CODEWITHSUNDEEP
cassemblygccx86inline-assembly
Vicent Marti
Vicent Marti

Reputation: 7315

GCC Inline Assembly: Jump to label outside block

When using inline assembly under MSVC, one is allowed to jump outside of the assembly block by referencing a label in the C/C++ code, as explained in this MSDN article.

Can such thing be done when using inline assembly under GCC?

Here's an example of what I'm trying to accomplish:

__asm__ __volatile__ (
"   /* assembly code */ "
"   jz external_label;  "
);

/* some C code */

external_label:
/* C code coninues... */

The compiler, however, complains about "external_label" not being defined.

Upvotes: 13

Views: 16425

Answers (2)

Bastien Léonard
Bastien Léonard

Reputation: 61713

The code in this answer happens to work, but is undefined behaviour and will in general break things with optimization enabled. It's only safe to jmp out of an inline asm statement with asm goto, or under limited circumstances when the asm statement is followed by __builtin_unreachable();

(That's not usable here: it's never safe to jump into the middle of an inline asm statement and then fall out into compiler-generated code again inside a function.)

What if you define the label with the assembler?

asm("external_label:");

Update: this code seems to work:

#include <stdio.h>

int
main(void)
{
  asm("jmp label");
  puts("You should not see this.");
  asm("label:");

  return 0;
}

Upvotes: 11

chue x
chue x

Reputation: 18803

As of GCC 4.5, you can also use asm goto. The following example jumps to a C label:

#include <stdio.h>

int main(void) {
    asm goto (
        "jmp %l[done]"  // %l == lowercase L
        :
        :
        :
        : done          // specify c label(s) here
    );
    printf("Should not see this\n");

done:
    printf("Exiting\n");
    return 0;
}

Upvotes: 9

Related Questions