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Floating-point Power of 2 (powf2 & powd2)
Posted: Fri Mar 04, 2016 9:33 pm
by Keya
floating-point Pow2() functions for Float and Double, a bit faster than PB's Pow(n,2)
(not a fair competition though as PB's Pow() is general-purpose supporting both integer+float and variable exponent)
Code: Select all
CompilerIf #PB_Compiler_Debugger
CompilerError("Error - Turn off debugger for timings")
CompilerEndIf
Procedure.f Powf2(value.f)
! fld dword [p.v_value] ;load 32bit Float
! fmul st, st ;pow2 = value * value
! fstp dword [p.v_value] ;store
ProcedureReturn value
EndProcedure
Procedure.d Powd2(value.d)
! fld qword [p.v_value] ;load 64bit Double
! fmul st, st ;pow2 = value * value
! fstp qword [p.v_value] ;store
ProcedureReturn value
EndProcedure
Define result.f, ftest.f = 7
Time1 = ElapsedMilliseconds()
For i = 1 To 9000000
result = Pow(ftest, 2)
Next i
Time2 = ElapsedMilliseconds()
MessageRequester("Purebasic Pow()", Str(Time2-Time1))
Time1 = ElapsedMilliseconds()
For i = 1 To 9000000
result = Powf2(ftest)
Next i
Time2 = ElapsedMilliseconds()
MessageRequester("asm Powf2()", Str(Time2-Time1))
Re: Floating-point Power of 2 (powf2 & powd2)
Posted: Fri Mar 04, 2016 9:52 pm
by StarBootics
A Macro is much faster :
Code: Select all
Procedure.f Powf2(value.f)
! fld dword [p.v_value] ;load 32bit Float
! fmul st, st ;pow2 = value * value
! fstp dword [p.v_value] ;store
ProcedureReturn value
EndProcedure
Procedure.d Powd2(value.d)
! fld qword [p.v_value] ;load 64bit Double
! fmul st, st ;pow2 = value * value
! fstp qword [p.v_value] ;store
ProcedureReturn value
EndProcedure
Macro Powr2(value)
(value * Value)
EndMacro
Define result.f, ftest.f = 7
Time1 = ElapsedMilliseconds()
For i = 1 To 9000000
result = Pow(ftest, 2)
Next i
Time2 = ElapsedMilliseconds()
MessageRequester("Purebasic Pow()", Str(Time2-Time1))
Time1 = ElapsedMilliseconds()
For i = 1 To 9000000
result = Powf2(ftest)
Next i
Time2 = ElapsedMilliseconds()
MessageRequester("asm Powf2()", Str(Time2-Time1))
Time1 = ElapsedMilliseconds()
For i = 1 To 9000000
result = Powr2(ftest)
Next i
Time2 = ElapsedMilliseconds()
MessageRequester("asm Powr2()", Str(Time2-Time1))
On my computer I got 96 ms, 77 ms and 31 ms
Best regards
StarBootics
Re: Floating-point Power of 2 (powf2 & powd2)
Posted: Fri Mar 04, 2016 10:02 pm
by Keya
yes macro would be better especially seeing as its so tiny
Im not so sure about your multiply being faster though ...
What I'm doing is the following line (shown in three variations), in a "For i = 1 to 1.4 million" loop... note the timings!
Code: Select all
ft = Sqr(Abs(Pow(*p1\a,2) - Pow(*p2\a,2))) ;=128, 127, 127
ft = Sqr(Abs(Powf2(*p1\a) - Powf2(*p2\a))) ;=19, 18, 18
ft = Sqr(Abs(*p1\a * *p1\a) - (*p2\a * *p2\a)) ;=538, 540, 538
Re: Floating-point Power of 2 (powf2 & powd2)
Posted: Fri Mar 04, 2016 10:07 pm
by Keya
Hmm, i made a cut-down demo of what im doing, but this one agrees with your finding that the straight multiply is faster:
Code: Select all
Procedure.f Powf2(value.f)
! fld dword [p.v_value] ;load 32bit Float
! fmul st, st ;pow2 = value * value
! fstp dword [p.v_value] ;store
ProcedureReturn value
EndProcedure
bufa = AllocateMemory(9000000)
bufb = AllocateMemory(9000000)
*p1.Ascii = bufa
*p2.Ascii = bufb
Time1 = ElapsedMilliseconds()
For i = 1 To 9000000
;ft = Sqr(Abs(Pow(*p1\a,2) - Pow(*p2\a,2))) ;=257, 248, 249
;ft = Sqr(Abs(Powf2(*p1\a) - Powf2(*p2\a))) ;=48, 50, 48
ft = Sqr(Abs(*p1\a * *p1\a) - (*p2\a * *p2\a)) ;=34, 41, 33
*p1+1: *p2+1
Next i
Time2 = ElapsedMilliseconds()
MessageRequester("Time", Str(Time2-Time1))
but that leaves me perplexed as to why it's performing differently in my other code! completely different timings from what should be identical code. Well... at the end of the day i'll be saving a lot of time (have to call this a lot), that's the main thing lol
Re: Floating-point Power of 2 (powf2 & powd2)
Posted: Fri Mar 04, 2016 10:25 pm
by wilbert
Compared with the other two lines, your line should probably be
Code: Select all
ft = Sqr(Abs(*p1\a * *p1\a - *p2\a * *p2\a))
instead of
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ft = Sqr(Abs(*p1\a * *p1\a) - (*p2\a * *p2\a))
If you really need to do this calculation so often, you might consider converting the whole equation to asm.
Re: Floating-point Power of 2 (powf2 & powd2)
Posted: Fri Mar 04, 2016 10:49 pm
by Keya
ahhh yes! good catch thankyou
caffeine levels not good here for debugging
the speed of the procedure is actually really good so far, Pow() seemingly the only bottleneck. I wasnt sure how Sqr() would go but it seems fast, and likewise no hiccups from Abs()
Re: Floating-point Power of 2 (powf2 & powd2)
Posted: Sat Mar 05, 2016 6:41 am
by wilbert
Keya wrote:I wasnt sure how Sqr() would go but it seems fast, and likewise no hiccups from Abs()
Pow() requires multiple instructions, Sqr() only one (fsqrt). How fast that fsqrt instruction is, depends a lot on the cpu architecture but on a modern Intel cpu it's quite fast.
Re: Floating-point Power of 2 (powf2 & powd2)
Posted: Sat Mar 05, 2016 3:19 pm
by Joris
I'm not into ASM, but with your Powf2(value.f) you can't do a Pow(value, 1.3) or whatever 'power value', or am I missing something ?
OOoops yeah sorry saw it just a litle later "Power of 2"...