Port of linux kernel jump labels to userspace. Uses GCC goto asm extension and some other gcc/gas specific extensions (always inline, .pushsection, section attributes).

Jump labels described in the kernel's jump_label.h:

Jump labels provide an interface to generate dynamic branches using self-modifying code. Assuming toolchain and architecture support the result of a "if (static_key_false(&key))" statement is a unconditional branch (which defaults to false - and the true block is placed out of line).

Original authors Jason Baron and Peter Zijlstra.

See these two LWN articles for an introduction:

http://lwn.net/Articles/412072/

http://lwn.net/Articles/436041/

Another good one on kernel elf special sections:

http://lwn.net/Articles/531148/

And the kernel documentation:

https://www.kernel.org/doc/Documentation/static-keys.txt

Installing:

git clone https://github.com/computee/s-keys.git
cd s-keys
make

To use static keys, just include the header and initialize like this:

#include <s-keys.h>

// must declare globally:
struct static_key key = STATIC_KEY_INIT_TRUE;

int main (int argc, char *argv[])
{
    jump_label_init();

    if (static_key_true(&key))
        printf("A\n");
    else
        printf("B\n");

}

pull requests / patches welcome

• Ideal nops: possibly re-implement this system
• Cross compilation: need to test
• RPM/deb skeleton files
• header-only arrangement?
• unit tests -like make targets and programs

Patrick McCormick patm()pdx.edu