You must be using my old example, with gcc -O3 my new example does access
the array and is 20x faster than the ghc -O code. Scroll down for
"-->" that I inserted beside the instruction to see the array access.
.file "arrayloop.c"
.version "01.01"
gcc2_compiled.:
.text
.p2align 2,0x90
.globl main
.type main,@function
main:
pushl %ebp
movl %esp,%ebp
subl $80,%esp
pushl %esi
pushl %ebx
xorl %ebx,%ebx
xorl %ecx,%ecx
leal -64(%ebp),%esi
.p2align 2,0x90
.L6:
movl %ecx,%edx
testl %ecx,%ecx
jge .L7
leal 15(%ecx),%edx
.L7:
andl $-16,%edx
movl %ecx,%eax
subl %edx,%eax
--> addl (%esi,%eax,4),%ebx
incl %ecx
cmpl $99999999,%ecx
jle .L6
movl %ebx,%eax
popl %ebx
popl %esi
leave
ret
.Lfe1:
.size main,.Lfe1-main
.ident "GCC: (GNU) c 2.95.4 20020320 [FreeBSD]"
On Thu, 26 Jun 2003, Hal Daume wrote:
> This has been hinted to before: gcc gets rid of the loop.
>
> bash-2.05b$ 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
> > >
> > _______________________________________________
> > Haskell-Cafe mailing list
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> >
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