[gambit-list] compiling fib
Marc Feeley
feeley at iro.umontreal.ca
Fri Jan 21 19:40:32 EST 2011
On 2011-01-21, at 4:34 PM, Bradley Lucier wrote:
> I don't know what the various optimization levels of chicken mean precisely, or which options in -O5 makes the biggest difference over -O4:
> -optimize-level 0 is equivalent to -no-usual-integrations -no-compiler-syntax
> -optimize-level 1 is equivalent to -optimize-leaf-routine
> -optimize-level 2 is equivalent to -optimize-leaf-routines -inline
> -optimize-level 3 is equivalent to -optimize-leaf-routines -local -inline -inline-global
> -optimize-level 4 is equivalent to -optimize-leaf-routines -local -inline -unsafe -unboxing
> -optimize-level 5 is equivalent to -optimize-leaf-routines -block -inline -unsafe -unboxing -lambda-lift -disable-interrupts -no-trace -no
> -lambda-info
>
> Marc, you know that with me and Gambit it's like the old Velvet Underground song: "I'm sticking with you, cause I'm made out of glue ..."
Perhaps this will be a more rational argument for sticking with Gambit...
Upon further investigation, compiling fib_scm with Chicken with -O5 gives this C code for the fib function:
/* fib in k32 in k29 */
static C_word C_fcall f_36(C_word t1){
C_word tmp;
C_word t2;
C_word t3;
C_word t4;
C_word t5;
C_word t6;
C_word t7;
if(C_truep(C_fixnum_less_or_equal_p(t1,C_fix(1)))){
t2=t1;
return(t2);}
else{
t2=C_u_fixnum_difference(t1,C_fix(1));
t3=f_36(t2);
t4=C_u_fixnum_difference(t1,C_fix(2));
t5=f_36(t4);
return(C_u_fixnum_plus(t3,t5));}}
This is basically the same code as fib_c once the C macros are expanded. Note that the recursive calls to fib are translated to direct C calls. Not only are interrupts no longer checked (due to the -disable-interrupts implied by -O5) but there is no stack overflow check! So the code no longer supports preemptive multithreading, and if you are close to the C stack limit the program will crash. The programmer has to be concerned with avoiding deep recursions which could crash the program. In other words, you're no longer dealing with a high-level language with graceful support for recursion.
Here's an example where that matters:
% cat deep.scm
(declare
(standard-bindings)
(extended-bindings)
(block)
(not safe)
)
;; Recursive algorithm for computing (even? n). The depth of
;; recursion is equal to n.
(define (even n)
(if (fx= n 0)
#t
(not (even (fx- n 1)))))
(display (even 10000000))
% csc -O5 deep.scm
% ./deep
Segmentation fault
Gambit has not problem with deep recursions. It can fill the whole heap with continuation frames if needed:
% gsc -exe deep.scm
% ./deep
#t%
If the Gambit heap overflows, you'll get an exception that your code can catch and act upon gracefully, not a segmentation fault.
So I'm not sure comparing Gambit against Chicken with -O5 is very meaningful. Perhaps -O4 is more in line with the expectations of a Scheme programmer, but I don't know enough about the meaning of Chicken's optimization levels to really tell what is reasonable.
Marc
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