(constant -4 of type uintptr) overflows uintptr

[kyleconroy]

TinyGo can't compile the the pointer_unsafe.go file from https://github.com/protocolbuffers/protobuf-go. I get the following error message:

unsafe.Sizeof(unsafe.Pointer(nil)) - unsafe.Sizeof((MessageState literal)) (constant -4 of type uintptr) overflows uintptr

[aykevl]

This comment above it is relevant:

// Static check that MessageState does not exceed the size of a pointer.

So it appears that MessageState is larger than a pointer.
Without further information, this would be hard to debug. How can I reproduce this?

---

That said, this is most certainly unsafe code so that throws away all promises of portability. But quickly looking at it, I don't see why it fails.

[skabbes]

I had a comment previously that I deleted, I did not read aykevl's comment closely enough.

The MessageState struct embeds a few structs with 0 fields.

My new guess is that these structs take space in TinyGo, but do not take space in "regular Go".

[kyleconroy]

Without further information, this would be hard to debug. How can I reproduce this?

Sorry about that. Here's the reproduction.

go mod init github.com/tinygo-issues
go get google.golang.org/protobuf

package main

import (
	"fmt"

	_ "google.golang.org/protobuf/runtime/protoimpl"
)

func main() {
	fmt.Println("protoc")
}

tinygo build -o example.wasm -gc=leaking -scheduler=none -wasm-abi=generic -target=wasi
# google.golang.org/protobuf/internal/impl
../../go/pkg/mod/google.golang.org/protobuf@v1.28.0/internal/impl/pointer_unsafe.go:142:16: unsafe.Sizeof(unsafe.Pointer(nil)) - unsafe.Sizeof((MessageState literal)) (constant -4 of type uintptr) overflows uintptr
make: *** [run] Error 1

[skabbes]

I trimmed down the minimal repro - to be something even more minimal.

I think the bug can be restated as "sizeof 0-length arrays of functions is greater than 0.

package main

import (
	"unsafe"
)

// DoNotCompare can be embedded in a struct to prevent comparability.
// This struct, when embedded takes non-zero space
type DoNotCompare [0]func()

type MessageState struct {
	DoNotCompare
	data *int32
}

// Static check that MessageState does not exceed the size of a pointer.
const _ = unsafe.Sizeof(unsafe.Pointer(nil)) - unsafe.Sizeof((MessageState{}))

func main() {
	println("protoc minimal example")
}

[skabbes]

FYI @kyleconroy #2854 fixed this issue, it is merged into the dev branch. I think you can probably close this issue.

[deadprogram]

I have tagged to close on next release.