cmd/compile: internal compiler error: assertion failed (on 1.18 and tip)

[ALTree]

This program:

package main

import (
	"reflect"
	"unsafe"
)

func F[T any](v T) uintptr {
	return unsafe.Alignof(func() T {
		_ = reflect.DeepEqual(struct{ _ T }{}, nil)
		return v
	}())
}

func main() {
	F(0)
}

crashes both the 1.18.3 and the tip compiler with this error:

$ go build crash.go

# command-line-arguments
.\crash.go:16:3: internal compiler error: assertion failed

cc @mdempsky @randall77

[mdempsky]

Thanks for the report! 'm guessing the problem is Go 1.18 frontend is being inconsistent in when/whether it visits the argument to unsafe.Alignof, since it isn't actually needed.

I think the obvious workaround is to just write unsafe.Alignof(*new(T)) instead of unsafe.Alignof(func() T { ... }()).

The code is also already working for Unified IR, and I'll add it as a test case to make sure it stays working.

I think it's probably an easy fix for non-unified mode, if anyone wants to investigate that (@cuonglm ?). But I don't think it's terribly important, and probably not worth backporting any fixes for, unless we can reproduce the issue with more realistic code.

[gopherbot]

Change https://go.dev/cl/412534 mentions this issue: [dev.unified] test: add regress test for #53390

[cuonglm]

@mdempsky I looked into this yesterday, but haven't had time to send CL yet, since when I'm not sure whether we should fix this for 1.19 on master, or fix for 1.20 in dev.unified branch.

You're right that the Go 1.18 frontend is not consistent, in process of generate instantiation for generic function, when doing substitution for unsafe.Alignof, it's transformed unsafe.Alignof to a OLITERAL node, but then still visit old unsafe.Alignof's children nodes.

[gopherbot]

Change https://go.dev/cl/412794 mentions this issue: cmd/compile: skip substituting closures in unsafe builtins arguments