Polaris

a UCI chess and chess960 engine

active development on Polaris has ceased in favour of its NNUE successor Stormphrax

Strength

Version	CCRL 40/15	CCRL Blitz	CCRL 40/2 FRC	MCERL
1.8.x	testing ongoing	3048	2982	-
1.7.0	2927	2953	2839	-
1.6.x	2806	2884	2745	-
1.5.0	2678	2749	2508	2713
1.4.x	2639	-	2455	-
1.3.0	2450	2512	N/A	-
1.2.0	~2300 (very few games played)	2330	N/A	-

Features

• standard PVS with quiescence search and iterative deepening
  • aspiration windows
  • check extensions
  • countermoves
  • futility pruning
  • history
    • countermove history (1-ply continuation history)
    • follow-up history (2-ply continuation history)
    • capture history
  • internal iterative reduction
  • killers (1 per ply)
  • late move reductions
  • mate distance pruning
  • nullmove pruning
  • reverse futility pruning
  • SEE move ordering and pruning
  • Syzygy tablebase support
• Texel-tuned HCE (private tuner because that code hurts me to reread)
  • tuner based on Andrew Grant's paper
  • tuned on a combination of the Zurichess and lichess-big3-resolved datasets
• BMI2 attacks in the bmi2 build, otherwise fancy black magic
  • pext/pdep for rooks
  • pext for bishops
• lazy SMP

To-do

• tune search constants
• contempt
• make it stronger uwu

UCI options

Name	Type	Default value	Valid values	Description
Hash	integer	64	[1, 131072]	Memory allocated to the transposition table (in MB). Rounded down internally to the next-lowest power of 2.
Clear Hash	button	N/A	N/A	Clears the transposition table.
Threads	integer	1	[1, 2048]	Number of threads used to search.
UCI_Chess960	check	false	false, true	Whether Polaris plays Chess960 instead of standard chess.
Move Overhead	integer	10	[0, 50000]	Amount of time Polaris assumes to be lost to overhead when making a move (in ms).
SyzygyPath	string	<empty>	any path	Location of Syzygy tablebases to probe during search.
SyzygyProbeDepth	spin	1	[1, 255]	Minimum depth to probe Syzygy tablebases at.
SyzygyProbeLimit	spin	7	[0, 7]	Maximum number of pieces on the board to probe Syzygy tablebases with.

Builds

bmi2: requires BMI2 and assumes fast pext and pdep (i.e. no Zen 1 and 2)
modern: requires BMI (blsi, blsr, tzcnt) - primarily useful for pre-Zen 3 AMD CPUs back to Piledriver
popcnt: just needs popcnt
compat: should run on anything back to an original Core 2

Alternatively, build the CMake target polaris-native for a binary tuned for your specific CPU (see below)
(note that this does not automatically disable pext and pdep for pre-Zen 3 AMD CPUs that implement them in microcode)

Note:

• If you have an AMD Zen 1 (Ryzen x 1xxx) or 2 (Ryzen x 2xxx) CPU, use the modern build even though your CPU supports BMI2. These CPUs implement the BMI2 instructions pext and pdep in microcode, which makes them unusably slow for Polaris' purposes.
• Builds other than bmi2 are untested and might crash on CPUs lacking newer instructions; I don't have older hardware to test them on.

Building

The makefile is not intended for building by users. It exists purely for OpenBench compliance.
Requires CMake and a competent C++20 compiler (tested with Clang 15 and 16 on Windows, GCC 11 and Clang 15 and 16 on Linux, and Apple Clang 14 on macOS on Apple Silicon)

> cmake -DCMAKE_BUILD_TYPE=Release -S . -B build/
> cmake --build build/ --target polaris-<TARGET>

(replace <TARGET> with your preferred target - native/bmi2/modern/popcnt/compat)

If you have a pre-Zen 3 AMD Ryzen CPU (see the notes in Builds above) and want to build the native target, use these commands instead (the second is unchanged):

> cmake -DCMAKE_BUILD_TYPE=Release -DPS_FAST_PEXT=OFF -S . -B build/
> cmake --build build/ --target polaris-native

Disabling the CMake option PS_FAST_PEXT builds the non-BMI2 attack getters.

Credit

Polaris uses Fathom for tablebase probing, licensed under the MIT license.