Khronos Vulkan

Using Debug Printf

Introduction

Debugging Vulkan shaders, especially compute shaders, can be very difficult to do even with the aid of a powerful debugging tool like RenderDoc. Debug Printf is a recent Vulkan feature that allows developers to debug their shaders by inserting Debug Print statements. This feature is now supported within RenderDoc in a way that allows for per-invocation inspection of values in a shader. This article describes how to instrument your GLSL or HLSL shaders with Debug Printf and how to inspect and debug with them in RenderDoc, using vkconfig, or with environment variables.

Configuring Debug Printf in Validation Layer

For an overview of how to configure layers, refer to the Layers Overview and Configuration document.

The Debug Printf settings are managed by configuring the Validation Layer. These settings are described in the VK_LAYER_KHRONOS_validation document.

Debug Printf settings can also be managed using the Vulkan Configurator included with the Vulkan SDK.

For those who “just need to quick use it” there is also a set VK_LAYER_PRINTF_ONLY_PRESET=1 environment variable that will turn on DebugPrintf and turn off all of the other validation logic.

Settings

All settings are found in Vulkan Configurator (VkConfig)

Even if you use VK_LAYER_PRINTF_ONLY_PRESET you need to set those settings yourself as desired

There are currently 3 environment variables that are used for settings in Debug Printf

  • VK_LAYER_PRINTF_TO_STDOUT will print to stdout instead of the normal Debug Callback
    • VK_LAYER_PRINTF_TO_STDOUT=1 turn on
    • VK_LAYER_PRINTF_TO_STDOUT=0 turn off
  • VK_LAYER_PRINTF_VERBOSE will print extra information (pipeline, shader, command, etc)
    • VK_LAYER_PRINTF_VERBOSE=1 turn on
    • VK_LAYER_PRINTF_VERBOSE=0 turn off
  • VK_LAYER_PRINTF_BUFFER_SIZE size of the buffer used to store Printf messages (buffer is shared across all calls in a single vkQueueSubmit).
    • Default: 1024 bytes
    • set VK_LAYER_PRINTF_BUFFER_SIZE=4096 (example of making it larger)

Using Debug Printf in GLSL Shaders

To use Debug Printf in GLSL shaders, you need to enable the GL_EXT_debug_printf extension. Then add debugPrintfEXT calls at the locations in your shader where you want to print messages and/or values Here is a very simple example (Try Online):

#version 450
#extension GL_EXT_debug_printf : enable
void main() {
    float myfloat = 3.1415f;
    debugPrintfEXT("My float is %f", myfloat);
}

Then use glslangValidator to generate SPIR-V to use in vkCreateShaderModule. glslangvalidator --target-env vulkan1.2 -e main -o shader.vert.spv shader.vert would be one example of compiling shader.vert

Note that every time this shader is executed, My float is 3.141500 will be printed. If this were in a vertex shader and a triangle was drawn, it would be printed 3 times.

Using Debug Printf in HLSL Shaders

In HLSL, debug printf can be invoked as follows (Try Online):

void main() {
    float myfloat = 3.1415;
    printf("My float is %f", myfloat);
}

Use glslangValidator or dxc to generate SPIR-V for this shader. For instance:

  • glslangValidator.exe -D --target-env vulkan1.2 -e main -o shader.vert.spv shader.vert
  • dxc.exe -spirv -E main -T ps_6_0 -fspv-target-env=vulkan1.2 shader.vert -Fo shader.vert.spv

Using Debug Printf in SPIR-V Shaders

Normally, developers will use a high-level language like HLSL or GLSL to generate SPIR-V. However, in some cases, developers may wish to insert Debug Printfs directly into SPIR-V

To execute debug printfs in a SPIR-V shader, a developer will need the following two instructions specified:

OpExtension "SPV_KHR_non_semantic_info"
%N0 = OpExtInstImport NonSemantic.DebugPrintf

Debug printf operations can then be specified in any function with the following instruction: %NN = OpExtInst %void %N0 1 %N1 %N2 %N3 ... where:

  • N0 is the result id of the OpExtInstImport NonSemantic.DebugPrintf
  • 1 is the opcode of the DebugPrintf instruction in NonSemantic.DebugPrintf
  • N1 is the result of an OpString containing the format for the debug printf
  • N2, N3, ... are result ids of scalar and vector values to be printed
  • NN is the result id of the debug printf operation. This value is undefined.

OpExtInstImport of any NonSemantic* is properly supported with the VK_KHR_shader_non_semantic_info device extension. Some older compiler stacks might not handle these unknown instructions well, some will ignore it as desired.

Debug Printf Output

The strings resulting from a Debug Printf will, by default, be sent to the debug callback which is either specified by the app, or by default sent to stdout. They are sent at the VK_DEBUG_REPORT_INFORMATION_BIT_EXT or VK_DEBUG_UTILS_MESSAGE_SEVERITY_INFO_BIT_EXT level.

