SMOL-V: like Vulkan/Khronos SPIR-V, but smaller.
C++ Makefile
Switch branches/tags
Nothing to show
Clone or download
Fetching latest commit…
Cannot retrieve the latest commit at this time.

SMOL-V: like Vulkan/Khronos SPIR-V, but smaller.


SMOL-V encodes Vulkan/Khronos SPIR-V format programs into a form that is smoller, and is more compressible. Normally no changes to the programs are done; they decode into exactly same program as what was encoded. Optionally, debug information can be removed too.

SPIR-V is a very verbose format, several times larger than same programs expressed in other shader formats (e.g. DX11 bytecode, GLSL, DX9 bytecode etc.). The SSA-form with ever increasing IDs is not very appreciated by regular data compressors either. SMOL-V does several things to improve this:

  • Many words, especially ones that most often have small values, are encoded using "varint" scheme (1-5 bytes per word, with just one byte for values in 0..127 range).
  • Some IDs used in the program are delta-encoded, relative to previously seen IDs (e.g. Result IDs). Often instructions reference things that were computed just before, so this results in small deltas. These values are also encoded using "varint" scheme.
  • Reordering instruction opcodes so that the most common ones are the smallest values, for smaller varint encoding.
  • Encoding several instructions in a more compact form, e.g. the "typical <=4 component swizzle" shape of a VectorShuffle instruction, or sequences of MemberDecorate instructions.

A somewhat similar utility is spirv-remap from glslang.

See this blog post for more information about how I did SMOL-V.


Add source/smolv.h and source/smolv.cpp to your C++ project build. It might require C++11 or somesuch; I only tested with Visual Studio 2010, 2015 and Mac Xcode 7.3.

smolv::Encode and smolv::Decode is the basic functionality. See smolv.h.

Other functions are for development/statistics purposes, to figure out frequencies and distributions of the instructions.

There's a test + compression benchmarking suite in testing/testmain.cpp, using that needs adding other files under testing/external to the build too (3rd party code: glslang remapper, Zstd, LZ4, miniz).


See Changelog.

Limitations / TODO

  • SPIR-V where the words got stored in big-endian layout is not supported yet.
  • The whole thing might not work on Big-Endian CPUs. It might, but I'm not 100% sure.
  • Not much prevention is done against malformed/corrupted inputs, TODO.
  • Out of memory cases are not handled. The code will either throw exception or crash, depending on your compilation flags.


Code itself: Public Domain.

There is 3rd party code under the testing framework (testing/external); it is not required for using SMOL-V. Most of that code (glslang, LZ4, Zstd) is BSD-licensed, and taken from github repositories of the respective projects. miniz is public domain.

There are SPIR-V binary shader dumps under tests/spirv-dumps for compression testing; these are not required for using SMOL-V. Not sure how to appropriately "license" them (but hey they are kinda useless by themselves out of context), so I'll go with this: "Binary shader dumps under 'tests' folder are only to be used for SMOL-V testing". Details on them:

  • tests/spirv-dumps/dota2 - some shaders from DOTA2, Copyright Valve Corporation, all rights reserved.
  • tests/spirv-dumps/shadertoy - several most popular shaders from Shadertoy, converted to Vulkan SPIR-V via glslang. Copyrights by their individual authors (filename matches last component of shadertoy URL).
  • tests/spirv-dumps/talos - some shaders from The Talos Principle, Copyright (c) 2002-2016 Croteam All rights reserved.
  • tests/spirv-dumps/unity - various Unity shaders, produced through a HLSL -> DX11 bytecode -> HLSLcc -> glslang toolchain.


As of 2016 September 1, results on 323 shaders (under tests/spirv-dumps) are:

0 Remap       4540.2KB  93.3%
0 SMOL-V      1629.1KB  33.5%
1    zlib     1212.7KB  24.9%
1 re+zlib     1079.4KB  22.2%
1 sm+zlib      601.8KB  12.4%
2    LZ4HC    1342.6KB  27.6%
2 re+LZ4HC    1147.9KB  23.6%
2 sm+LZ4HC     606.0KB  12.4%
3    Zstd      898.7KB  18.5%
3 re+Zstd      742.6KB  15.3%
3 sm+Zstd      445.3KB   9.1%
4    Zstd20    589.3KB  12.1%
4 re+Zstd20    508.8KB  10.5%
4 sm+Zstd20    347.8KB   7.1%
  • "Remap" is spirv-remap from glslang, with debug info stripping.
  • SMOL-V is what you're looking at, with debug info stripping too.
  • zlib, LZ4HC and Zstd are general compression algorithms at default settings (Zstd20 is Zstd compression with almost max setting of 20).
  • "re+" is "remapper + compression", "sm+" is "SMOL-V + compression".
  • Compression is done on the whole blob of all the test programs (not individually for each program).