[Haskell-cafe] gcc as a portable assembler: tail calls vs trampoline

[Loup Vaillant]

Hello, everyone,
I'm not sure this is the good list for this posting: I plan to write
yet another lazy graph reduction machine, using gcc, mainly for fun.
My concern right now is about one particular aspect of gcc: the tail
calls vs trampoline deathmatch.

First some clarifications:
-> By tail call, I mean a properly optimized one, which doesn't grow
the stack. Basically a jump.
-> By trampoline, I mean the infinite loop referred to as a "tiny
interpreter" in the STG paper [1]. Instead of (tail) calling a
function, we return its pointer, which will then be used for the
actual call.

I've read that trampoline calls are about 3 times as expensive as tail
calls. Instead of a (direct) jump, you have a return, followed by an
indirect call. However, testing this with gcc, I didn't manage to show
any measurable difference. What am I missing? I used -O2 on both
benchmarks, and turned off inlining (using the __noinline__
attribute). I'm running the program on a core2Duo in 32 bits mode,
using GNU/Linux Ubuntu. Should I also make separate modules to prevent
whole program analysis from gcc?

Anyway, I send you the code below, so you can test and review it. Enjoy!

[1]: http://research.microsoft.com/Users/simonpj/Papers/spineless-tagless-gmachine.ps.gz



/* This program tests the performance difference between trampoline
 * calls and tail calls.  (In the STG and related papers, trampoline
 * is referred to as "tiny interpreter")
 *
 * depending on the mode (tail call or trampoline), some code changes:
 * the return type of the code blocks, and the calls to other blocks
 * (direct tail calls or return to trampoline). Hence the macros ENTER
 * and RETURN.
 *
 * To compile it,type one of the two following commands:
 * gcc -O2 sibcall.c -D SIBCALL
 * gcc -O2 sibcall.c
 *
 * The first one will make the program use sibling calls (enabled at
 * O2), and the second one will fall back to trampoline calls
 */

#ifdef SIBCALL // tail call mode

#define ENTER(f) (f())
#define RETURN void
#define MAIN(start) start()

#else // trampoline call mode

#define ENTER(f) return f
#define RETURN void*
#define MAIN(start)  do{ void * (*f)(void) = start; \
                         while (1) \
                           f = (*f)(); \
                       } while (0)
#endif

// Now, we can begin the generic code
#include <stdlib.h>
#define LIMIT 1000000000 // one billion

int counter = 0;

RETURN closure0(void);
RETURN closure1(void);


int main(int argc, char ** argv)
{
  MAIN(closure0);
  return 0;
}

__attribute__ ((__noinline__))
RETURN closure0(void)
{
  if (counter >= LIMIT) exit(0);
  counter++;
  ENTER(closure1);
}
__attribute__ ((__noinline__))
RETURN closure1(void)
{
  if (counter >= LIMIT) exit(0);
  counter++;
  ENTER(closure0);
}