Is it possible to store the address of a label in a variable and use goto to jump to it?

Question

I know everyone hates gotos. In my code, for reasons I have considered and am comfortable with, they provide an effective solution (ie I'm not looking for "don't do that" as an answer, I understand your reservations, and understand why I am using them anyway).

So far they have been fantastic, but I want to expand the functionality in such a way that requires me to essentially be able to store pointers to the labels, then go to them later.

If this code worked, it would represent the type of functionality that I need. But it doesn't work, and 30 min of googling hasn't revealed anything. Does anyone have any ideas?

int main (void)
{
  int i=1;
  void* the_label_pointer;

  the_label:

  the_label_pointer = &the_label;

  if( i-- )
    goto *the_label_pointer;

  return 0;
}

Answer 1

The C and C++ standards do not support this feature.

However, the GNU Compiler Collection (GCC) includes a non-standard extension for doing this, as described in the Labels as Values section of the Using the GNU Compiler Collection manual.

Essentially, they have added a special unary operator && that reports the address of the label as type void *. See the article for details. With that extension, you could just use && instead of & in your example, and it would work on GCC.

P.S. I know you don’t want me to say it, but I’ll say it anyway… DON’T DO THAT!!!

Answer 2

I know the feeling then everybody says it shouldn't be done; it just has to be done. In GNU C use &&the_label; to take the address of a label. (https://gcc.gnu.org/onlinedocs/gcc/Labels-as-Values.html) The syntax you guessed, goto *ptr on a void*, is actually what GNU C uses.

Or if you want to use inline assembly for some reason, here's how to do it with GNU C asm goto

// unsafe: this needs to use  asm goto so the compiler knows
// execution might not come out the other side
#define unsafe_jumpto(a) asm("jmp *%0"::"r"(a):)

// target pointer, possible targets
#define jumpto(a, ...) asm goto("jmp *%0" : : "r"(a) : : __VA_ARGS__)

int main (void)
{
  int i=1;
  void* the_label_pointer;

  the_label:
  the_label_pointer = &&the_label;

label2:

  if( i-- )
    jumpto(the_label_pointer, the_label, label2, label3);

label3:
  return 0;
}

The list of labels must include every possible value for the_label_pointer.

The macro expansion will be something like

asm goto("jmp *%0" : : "ri"(the_label_pointer) : : the_label, label2, label3);

This compiles with gcc 4.5 and later, and with the latest clang which just got asm goto support some time after clang 8.0. https://godbolt.org/z/BzhckE. The resulting asm looks like this for GCC9.1, which optimized away the "loop" of i=i / i-- and just put the the_label after the jumpto. So it still runs exactly once, like in the C source.

# gcc9.1 -O3 -fpie
main:
    leaq    .L2(%rip), %rax     # ptr = &&label
    jmp *%rax                     # from inline asm
.L2:
    xorl    %eax, %eax          # return 0
    ret

But clang didn't do that optimization and still has the loop:

# clang -O3 -fpie
main:
    movl    $1, %eax
    leaq    .Ltmp1(%rip), %rcx
.Ltmp1:                                 # Block address taken
    subl    $1, %eax
    jb      .LBB0_4                  # jump over the JMP if i was < 1 (unsigned) before SUB.  i.e. skip the backwards jump if i wrapped
    jmpq    *%rcx                   # from inline asm
.LBB0_4:
    xorl    %eax, %eax              # return 0
    retq

---

The label address operator && will only work with gcc. And obviously the jumpto assembly macro needs to be implemented specifically for each processor (this one works with both 32 and 64 bit x86).

Also keep in mind that (without asm goto) there would be no guarantee that the state of the stack is the same at two different points in the same function. And at least with some optimization turned on it's possible that the compiler assumes some registers to contain some value at the point after the label. These kind of things can easily get screwed up then doing crazy shit the compiler doesn't expect. Be sure to proof read the compiled code.

These are why asm goto is necessary to make it safe by letting the compiler know where you will / might jump, getting consistent code-gen for the jump and the destination.