This has been hinted to before: gcc gets rid of the loop.
bash-2.05b$ gcc -O3 arr.c -S -o arr.s
bash-2.05b$ cat arr.s
.file "arr.c"
.def ___main; .scl 2; .type 32; .endef
.text
.align 2
.align 16
.globl _main
.def _main; .scl 2; .type 32; .endef
_main:
pushl %ebp
movl %esp, %ebp
subl $72, %esp
xorl %eax, %eax
andl $-16, %esp
call __alloca
call ___main
xorl %eax, %eax
.align 16
L7:
incl %eax
cmpl $100000000, %eax
jle L7
leave
ret
the inside of the loop is exactly:
L7:
incl %eax
cmpl $100000000, %eax
jle L7
which doesn't read the array! essentially you are being unfare to ghc
by making it seq the array element, but not doing the same to GCC. even
with -O0, gcc gets rid of the loop body.
one solution: add in the assembly to actually read the array (i'm too
lazy and busy to do this).
- hal
--
Hal Daume III | [EMAIL PROTECTED]
"Arrest this man, he talks in maths." | www.isi.edu/~hdaume
> -----Original Message-----
> From: [EMAIL PROTECTED]
> [mailto:[EMAIL PROTECTED] On Behalf Of Lex Stein
> Sent: Thursday, June 26, 2003 3:46 PM
> To: Sven Panne
> Cc: [EMAIL PROTECTED]
> Subject: Re: haskell array access
>
>
>
> Great, thanks. Those suggestions narrow the gap from GHC -O being 330x
> slower than GCC -O3 to it being 20x slower. Here are the new results:
>
> gcc -O3 0.54s
> ocamlopt 1.11s
> ghc -O 10.76s
> ocamlc 14.10s
>
> GHC is still pretty slow for native x86 instruction code. Is
> there any way
> to further explain the performance gap ? (new code below)
>
> Thanks!
> Lex
>
> Haskell (GHC) source:
>
> import Array
> k :: Array Int Int
> k = Array.array (0,15) [(i, i+1) | i <- [0 .. 15] ]
> acc :: Int -> Int
> acc 0 = 0
> acc n = seq (k Array.! (n `mod` 16)) (acc (n-1))
> main = do
> print (acc 100000000)
>
> Caml test source:
>
> let a = Array.make 16 0;;
> let rec do1 i z =
> if (i>100000000) then z else
> do1 (i+1) (z + a.(i mod 16));;
> do1 0 0;;
>
> C (gcc) test source:
>
> int main () {
> int i, k = 0;
> int a [16];
> for (i=0; i<100000000; i++) {
> k += a [i % 16];
> }
> return (k);
> }
>
>
> On Fri, 27 Jun 2003, Sven Panne wrote:
>
> > Well, part of the answer is definitely that the Haskell
> program is the
> > *only* one which really uses the array elements. :-) I
> guess that the
> > compilers for the other languages simply remove the array
> access from
> > the generated code (gcc definitely does, only an empty loop
> remains).
> >
> > Another reason is that the type signatures are missing, so
> Integer is
> > used instead of Int, which is a bit unfair. Adding
> >
> > k :: Array Int Int
> > acc :: Int -> Int
> >
> > cuts down the time by more than factor 5 on my machine.
> >
> > Cheers,
> > S.
> >
> >
> > _______________________________________________
> > Haskell-Cafe mailing list
> > [EMAIL PROTECTED]
> > http://www.haskell.org/mailman/listinfo/haskell-cafe
> >
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