Author: Gerd Isenberg
Date: 08:35:42 01/06/04
Go up one level in this thread
On January 06, 2004 at 10:56:00, martin fierz wrote:
>On January 06, 2004 at 09:40:33, Robert Hyatt wrote:
>
>>Maybe there is some confusion here. There is _no_ cost for "allocating"
>>local variables in a function. And I _do_ mean _NO_. These values are
>>simply "on the stack" and when the function is entered, the stack pointer has
>>some constant subtracted from it to leave a "hole" that is used to hold local
>>function variables. no malloc() or anything like that is done by modern
>>compilers, hence zero cost.
>
>hi bob,
>
>is this really true? i used to have a HUGE structure describing a move in the
>first version of my chess program (don't ask, please...). in my negamax function
>i had this
>
>int negamax(int alpha, int beta, int depth)
> {
> MOVE movelist[MAXMOVES];
> ...other variables...
>
> ...typical negamax code...
>
> }
>
>i saw clear speed differences depending on the value i used for MAXMOVES - and
>in my tests, i made sure that using 100 would never overflow in the positions i
>was searching, then i set it to e.g. 200 and the program would get a few %
>slower, although these additional 100 array elements were never used.
>
>how can this be explained? i'm not memsetting the array to 0 or anything.
Hi Martin,
that was my point in my reply to Bob - instruction length.
Did you expect the assembly, specially the bp-relative offsets of the other
locals load/stores? In C, per default all locals of a function on the stack,
including actual parameters are addressed relative to the base pointer register.
As long those offsets are in a signed byte range (-128..+127), with X86-32 only
one byte (to sign extend) is necessary, otherwise the complete 32-bit offset is
needed.
Gerd
>
>like uri, i thought the program was allocating memory every time it entered the
>function, and that that was taking time. i ended up making my MOVE variable much
>smaller - although it's still much larger than crafty's :-)
>
>i thought perhaps i should allocate the movelist beforehand as a global, like
>this:
>
>MOVE *movelist[MAX_SEARCH_DEPTH]
>for(i=0;i<MAX_SEARCH_DEPTH;i++)
> movelist[i] = malloc(sizeof(MOVE)*MAXMOVES)
>
>and then in the negamax function use the appropriate movelist for that search
>depth. that turned out to be significantly slower than the local variable
>approach. again, i don't understand why that would be... - anyone?
>
>cheers
> martin
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