Re: Skybuck presents Faster Div 7

"Skybuck Flying" <BloodyShame@[EMAIL PROTECTED]
> ha scritto nel messaggio
news:22c72$48142edd$541983fa$11360@[EMAIL PROTECTED]
> Close but no cooky.
>
> // verification failure after: 3435973841 loops
> function AsmDiv7v2( Value : longword ) : longword;
> asm
> mov edx, Value
> mov eax, $92492493
> mul edx
> mov eax, edx
> shr eax, 2
> end;
>
> Verification failures at:
>
> 3435973841
> 3435973848
> 3435973855
> 3435973862
> 3435973869
> 3435973876
> etc.
>
> Bye,
>  Skybuck.

you can try to find "failures" for div7
in 'pseudocode'
div7: c=D[s+4]; a=0x24924925; D[s+4]=0;
..0: c<=1431655764#.1; c-=1431655764; D[s+4]+=204522252; #.0;
..1: mul c; a=r; a+=D[s+4]; ret

but it seem to me not the fast one :)

for the code in asm and C
---------------------------------------------------------
section DATA
global div7
section TEXT

div7:
      mov ecx, dword[esp+4]
      mov eax, 0x24924925
      mov dword[esp+4], 0
..0:
      cmp ecx, 1431655764
      jbe .1
      sub ecx, 1431655764
      add dword[esp+4], 204522252
      jmp short .0
..1:
      mul ecx
      mov eax, edx
      add eax, dword[esp+4]
ret

--------------------------------------------------------

#include <stdio.h>

unsigned div7(unsigned);

int main(void)
{unsigned a=1, b, bb, i=0;

  printf("0 for end\n\n");
l0:;
  b =div7(a);
  bb=a/7;
  if(bb!=b)
     {++i;
      printf("%u/7=%u but it should be %u\n", a, b, bb);
      fflush(stdout);
     }

  if(a%0x10000000==0) {printf(" %u ", a); fflush(stdout);}
  if(i>=15) goto end;
  ++a;
  if(a) goto l0;
  else printf("\n\na==0 all is ok\n");
end:;
  return 0;
}