foreign export + unsafeCoerce# = non-termination

Summary

On aarch64 macOS I have a module which foreign exports a function which uses unsafeCoerce# within. Running this Haskell function from an external main (e.g. a C main) doesn't terminate, the function looks like it deadlocks and never returns. On x86_64 linux it does terminate and return.

Steps to reproduce

main.c:

#include <HsFFI.h>
#include "X_stub.h"

int main(void) {
    hs_init(NULL, NULL);
    bad();
    hs_exit();
    return 0;
}

X.hs:

{-# LANGUAGE MagicHash #-}
module X where
import GHC.Exts

n = 5
bad = return (Ptr (unsafeCoerce# n))

foreign export ccall bad :: IO (Ptr ())

Then

ghc -c X.hs
ghc -o main -no-hs-main main.c X.o
./main

Expected behavior

The program should terminate. I don't see why interpreting whatever as an Addr# could cause the program to deadlock. This behaviour on aarch64 macOS is also different from what @bgamari reported happened in Linux x86_64 where the program terminates.

Environment

• GHC version used: 9.8.2

Optional:

• Operating System: macOS
• System Architecture: M2/aarch64