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Subject: Re: or even better...

Author: Gerd Isenberg

Date: 09:41:04 12/13/05

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On December 13, 2005 at 10:36:19, Chrilly Donninger wrote:

>On December 12, 2005 at 15:31:07, Gerd Isenberg wrote:
>
>>of course also saving the bestscore assignment in that case:
>>
>>if (val > bestscore)
>>{
>>  if( val >= beta )
>>    return val; // Actually goto return-sequence.
>>  bestscore = val;
>>  if(val > alpha)
>>    alpha = val;
>>}
>>
>>or what about improving branch-prediction in some way:
>>
>>if( val >= beta )
>>   return val; // Actually goto return-sequence.
>>if (val > bestscore)
>>{ // most likely not taken
>>  bestscore = val;
>>  if(val > alpha)
>>    alpha = val;
>>}
>
>
>The optimizing compilers are shuffling the statements around. Sometimes they
>implement some very clever tricks to avoid a jumpt at all. I have not seen this
>from gcc output, put Visual-C and especially the Intel compiler do amazing
>tricks to avoid jumps.
>As a C-programmer one has no real control. I think the only way is to run the
>Code through VTune and look on the reprots.
>As a rule of thumb, the condition is usually reversed by the compiler.
>
>Chrilly

Yes, there are a lot of techniques to avoid branches.

Otoh branch prediction heuristics of todays processors are really sophisticated,
and most often - if no "really" random pattern occur -
and there are not too many branches that the btb got trashed, conditional
branches are still fine and produce most often shortes/fastest code.

We had some discussions here recently about that - a brief summary:

First to mention is cmovxx.
Then there are all kind of "boolean" multiplications by anding a value with a
{0,-1}-mask. One trick, often seen in msvc is using setCC with a formerly
cleared 32-bit register:

cmp/test... ; set some flags
setCC   ; 0,1
neg     ; 0,-1

cmp/test...; set some flags
setCC   ; 0,1
dec     ; -1,0

With carry flag one has sbb reg32,reg32 to build the mask.

With sign flag (eg. after a subtraction), one may use sar 31 or cdq to build a
{0,-1}-mask instead of setcc, neg/dec.

With some care and knowledge about reduced value-ranges, where the compiler is
ususally not aware of, the setCC-Trick and specially the sign-trick may also be
applied at C-level to force branchless code:

1) if ( booleanCondition ) x += y;
-> x += y & -( booleanCondition );
-> x += y &  (!booleanCondition - 1);

2) if (depth < 0) fm = fm1; else fm = fm2;
-> fm = fm2 +((depth>>31) & (fm1-fm2));

3) fm = depth < -1 ? fm1 : depth < 0 ? fm2 : fm3;
-> fm = fm3 + ( (depth   >>31) & (fm2-fm3))
            + (((depth+1)>>31) & (fm1-fm2));

// signed max/min/abs:
max(x,y) := x - ((x-y) & ((x-y)>>31));
min(x,y) := y + ((x-y) & ((x-y)>>31));
abs(x)   := ( x ^ (x>>31) ) - (x>>31); // patented by sun!
abs(x)   := ( x + (x>>31) ) ^ (x>>31);

// abs with sign extension via cdq:
int abs(int x)
{
  int m = (int)( (unsigned __int64)((__int64)x) >> 32 );
  return (x + m) ^ m;
}
// msvc 6.0
?abs@@YIHH@Z PROC NEAR					; abs, COMDAT
  00000	8b c1		 mov	 eax, ecx
  00002	99		 cdq
  00003	8d 04 0a	 lea	 eax, [edx+ecx]
  00006	33 c2		 xor	 eax, edx
  00008	c3		 ret	 0
?abs@@YIHH@Z ENDP					; abs

For another branchless domain, a SIMD-wise bool[64]*byte[64] dot-prduct, you may
have a look the ccc-shadow archives:
http://hornid.com/cgi-bin/ccc/topic_show.pl?pid=399116;hl=#pid399116

For the val > bestScore,alpha and >= beta case, i prefere not to fool the
compiler. If val >= beta is not true, val > alpha becomes never true with
nullwindows. Therefor i don't think a branchless alpha = max(val,alpha) makes
sense here:

if (val > bestscore)
{
  if( val >= beta )
    return val; // Actually goto return-sequence.
  bestscore = val;
  if(val > alpha)
    alpha = val; // not taken if beta == alpha + 1
}

Gerd



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