A serialization format for various persistent Vulkan object types.
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rapidjson @ f0177eb
test Use pimpl idiom to hide rapidjson include from global scope. May 2, 2018
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varint.hpp Use varint for serialization format. Apr 16, 2018



Build Status Build Status

Fossilize is a simple library for serializing various persistent Vulkan objects which typically end up in hashmaps. CreateInfo structs for these Vulkan objects can be recorded and replayed.

  • VkSampler (immutable samplers in set layouts)
  • VkDescriptorSetLayout
  • VkPipelineLayout
  • VkRenderPass
  • VkShaderModule
  • VkPipeline (compute/graphics)

The goal for this project is to cover some main use cases:

High priority

  • For internal engine use. Extend the notion of VkPipelineCache to also include these persistent objects, so they can be automatically created in load time rather than manually declaring everything up front. Ideally, this serialized cache could be shipped, and applications can assume all persistent objects are already created.
  • Create a Vulkan layer which can capture this cache for repro purposes. A paranoid mode would serialize the cache before every pipeline creation, to deal with crashing drivers.
  • Easy way of sending shader compilation repros to conformance. Capture internally or via a Vulkan layer and send it off. Normally, this is very difficult with normal debuggers because they generally rely on capturing frames or similar, which doesn't work if compilation segfaults the driver. Shader compilation in Vulkan requires a lot of state, which requires sending more complete repro applications.

Mid priority

  • Some convenience tools to modify/disassemble/spirv-opt parts of the cache.

Low priority

  • Serialize state in application once, replay on N devices to build up VkPipelineCache objects without having to run application.
  • Benchmark a pipeline offline by feeding it fake input.


Supported compilers

  • GCC 4.8+
  • Clang
  • MSVC 2013/2015/2017+

If rapidjson is not already bundled in your project, you need to check out the submodules.

git submodule update --init

otherwise, you can set FOSSILIZE_RAPIDJSON_INCLUDE_PATH if building this library as part of your project. It is also possible to use FOSSILIZE_VULKAN_INCLUDE_PATH to override Vulkan header include paths.

Normally, the CLI tools will be built. These require SPIRV-Tools and SPIRV-Cross submodules to be initialized, however, if you're only building Fossilize as a library/layer, you can use CMake options -DFOSSILIZE_CLI=OFF and -DFOSSILIZE_TESTS=OFF to disable all those requirements for submodules (assuming you have custom include path for rapidjson). Standalone build:

mkdir build
cd build
cmake ..
cmake --build .

Link as part of other project:

add_subdirectory(fossilize EXCLUDE_FROM_ALL)
target_link_library(your-target fossilize)

For Android, you can use the android_build.sh script. It builds the layer for armeabi-v7a and arm64-v8a. See the script for more details.

Serialization format

Overall, a binary format which combines JSON with varint-encoded SPIR-V (light compression).

  • Magic "FOSSILIZE0000001" (16 bytes ASCII)
  • Size of entire binary (64-bit LE)
  • JSON magic "JSON " (8 bytes ASCII)
  • JSON size (64-bit LE)
  • JSON data (JSON size bytes)
  • SPIR-V magic "SPIR-V " (8 bytes ASCII)
  • SPIR-V size (64-bit LE)
  • Varint-encoded SPIR-V words (SPIR-V size bytes)

64-bit little-endian values are not necessarily aligned to 8 bytes.

The JSON is a simple format which represents the various Vk*CreateInfo structures. pNext is currently not supported. When referring to other VK handle types like pImmutableSamplers in VkDescriptorSetLayout, or VkRenderPass in VkPipeline, a 1-indexed format is used. 0 represents VK_NULL_HANDLE and 1+, represents an array index into the respective array (off-by-one). Data blobs (specialization constant data, SPIR-V) are encoded in base64, but I'll likely need something smarter to deal with large applications which have half a trillion SPIR-V files. When recording or replaying, a mapping from and to real Vk object handles must be provided by the application so the offset-based indexing scheme can be resolved to real handles.

VkShaderModuleCreateInfo refers to an encoded buffer in the SPIR-V block by codeBinaryOffset and codeBinarySize.

The varint encoding scheme encodes every 32-bit SPIR-V word by encoding 7 bits at a time starting with the LSBs, the MSB bit in an encoded byte is set if another byte needs to be read (7 bit) for the same SPIR-V word. Each SPIR-V word takes from 1 to 5 bytes with this scheme.

Sample API usage

Recording state

// Note that fossilize.hpp will include Vulkan headers, so make sure you include vulkan.h before
// this one if you care about which Vulkan headers you use.
#include "fossilize.hpp"

void create_state()
        Fossilize::StateRecorder recorder;
        // TODO here: Add way to capture which extensions/physical device features were used to deal with exotic things
        // which require extensions when making repro cases.

        VkDescriptorSetLayoutCreateInfo info = {

        // Fill in stuff.

        Fossilize::Hash hash = Fossilize::Hashing::compute_hash_descriptor_set_layout(recorder, info);
        // Or use your own hash. This 64-bit hash will be provided back to application when replaying the state.
        // The builtin hashing functions are mostly useful for a Vulkan capturing layer, testing or similar.
        unsigned set_index = recorder.register_descriptor_set_layout(hash, info);

        vkCreateDescriptorSetLayout(..., &layout);

        // Register the true handle of the set layout so that the recorder can map
        // layout to an internal index when using register_pipeline_layout for example.
        // Setting handles are only required when other Vk*CreateInfo calls refer to any other Vulkan handles.
        recorder.set_descriptor_set_layout_handle(set_index, layout);

        // Do the same for render passes, pipelines, shader modules, samplers (if using immutable samplers) as necessary.

        std::vector<uint8_t> serialized = recorder.serialize();
    catch (const std::exception &e)
        // Can throw exception on API misuse.

