[gambit-list] compiling fib
Marc Feeley
feeley at iro.umontreal.ca
Mon Jan 24 10:16:06 EST 2011
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
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