Stormphrax

a work-in-progress UCI chess and chess960 engine, with NNUE evaluation trained from zero knowledge starting with random weights

this project is a continuation of my HCE engine Polaris

Strength

Version	CCRL 40/15	CCRL Blitz	CCRL 40/2 FRC	CEGT 40/4	MCERL
4.1.0	3484	-	3812	-	-
4.0.0	3475	3565	3781	3442	3542
3.0.0	3407	3491	3696	-	3495
2.0.0	3397	3481	3675	3339	3482
1.0.0	3317	3374	3543	3235	3346

Features

• standard PVS with quiescence search and iterative deepening
  • aspiration windows
  • check extensions
  • countermoves
  • futility pruning
  • history
    • capture history
    • 1-ply continuation history (countermove history)
    • 2-ply continuation history (follow-up history)
    • 4-ply continuation history
  • internal iterative reduction
  • killers (1 per ply)
  • late move reductions
  • late move pruning
  • mate distance pruning
  • multicut
  • nullmove pruning
  • reverse futility pruning
  • SEE move ordering and pruning
  • singular extensions
    • double extensions
    • triple extensions
    • various negative extensions
  • Syzygy tablebase support
• NNUE
  • (768x8->1024)x2->1x8 architecture
  • trained from zero knowledge with reinforcement learning from a randomly-initialised network
• BMI2 attacks in the bmi2 build, otherwise fancy black magic
  • pext/pdep for rooks
  • pext for bishops
• lazy SMP
• static contempt

To-do

• 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).
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 Stormphrax plays Chess960 instead of standard chess.
UCI_ShowWDL	check	true	false, true	Whether Stormphrax displays predicted win/draw/loss probabilities in UCI output.
Move Overhead	integer	10	[0, 50000]	Amount of time Stormphrax assumes to be lost to overhead when making a move (in ms).
SyzygyPath	string	<empty>	any path, or <empty>	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.
EvalFile	string	<internal>	any path, or <internal>	NNUE file to use for evaluation.

Builds

avx512: requires AVX-512 (Zen 4, Skylake-X)
avx2-bmi2: requires BMI2 and AVX2 and assumes fast pext and pdep (i.e. no Bulldozer, Piledriver, Steamroller, Excavator, Zen 1, Zen+ or Zen 2)
avx2: requires BMI and AVX2 - primarily useful for pre-Zen 3 AMD CPUs back to Excavator
sse41-popcnt: needs SSE 4.1 and popcnt - for older x64 CPUs

Alternatively, build the makefile target native for a binary tuned for your specific CPU (see below)

Note:

• If you have an AMD Zen 1 (Ryzen x 1xxx), Zen+ (Ryzen x 2xxx) or Zen 2 (Ryzen x 3xxx) CPU, use the avx2 build even though your CPU supports BMI2. These CPUs implement the BMI2 instructions pext and pdep in microcode, which makes them unusably slow for Stormphrax's purposes.

Building

Requires Make and a competent C++20 compiler that optionally supports LTO. GCC is not currently supported, so the usual compiler is Clang.

> make <BUILD> CXX=<COMPILER>

• replace <COMPILER> with your preferred compiler - for example, clang++ or icpx
  • if not specified, the compiler defaults to clang++
• replace <BUILD> with the binary you wish to build - native/avx512/avx2-bmi2/avx2/sse41-popcnt
  • if not specified, the default build is native
• if you wish, you can have Stormphrax include the current git commit hash in its UCI version string - pass COMMIT_HASH=on

By default, the makefile builds binaries with profile-guided optimisation (PGO). To disable this, pass PGO=off. When using Clang with PGO enabled, llvm-profdata must be in your PATH.

Credit

Stormphrax uses Fathom for tablebase probing, licensed under the MIT license, and a slightly modified version of incbin for embedding neural network files, under the Unlicense.

Stormphrax is tested on this OpenBench instance - thanks to all the people there, SP would be much weaker without your support :3

In no particular order, these engines have been notable sources of ideas or inspiration:

• Viridithas
• Svart
• Stockfish
• Ethereal
• Berserk
• Weiss
• Koivisto

Stormphrax's current networks are trained with bullet. Previous networks were trained with Marlinflow.

The name "Stormphrax" is a reference to the excellent Edge Chronicles :)