## Thursday, April 12, 2012

### Meteor Contest Part 6 - Visualworks solution

The shootout benchmark is written for Visualworks, so I tried to port the meteor contest in VW7.8 non commercial.
Here are the required modifications:
1. << and >> are not understood by Integer and must be replaced by proper signed bitShift:
2. bitsDo: bitReverse: and bitCount operations are missing and must be added as extensions
3. SequenceableCollection>>reversed is not understood, the VW version is reverse.
Surprise, VW did not perform better than Cog on my mac mini. 1.5 seconds instead of 1.3s for solving the board.

Yes, but... Unlike Squeak, VW positive SmallInteger don't have 30 bits but only 29. The Squeak solution was thus a bit sub-optimal in this context (I like to read a bit in double sense).
Oh, let's just change that! The major generator of 30-bits Integer is the bitReverse: operation when we turn the board 180° once first 6 rows were filled. After the reversal, the boardMask has two rows of barrier toward north, remember:
1 1 1 1 1
1 1 1 1 1
1 0 1 1 0
1 0 0 0 0
0 1 0 0 0
0 0 0 0 0
Yes, that's one more row than necessary, this would work too:
0 0 0 0 0
1 1 1 1 1
1 0 1 1 0
1 0 0 0 0
0 1 0 0 0
0 0 0 0 0
The variant is quite easy in:
| nextFreeCell possibles colorBit iRow boardMask rowStatus |
colorMask = 0 ifTrue: [ ^solutionBlock value: (self boardStringWithPieces: pieces) ].
loopCount := loopCount + 1.
iRow := rowOff.
[(nextFreeCell := (boardMask + 1) lowBit) > twoRows]
whileTrue:
[boardMask := (iRow := iRow + 2) = 6
ifTrue: [boardMask bitReverse: sixRows]
ifFalse: [boardMask bitShift: 0 - twoRows]].
...snip...
just change the last but one line with:
ifTrue: [(boardMask bitShift: 0 - ncol) bitReverse: sixRows - ncol]
This change makes VW slightly faster than Cog/Squeak again.

The good thing with VW is that we can play with:
TimeProfiler new
samplingInterval: 3;
profile:[ShootoutMeteorBoard solveDefault].
And this one tells us we waste a bunch of time in SequenceableCollection>>do:
Ah yes, Squeak MessageTally told that too, but I didn't trust it enough.
Deceivingly, we will have to inline this do: loop by ourselves. So we also change the end of above method:
...snip...
rowStatus :=
(iRow < 6 ifTrue: [0] ifFalse: [2]) +
(((iRow = 0 or: [iRow = 6]) and: [nextFreeCell <= ncol]) ifTrue: [1] ifFalse: [2]).
possibles := (positionsPerPiece at: rowStatus) at: nextFreeCell.
colorBit := 1.
1 to: pieces size do: [:pieceNumber |
(colorMask bitAnd: colorBit) = 0
ifFalse:
[ | positions |
positions := possibles at: pieceNumber.
1 to: positions size do: [:i |
| aPiece |
((aPiece := positions at: i) fitOnBoard: boardMask)
ifTrue:
[pieces at: pieceNumber put: (aPiece forRow: iRow).
colorBit := colorBit * 2].
^nil

And measure the time again:
[ShootoutMeteorBoard solveDefault] timeToRun.
-> 708 milliseconds (fraction omitted).

Good thing, this applies to Cog/Squeak too:
[ShootoutMeteorBoard solveDefault] timeToRun.
-> 746  (milliseconds is implicit in Squeak).

For 885075 loops, that's less than 1 microsecond per loop, not that bad after all.

For single-core oriented programs, my machine has more or less same performance than shootout reference machine, as can be verified with the reference g++ solution (which runs in 80ms as reported on the shootout site).
/usr/bin/g++ --version
i686-apple-darwin10-g++-4.2.1 (GCC) 4.2.1 (Apple Inc. build 5666) (dot 3)
Copyright (C) 2007 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

/usr/bin/g++ -m32 -c -pipe -O3 -fomit-frame-pointer meteor.gpp-4.c++ -o meteor.gpp-4.c++.o &&  \
/usr/bin/g++ -m32 meteor.gpp-4.c++.o -o meteor.gpp-4.gpp_run

time ./meteor.gpp-4.gpp_run 2098
real    0m0.103s
user    0m0.090s
sys    0m0.002s

And with -m64

real    0m0.089s
user    0m0.079s
sys    0m0.002s

I'm reluctant to play with all the g++ code generation options, but that makes roughly a factor 10 between C++ and Smalltalk for that one (not accounting for image startup time).
Sure, I played with very low level bit tricks, but hey, that was the spirit of the game.

#### 1 comment:

1. very nice thread. Pun intended!