[Bug target/107843] error: incompatible type for argument in ___bpf_ctx_cast2

[david.faust at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107843

--- Comment #1 from David Faust <david.faust at oracle dot com> ---
Looks like this is an issue with passing void* where an enum type is
expected in a function call. This is not specific to the BPF backend.

Not entirely clear to me whether this is expected or a bug, but it
does differ from llvm behavior.

Reproducer below, tried with a few gccs, same behavior:
today (6 Dec 2022)'s master 81476bc4f4a20bcf3af7ac2548c2322d48499402
gcc-12 (Debian 12.2.0-9) 12.2.0
gcc-10 (Debian 10.4.0-5) 10.4.0
gcc-8 (Debian 8.4.0-7) 8.4.0

$ cat enumcast.c
enum E {
  E_FOO = 0,
  E_BAR = 1,
};

int foo_enum (enum E e);

int bar_enum (enum E e) {
  return foo_enum ((void *) e);
}

int foo_int (int x);

int bar_int (int x) {
  return foo_int ((void *) x);
}


$ gcc -c enumcast.c -o enumcast.o
enumcast.c: In function ‘bar_enum’:
enumcast.c:10:20: error: incompatible type for argument 1 of ‘foo_enum’
   10 |   return foo_enum ((void *) e);
      |                    ^~~~~~~~~~
      |                    |
      |                    void *
enumcast.c:7:22: note: expected ‘enum E’ but argument is of type ‘void *’
    7 | int foo_enum (enum E e);
      |               ~~~~~~~^
enumcast.c: In function ‘bar_int’:
enumcast.c:16:19: warning: cast to pointer from integer of different size
[-Wint-to-pointer-cast]
   16 |   return foo_int ((void *) x);
      |                   ^
enumcast.c:16:19: warning: passing argument 1 of ‘foo_int’ makes integer from
pointer without a cast [-Wint-conversion]
   16 |   return foo_int ((void *) x);
      |                   ^~~~~~~~~~
      |                   |
      |                   void *
enumcast.c:13:18: note: expected ‘int’ but argument is of type ‘void *’
   13 | int foo_int (int x);
      |              ~~~~^


$ clang -c enumcast.c -o enumcast.o
enumcast.c:10:20: warning: incompatible pointer to integer conversion passing
'void *' to parameter of type 'enum E' [-Wint-conversion]
  return foo_enum ((void *) e);
                   ^~~~~~~~~~
enumcast.c:7:22: note: passing argument to parameter 'e' here
int foo_enum (enum E e);
                     ^
enumcast.c:16:19: warning: cast to 'void *' from smaller integer type 'int'
[-Wint-to-void-pointer-cast]
  return foo_int ((void *) x);
                  ^~~~~~~~~~
enumcast.c:16:19: warning: incompatible pointer to integer conversion passing
'void *' to parameter of type 'int' [-Wint-conversion]
  return foo_int ((void *) x);
                  ^~~~~~~~~~
enumcast.c:13:18: note: passing argument to parameter 'x' here
int foo_int (int x);
                 ^
3 warnings generated.