On 2011-01-21, at 7:40 PM, Marc Feeley wrote:
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
I have done some more experimenting with fib_scm. I analyzed the machine code generated by gcc for the jump to fib's return address. It turns out that gcc is failing to do a simple constant folding operation (combining two offsets into one for an indirect memory access). I've now committed a patch to do the constant folding explicitly (at the C level).
The new code decreases the run time by about 10% at the default optimization level (gcc -O1). If the optimization level is increased to -O2 with
gsc -exe -cc-options "-O2" fib_scm.scm
then the run time drops by 24%.
Here are the run times in seconds on my MacBook Pro:
x86-32 (i.e. "gcc -m32") old : 1.859 new : 1.654 new -O2 : 1.437
x86-64 old : 1.914 new : 1.728 new -O2 : 1.449
With gcc -O2 fib_scm is still not as fast as fib_c compiled with gcc -O3, but the difference is smaller (fib_scm takes 50% longer to execute than fib_c, rather than 100% longer). On the other hand, with gcc -O2 fib_scm is about twice as fast as fib_c compiled with gcc -O2. Note that for all these measurements the declaration (not interrupts-enabled) was used.
Marc