is more likely to tickle this bug. Maybe cgocheck2 makes more irreducible graphs, and irreducible graphs are the problem?

[gopherbot]

Change https://go.dev/cl/706355 mentions this issue: cmd/compile: don't rely on loop info when there are irreducible loops