Debug Printf messages in RenderDoc

As of RenderDoc release 1.14, Debug Printf statements can be added to shaders, and debug printf messages will be received and logged in the Event Browser window.

Using the debugmarker sample from Sascha Willems' Vulkan samples repository:

  1. Capture a frame: Rd Frame

  2. Edit the shader:

  • Add #extension GL_EXT_debug_printf : enable to beginning of shader
  • Add debugPrintfEXT("Position = %v4f", pos); to shader after pos definition
  • Hit Refresh

Refresh

The vkCmdDrawIndexed in question now has 51 messages.

  1. Click on msg(s) to see Debug Printf output per draw: Values

Debug Printf messages from Validation Layers via VkConfig (Vulkan Configurator)

Here's an example of adding a Debug Printf statement to the shader in the vkcube demo (from the Vulkan-Tools repository), and then using VkConfig to enable Debug Printf, launch vkcube, and see the Debug Printf output.

  1. Add Debug Printf to the vkcube demo:
  • Add VK_KHR_shader_non_semantic_info to cube's CreateDevice function
  • Add extension and debugPrintfEXT call to the shader
  • Use glslangvalidator to compile the new shader
  • (Offscreen) Rebuild vkcube

Add Dbpf

  1. Configure VkConfig to enable Debug Printf
  • Set Shader Printf Preset
  • Set the executable path to the vkcube demo and add --c 1 to the command line to render one frame
  • Click the “Launch” button

Vkconfig Setup

  1. See the Debug Printf output in Launcher window: Vkconfig Result

Debug Printf Format String

The format string for this implementation of debug printf is more restricted than the traditional printf format string.

Format for specifier is “%”precision <d, i, o, u, x, X, a, A, e, E, f, F, g, G, ul, lu, or lx>

Format for vector specifier is “%”precision“v” [2, 3, or 4] [specifiers list above]

  • The vector value separator is ", "
  • “%%” will print as “%”
  • No length modifiers. Everything except ul, lu, and lx is 32 bits, and ul and lx values are printed in hex
  • No strings or characters allowed
  • No flags or width specifications allowed
  • No error checking for invalid format strings is done.

For example:

float myfloat = 3.1415f;
vec4 floatvec = vec4(1.2f, 2.2f, 3.2f, 4.2f);
uint64_t bigvar = 0x2000000000000001ul;

debugPrintfEXT("Here's a float value to 2 decimals %1.2f", myfloat); Would print “Here's a float value to 2 decimals 3.14”

debugPrintfEXT("Here's a vector of floats %1.2v4f", floatvec); Would print “Here's a vector of floats 1.20, 2.20, 3.20, 4.20”

debugPrintfEXT("Unsigned long as decimal %lu and as hex 0x%lx", bigvar, bigvar); Would print “Unsigned long as decimal 2305843009213693953 and as hex 0x2000000000000001”

Limitations

  • Debug Printf consumes a descriptor set. If your application uses every last descriptor set on the GPU, Debug Printf will not work.
    • Suggest using VK_VALIDATION_FEATURE_ENABLE_GPU_ASSISTED_RESERVE_BINDING_SLOT_EXT
  • Debug Printf consumes device memory on the GPU. Large or numerous Debug Printf messages can exhaust device memory. See settings above to control buffer size.
    • Can be controlled with VK_LAYER_PRINTF_BUFFER_SIZE
  • Validation Layers version: 1.2.135.0 or later is required
  • Vulkan API version 1.1 or greater is required
  • When using Validation Layers, the fragmentStoresAndAtomics, vertexPipelineStoresAndAtomics, and timelineSemaphore features are required
  • The VK_KHR_shader_non_semantic_info extension must be supported and enabled
    • If using the Validation Layers, we attempt to strip it out to allow wider range of users to still use Debug Printf
  • RenderDoc release 1.14 or later
  • When using Debug Printf with a debug callback, it is recommended to disable validation, as the debug level of INFO or DEBUG causes the validation layers to produce many messages unrelated to Debug Printf, making it difficult to find the desired output.

Other References

Documentation for the GL_EXT_debug_printf extension can be found here

There are many validation layer tests that demonstrates the simple and programmatic use of Debug Printf. See tests/unit/debug_printf.cpp in the Vulkan-ValidationLayers repository.

Earlier implementations implicitly included stage specific built-in variables such as gl_InvocationID, gl_VertexID and gl_FragCoord in Debug Printf messages. This functionality has been removed because it made Debug Printf unusable in shader modules that defined entry points for multiple pipeline stages. If necessary, you can add these values to your printf statements explicitly. However, you must then make sure that the printf statement can only be executed from a pipeline stage where the built-in variable is available.