Replaying state

// Note that fossilize.hpp will include Vulkan headers, so make sure you include vulkan.h before
// this one if you care about which Vulkan headers you use.
#include "fossilize.hpp"

struct Device : Fossilize::StateCreatorInterface
    // See header for other functions.
    bool set_num_descriptor_set_layouts(unsigned count) override
        // Know a-head of time how many set layouts there will be, useful for multi-threading.
        return true;

    bool enqueue_create_descriptor_set_layout(Hash hash, unsigned index,
                                              const VkDescriptorSetLayoutCreateInfo *create_info,
                                              VkDescriptorSetLayout *layout) override
        // Can queue this up for threaded creation (useful for pipelines).
        // create_info persists as long as Fossilize::Replayer exists.
        VkDescriptorSetLayout set_layout = populate_internal_hash_map(hash, create_info);

        // Let the replayer know how to fill in VkDescriptorSetLayout in upcoming pipeline creation calls.
        // Can use dummy values here if we don't care about using the create_info structs verbatim.
        *layout = set_layout;
        return true;

    void wait_enqueue() override
        // If using multi-threaded creation, join all queued tasks here.

void replay_state(Device &device)
        Fossilize::Replayer replayer;
        replayer.parse(device, serialized_state, serialized_state_size);
        // Now internal hashmaps are warmed up, and all pipelines have been created.
    catch (const std::exception &e)
        // Can throw exception on API misuse.

Vulkan layer capture

Fossilize can also capture Vulkan application through the layer mechanism. The layer name is VK_LAYER_fossilize.

To build, enable FOSSILIZE_VULKAN_LAYER CMake option. This is enabled by default. The layer and JSON is placed in layer/ in the build folder.


By default the layer will serialize to fossilize.json in the working directory on vkDestroyDevice. However, due to the nature of some drivers, there might be crashes in-between. For this, there are two other modes.


Before every call to vkCreateComputePipelines and vkCreateGraphicsPipelines, data is serialized to disk. This can be quite slow for application with lots of pipelines, so only use it if the method below doesn't work ...


This only works on Linux. A SIGSEGV handler is registered, and the state is serialized to disk in the segfault handler. This is a bit sketchy, but should work well if drivers are crashing on pipeline creation (or just crashing in general).

On Windows, all vkCreate*Pipelines calls are always wrapped in SEH-style __try/__except blocks, which will catch access violations specifically inside those calls. If an access violation is triggered, a safety serialization is performed, a message box will appear, notifying user about this, and immediately terminate the process after. This functionality however, is only enabled when building with MSVC, as MinGW does not readily support the __try/__except extension. Patches welcome!

export FOSSILIZE_DUMP_PATH=/my/custom/path

Custom file path for capturing state.


By default the layer will serialize to /sdcard/fossilize.json on vkDestroyDevice. However, this path might not be writeable, so you will probably have to override your path to something like /sdcard/Android/data/<package.name>/capture.json. Make sure your app has external write permissions if using the default path.

Due to the nature of some drivers, there might be crashes in-between. For this, there are two other modes. Options can be set through setprop.

Setprop options

  • setprop debug.fossilize.dump_path /custom/path
  • setprop debug.fossilize.paranoid_mode 1
  • setprop debug.fossilize.dump_sigsegv 1

To force layer to be enabled outside application: setprop debug.vulkan.layers "VK_LAYER_fossilize". The layer .so needs to be part of the APK for the loader to find the layer.

Use adb logcat -s Fossilize to isolate log messages coming from Fossilize. You should see something like:

04-18 21:49:41.692 17444 17461 I Fossilize: Overriding serialization path: "/sdcard/fossilize.json".
04-18 21:49:43.741 17444 17461 I Fossilize: Serialized to "/sdcard/fossilize.json".

if capturing is working correctly.


The CLI currently has 3 tools available. These are found in cli/ after build.


This tool is for taking a capture, and replaying it on any device. Currently, all basic PhysicalDeviceFeatures (not PhysicalDeviceFeatures2 stuff) and extensions will be enabled to make sure a capture will validate properly. robustBufferAccess however is forced off.

This tool serves as the main "repro" tool. After you have a capture, you should ideally be able to repro crashes using this tool. To make replay faster, use --filter-compute and --filter-graphics to isolate which pipelines are actually compiled.


This tool can disassemble any pipeline into something human readable. Three modes are provided:

  • ASM (using SPIRV-Tools)
  • Vulkan GLSL (using SPIRV-Cross)
  • AMD ISA (using VK_AMD_shader_info if available)

TODO is disassembling more of the other state for quick introspection. Currently only SPIR-V disassembly is provided.


Runs spirv-opt over all shader modules in the capture and serializes out an optimized version. Useful to sanity check that an optimized capture can compile on your driver.


Running the CLI apps on Android is also supported. Push the binaries generated to /data/local/tmp, chmod +x them if needed, and use the binaries like regular Linux.

Submit shader failure repro cases


External modules