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question about symmertic evaluation
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Gerd Isenberg



Joined: 08 Mar 2006
Posts: 1785
Location: Hattingen, Germany
PostPost subject: Re: question about symmertic evaluation    Posted: Thu May 24, 2007 8:55 pm Reply to topic Reply with quote
bob wrote:

I actually don't know how (nor do I care) the compiler handles a/b for ints. All I care about is symmetrical answers, which >> won't give directly. Whatever it does the cost inside Crafty is nil. And I am not dividing by a power of 2 either. I have this in a couple of key places:

((s) * (62 - Min(TotalWhitePieces + TotalBlackPieces, 42)) / 86)

which is not exactly a power of 2...


You would care if really an idiv instruction would be generated Wink

But i agree that using div-operator "/" as C-instruction to divide by constant is fine and good for symmetrical scaling - since compiler are smart enough.

Here the output how to divide by 86 and the source from AMD64 optimization manual:

Code:
Signed division by constant
===========================

enter divisor: 86

; dividend: memory location or register other than EAX or EDX

MOV EAX, 02FA0BE83h
IMUL dividend
MOV EAX, dividend
SAR EDX, 4
SHR EAX, 31
ADD EDX, EAX

; quotient now in EDX


Code:

/* This program determines the algorithm (a), multiplier (m), and
shift factor (s) to be used to accomplish *signed* division by
a constant divisor. Compile with MSVC.
*/
#include <stdio.h>

typedef unsigned __int64 U64;
typedef unsigned long U32;


U32 log2(U32 i)
{
   U32 t = 0;
   i = i >> 1;
   while (i) {
      i = i >> 1;
      t++;
   }
   return(t);
}


int main(void)
{
   long e;
   U32 d, l, s, m, a;
   U64 m_low, m_high, j, k;
   fprintf(stderr, "\n");
   fprintf(stderr, "Signed division by constant\n");
   fprintf(stderr, "===========================\n\n");
   fprintf(stderr, "enter divisor: ");
   scanf("%ld", &d);
   fprintf(stderr, "\n");
   e = d;
   if ( e < 0 ) d = -d;
   if (d == 0) goto printed_code;
   if (e == (-1)) {
      printf("; dividend: register or memory location\n");
      printf("\n");
      printf("NEG dividend\n");
      printf("\n");
      printf("; quotient replaced dividend\n");
      goto printed_code;
   }
   if (d == 2) {
      printf("; dividend expected in EAX\n");
      printf("\n");
      printf("CMP EAX, 080000000h\n");
      printf("SBB EAX, -1\n");
      printf("SAR EAX, 1\n");
      if (e < 0) printf("NEG EAX\n");
      printf("\n");
      printf("; quotient now in EAX\n");
      goto printed_code;
   }
   if (!(d & (d - 1))) {
      printf("; dividend expected in EAX\n");
      printf("\n");
      printf("CDQ\n");
      printf("AND EDX, 0%08lXh\n", (d-1));
      printf("ADD EAX, EDX\n");
      if (log2(d)) printf("SAR EAX, %d\n", log2(d));
      if (e < 0) printf("NEG EAX\n");
      printf("\n");
      printf("; quotient now in EAX\n");
      goto printed_code;
   }

   /* Determine algorithm (a), multiplier (m), and shift factor (s) for 32-bit
   signed integer division. Based on: Granlund, T.; Montgomery, P.L.:
   "Division by Invariant Integers using Multiplication". SIGPLAN Notices,
   Vol. 29, June 1994, page 61.
   */

   l = log2(d);
   j = (((U64)(0x80000000)) % ((U64)(d)));
   k = (((U64)(1)) << (32 + l)) / ((U64)(0x80000000 - j));
   m_low = (((U64)(1)) << (32 + l)) / d;
   m_high = ((((U64)(1)) << (32 + l)) + k) / d;
   while (((m_low >> 1) < (m_high >> 1)) && (l > 0)) {
      m_low = m_low >> 1;
      m_high = m_high >> 1;
      l = l - 1;
   }
   m = ((U32)(m_high));
   s = l;
   a = (m_high >> 31) ? 1 : 0;

   if (a) {
      printf("; dividend: memory location or register other than EAX or EDX\n");
      printf("\n");
      printf("MOV EAX, 0%08LXh\n", m);
      printf("IMUL dividend\n");
      printf("MOV EAX, dividend\n");
      printf("ADD EDX, EAX\n");
      if (s) printf("SAR EDX, %d\n", s);
      printf("SHR EAX, 31\n");
      printf("ADD EDX, EAX\n");
      if (e < 0) printf("NEG EDX\n");
      printf("\n");
      printf("; quotient now in EDX\n");
   }

