[Caml-list] caml perfomance

[Anton Moscal <msk@post.tepkom.ru>]

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Hi!

I & my friend has made some investgations about O'Caml flotaing-point code quality. We 
discovered some strange code examples(all assembler code is for IA32 architecture):

--------------------------------------------------------------------------
1. "let _ = f a in" and "ignore (f a);" generates different code.

for example:

let _ =	ignore (sin 1.0); ()

produces the following:

1	Float2_entry:
2	        subl    $8, %esp
3	.L100:
4	        fld1
5	        subl    $8, %esp
6	        fstpl   0(%esp)
7 	        call    sin
8	        addl    $8, %esp
9	        fstpl   0(%esp)
;-----------------------! floating value boxed before discarding
10	       call    caml_alloc2
12   .L101:
13	       leal    4(%eax), %eax
14	       movl    $2301, -4(%eax)
15	       fldl    0(%esp)
16	       fstpl   (%eax)
;-----------------------------------------------------------------
17	       movl    $1, %eax
18	       addl    $8, %esp
19	       ret



let _ = let _ = sin 1.0 in ()

produces the following (much more better) code:

1	Float1_entry:
2	        subl    $8, %esp
3	.L100:
4	        fld1
5	        subl    $8, %esp
6	        fstpl   0(%esp)
7	        call    sin
8	        addl    $8, %esp
9	        fstpl   0(%esp)
;--- instead discarding value from FP stack O'Caml allocates
; space for result on the main stack, store this value to 
; stack and after all discards it.
10	       movl    $1, %eax
11	       addl    $8, %esp
12	       ret


-- 2. passing FP parameter to stack is less efficient than possible:

Ocaml currently generates this code for pushing float arguments.

fldl <address>
subl $8, %esp
fstpl 0(%esp)

While gcc generates two pushes

pushl <const>
pushl <const>

or

movl %address,%reg1
movl %address+4,%reg2
pushl %reg2
pushl %reg1

3. (less important) Ocaml never generate assembler trigonometric functions: fsin, 
fcos...

Yes they violates ANSI rules, but their usage can improve perfomance of an
application. Write simple C program with sin or cos usage and compile it
with and without "-ffast-math" switch. The difference is about 25%.

example.c:
#include <math.h>

int main()
{
    int i;
    volatile double d=1.0;
    for(i=0;i<10000000;i+=1) d=cos(sin(d));
    return 0;
}

Regards, 
Anton Moscal
Nickolay Kolchin

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