PureBasic Forum
http://forums.purebasic.com/english/

Population count
http://forums.purebasic.com/english/viewtopic.php?f=35&t=62235
Page 3 of 3

Author:  Bo Marchais [ Sun Apr 10, 2016 2:57 am ]
Post subject:  Re: Population count

There was once a stupid trick using local lookup tables for direct conversion, via xlat, or modern equivalent.

You can build a 256 entry table mapping 8 bits to # of bits, and then do an xlat. It's only useful in stupid cases...and nothing else.
With a swap and an add, you can do 16. Duplicate for 32, etc. I don't know modern latencies for memory access, so it will probably be horribly slow. Also, setup is very slow, so it will only look better if you count performance after the table is built.

Even if it works, I will be surprised. But it was the fast way when processors were slower and fewer instructions.

Author:  Michael Vogel [ Sun Apr 10, 2016 12:44 pm ]
Post subject:  Re: Population count

I don't have a better assembler code which is clear because wilbert & co are doing the best job...

...but I just want to add two ideas how to it also possible to achieve a fine speed:
1) create tables (PopCount9a is as fast as the best routines above)
2) use macros (PopCount9b only needs 1/3 of the time)

Code:
; Initialization
Global Dim pc9table.i($FFFF)
Global pc9
For pc9=0 To $FFFF
   pc9table(pc9)=PopCount0(pc9); any procedure you like here
Next pc9

Procedure.l PopCount9a (v.l)
   ProcedureReturn pc9table(v&$FFFF)+pc9table(v>>16)
EndProcedure
Macro PopCount9b (v)
   pc9table(v&$FFFF)+pc9table(v>>16)
EndMacro

Author:  Bo Marchais [ Sun Apr 10, 2016 7:08 pm ]
Post subject:  Re: Population count

Such tables also work well for trig, curves or "analog" functions.
The xlat/table thing was very handy for weighting in neural nets, fuzzy logic and weighted thresholds with multiple outputs.

Author:  Keya [ Wed Oct 05, 2016 10:43 pm ]
Post subject:  Re: Population count

Bo Marchais wrote:
There was once a stupid trick using local lookup tables for direct conversion, via xlat, or modern equivalent. You can build a 256 entry table mapping 8 bits to # of bits, and then do an xlat. It's only useful in stupid cases...and nothing else.

I was interested in this stupid trick!
Code:
Procedure.i GetBitCount(byte.a)
  CompilerIf #PB_Compiler_Processor = #PB_Processor_x64
  ! mov rax, [p.v_byte]
  ! mov rdx, rbx
  ! mov rbx, BitCounts
  ! xlatb
  ! mov rbx, rdx
CompilerElseIf #PB_Compiler_Processor = #PB_Processor_x86
  ! mov eax, [p.v_byte]
  ! mov edx, ebx
  ! mov ebx, BitCounts
  ! xlatb
  ! mov ebx, edx
  CompilerEndIf
  ProcedureReturn
  DataSection
    !BitCounts:
    Data.a 0,1,1,2,1,2,2,3,1,2,2,3,2,3,3,4,1,2,2,3,2,3,3,4,2,3,3,4,3,4,4,5,1,2,2,3,2,3,3,4,2,3,3,4,3,4,4,5,2,3,3,4,3,4,4,5,3,4,4,5,4,5,5,6
    Data.a 1,2,2,3,2,3,3,4,2,3,3,4,3,4,4,5,2,3,3,4,3,4,4,5,3,4,4,5,4,5,5,6,2,3,3,4,3,4,4,5,3,4,4,5,4,5,5,6,3,4,4,5,4,5,5,6,4,5,5,6,5,6,6,7
    Data.a 1,2,2,3,2,3,3,4,2,3,3,4,3,4,4,5,2,3,3,4,3,4,4,5,3,4,4,5,4,5,5,6,2,3,3,4,3,4,4,5,3,4,4,5,4,5,5,6,3,4,4,5,4,5,5,6,4,5,5,6,5,6,6,7
    Data.a 2,3,3,4,3,4,4,5,3,4,4,5,4,5,5,6,3,4,4,5,4,5,5,6,4,5,5,6,5,6,6,7,3,4,4,5,4,5,5,6,4,5,5,6,5,6,6,7,4,5,5,6,5,6,6,7,5,6,6,7,6,7,7,8 
  EndDataSection 
EndProcedure

It performs very well, i used John's test from the first page of this thread, but doubled the amount of tests to make the results a bit clearer as they're all so fast:
Code:
t0 = 36 ms     (10%)  <-- popcnt instruction
t1 = 350 ms   (100%)
t2 = 420 ms   (120%)
t3 = 1032 ms  (294%)
t4 = 1906 ms  (544%)
t5 = 1837 ms  (524%)
t6 = 1242 ms  (354%)
t7 = 57 ms     (16%)
t8 = 44 ms     (12%)  <-- this demo using xlatb

Author:  wilbert [ Thu Oct 06, 2016 6:19 am ]
Post subject:  Re: Population count

@Keya, your x64 code didn't work on MacOS.
I had to change it into this using lea
Code:
  ! mov rax, [p.v_byte]
  ! mov rdx, rbx
  ! lea rbx, [BitCounts]
  ! xlatb
  ! mov rbx, rdx


Lookups are fast but even faster without xlatb :wink:
Code:
Procedure.i GetBitCount(byte.a)
  CompilerIf #PB_Compiler_Processor = #PB_Processor_x64
    !movzx eax, byte [p.v_byte]
    !lea rdx, [BitCounts]
    !movzx eax, byte [rdx + rax]
  CompilerElseIf #PB_Compiler_Processor = #PB_Processor_x86
    !movzx eax, byte [p.v_byte]
    !movzx eax, byte [BitCounts + eax]
  CompilerEndIf
  ProcedureReturn
  DataSection
    !BitCounts:
    Data.a 0,1,1,2,1,2,2,3,1,2,2,3,2,3,3,4,1,2,2,3,2,3,3,4,2,3,3,4,3,4,4,5,1,2,2,3,2,3,3,4,2,3,3,4,3,4,4,5,2,3,3,4,3,4,4,5,3,4,4,5,4,5,5,6
    Data.a 1,2,2,3,2,3,3,4,2,3,3,4,3,4,4,5,2,3,3,4,3,4,4,5,3,4,4,5,4,5,5,6,2,3,3,4,3,4,4,5,3,4,4,5,4,5,5,6,3,4,4,5,4,5,5,6,4,5,5,6,5,6,6,7
    Data.a 1,2,2,3,2,3,3,4,2,3,3,4,3,4,4,5,2,3,3,4,3,4,4,5,3,4,4,5,4,5,5,6,2,3,3,4,3,4,4,5,3,4,4,5,4,5,5,6,3,4,4,5,4,5,5,6,4,5,5,6,5,6,6,7
    Data.a 2,3,3,4,3,4,4,5,3,4,4,5,4,5,5,6,3,4,4,5,4,5,5,6,4,5,5,6,5,6,6,7,3,4,4,5,4,5,5,6,4,5,5,6,5,6,6,7,4,5,5,6,5,6,6,7,5,6,6,7,6,7,7,8 
  EndDataSection 
EndProcedure

Author:  davido [ Mon Jan 06, 2020 7:45 pm ]
Post subject:  Re: Population count

Deleted!!
Added in Error, Sorry.

Page 3 of 3 All times are UTC + 1 hour
Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group
http://www.phpbb.com/