   else {
      printf("; dividend: memory location or register other than EAX or EDX\n");
      printf("\n");
      printf("MOV EAX, 0%08LXh\n", m);
      printf("IMUL dividend\n");
      printf("MOV EAX, dividend\n");
      if (s) printf("SAR EDX, %d\n", s);
      printf("SHR EAX, 31\n");
      printf("ADD EDX, EAX\n");
      if (e < 0) printf("NEG EDX\n");
      printf("\n");
      printf("; quotient now in EDX\n");
   }
   printed_code:
   fprintf(stderr, "\n");
   return 0;
}
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Display posts from previous:   
Subject Author Date/Time
question about symmertic evaluation Uri Blass Wed May 23, 2007 7:53 pm
      Re: question about symmertic evaluation Uri Blass Wed May 23, 2007 8:45 pm
      Re: question about symmertic evaluation Gerd Isenberg Wed May 23, 2007 8:46 pm
            Re: question about symmertic evaluation Uri Blass Wed May 23, 2007 9:04 pm
            Re: question about symmertic evaluation Robert Hyatt Wed May 23, 2007 9:51 pm
                  Re: question about symmertic evaluation Uri Blass Wed May 23, 2007 10:04 pm
                  Re: question about symmertic evaluation Gerd Isenberg Wed May 23, 2007 10:09 pm
                        Re: question about symmertic evaluation Robert Hyatt Thu May 24, 2007 12:01 am
                              Re: question about symmertic evaluation Gerd Isenberg Thu May 24, 2007 7:37 am
                                    Re: question about symmertic evaluation H.G.Muller Thu May 24, 2007 8:06 am
                                          Re: question about symmertic evaluation Gerd Isenberg Thu May 24, 2007 9:24 am
                                                Re: question about symmertic evaluation H.G.Muller Thu May 24, 2007 10:39 am
                                                Re: question about symmertic evaluation Gerd Isenberg Thu May 24, 2007 11:36 am
                                                      Re: question about symmertic evaluation H.G.Muller Thu May 24, 2007 11:55 am
                                                Re: question about symmertic evaluation Gerd Isenberg Thu May 24, 2007 7:46 pm
                                          Re: question about symmertic evaluation Robert Hyatt Thu May 24, 2007 8:18 pm
                                                Re: question about symmertic evaluation Gerd Isenberg Thu May 24, 2007 8:55 pm
                                                      Re: question about symmertic evaluation Uri Blass Fri May 25, 2007 10:44 am
                                                            Re: question about symmertic evaluation Gerd Isenberg Fri May 25, 2007 6:28 pm
                                                                  Re: question about symmertic evaluation Uri Blass Fri May 25, 2007 7:42 pm
                                                                        Re: question about symmertic evaluation Gerd Isenberg Fri May 25, 2007 8:00 pm
                                    Re: question about symmertic evaluation Robert Hyatt Thu May 24, 2007 8:13 pm
                                          Re: question about symmertic evaluation Gerd Isenberg Thu May 24, 2007 8:33 pm
                                                Re: question about symmertic evaluation Robert Hyatt Sat May 26, 2007 1:05 am
                                                      Re: question about symmertic evaluation Gerd Isenberg Sat May 26, 2007 7:18 am
                                                            Re: question about symmertic evaluation Robert Hyatt Sat May 26, 2007 5:08 pm
                                                                  Re: question about symmertic evaluation Robert Hyatt Sat May 26, 2007 5:25 pm
                                                                  Re: question about symmertic evaluation Gerd Isenberg Sat May 26, 2007 6:23 pm
                                                                        Re: question about symmertic evaluation Robert Hyatt Sun May 27, 2007 2:48 pm
                  Re: question about symmertic evaluation Steven Edwards Thu May 24, 2007 3:42 am
                        Re: question about symmertic evaluation Robert Hyatt Thu May 24, 2007 8:20 pm
      Re: question about symmertic evaluation Robert Hyatt Wed May 23, 2007 9:50 pm
      Re: question about symmertic evaluation William H. Rogers Fri May 25, 2007 7:48 pm
            Re: question about symmertic evaluation Uri Blass Sat May 26, 2007 11:38 am
                  Re: question about symmertic evaluation H.G.Muller Sat May 26, 2007 2:07 pm
      Re: question about symmertic evaluation Vincent Diepeveen Sat May 26, 2007 7:27 pm
            Re: question about symmertic evaluation Gerd Isenberg Sat May 26, 2007 8:20 pm
            Re: question about symmertic evaluation Uri Blass Sat May 26, 2007 9:29 pm
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