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Optimizing Dividing by 2,4,16 etc ?
Posted: Sat Feb 23, 2008 6:18 pm
by Franky
Hi, I just played a little bit around with PB and found out, that PB optimizes
to
But it doesn´t do this with division:
Is translated to
Code: Select all
MOV ebx,dword [v_a]
MOV eax,ebx
MOV ebx,16
CDQ
IDIV ebx
MOV ebx, eax
MOV dword [v_a],ebx
So,
1.) why first move the value of a t ebx and then move it from ebx to eax, why not directly to eax?
2.)Why not just use SAR instead of IDIV?
Posted: Sat Feb 23, 2008 6:23 pm
by Trond
Code: Select all
a = (-4) / 2
Debug a
a = (-4) >> 2
Debug a
Posted: Sat Feb 23, 2008 6:26 pm
by Franky
a = (-4) / 2
Debug a
a = (-4) >> 1
Debug a
Posted: Sat Feb 23, 2008 6:26 pm
by Dare
Trond wrote:Code: Select all
a = (-4) / 2
Debug a
a = (-4) >> 2
Debug a
Code: Select all
a = (-4) / 2
Debug a
a = (-4) >> 1
Debug a
Posted: Sat Feb 23, 2008 6:42 pm
by Trond
Posted: Sat Feb 23, 2008 8:02 pm
by Dare
Posted: Mon Mar 03, 2008 2:19 pm
by Franky
Noone makes me give up
So, this is the new version, rounding the same way, as PureBasic does:
My Test brought an optimization from 25Seconds to 11 Seconds:
Code: Select all
#Compare=0
Global a,b,c.l,*adr1.long,*adr2.long,adr1.l,adr2.l
adr1=AllocateMemory(80000)
adr2=AllocateMemory(80000)
;Rechnung: c=a/8
Zeit=GetTickCount_()
For b=1 To 99999
CompilerIf #compare
*adr1=adr1
CompilerEndIf
For a=-9999 To 9999
! MOV eax,dword [v_a]
; ! MOV ebx,dword [v_a]
;! MOV eax,ebx
! MOV ebx,8
! CDQ
! IDIV ebx
;! MOV ebx, eax
;! MOV dword [v_c],ebx
! MOV dword [v_c],eax
CompilerIf #Compare
*adr1\l=c
*adr1+4
CompilerEndIf
Next
Next
zeit2=GetTickCount_()
For b=1 To 99999
CompilerIf #compare
*adr2=adr2
CompilerEndIf
; CallDebugger
For a=-9999 To 9999
! MOV ebx,dword [v_a]
! SAR ebx,3
! MOV dword [v_c],ebx
! CMP ebx,0
! JNL l___div_end
! SAL ebx,3
! CMP dword[v_a],ebx
! JE l___div_end
! INC dword[v_c]
__div_end:
CompilerIf #compare
*adr2\l=c
*adr2+4
CompilerEndIf
Next
Next
zeit3=GetTickCount_()
CompilerIf #compare
*adr1=adr1
*adr2=adr2
dif=0
For a=0 To 19998
If *adr1\l<>*adr2\l
INC dif
Debug Str(a-9999)+"/8="+Str(*adr1\l)+" SAR a,3="+Str(*adr2\l)+" %8="+Str(a%8)
EndIf
*adr1+4
*adr2+4
Next
CompilerEndIf
MessageRequester("Ergebnis:","Freds Method: "+Str(zeit2-zeit)+"ms"+Chr(13)+"My Method: "+Str(zeit3-zeit2)+"ms"+Chr(13)+"Menge der Unterschiede: "+Str(dif))
Posted: Tue Mar 04, 2008 4:59 pm
by va!n
@Franky:
respect! *thumbs up*
i would like to see such speed optimisations directly done by the compiler... your code is surely a nice contribution to the pb team
Posted: Tue Mar 04, 2008 5:37 pm
by Psychophanta
...and what makes you think that SAR or SAL is more optimized instruction than IDIV, or DIV or IMUL or MUL
Don't be so sure that it is faster in modern processors
Posted: Tue Mar 04, 2008 6:07 pm
by hellhound66
Removed.
