SIGILL on MAC and g++

ub3.c

int func(int a){
}

int func2(int a){
  return func(a);
}

int main(){
  func2(1);
}

$ gcc --version
gcc (Homebrew GCC 8.2.0) 8.2.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ g++ --version
g++ (Homebrew GCC 8.2.0) 8.2.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ gcc -O1 ub3.c; ./a.out; echo $?
0

$ g++ -O1 ub3.c; ./a.out; echo $?
ub3.c: In function 'int func(int)':
ub3.c:2:1: warning: no return statement in function returning non-void [-Wreturn-type]
 }
 ^
zsh: illegal hardware instruction  ./a.out
132