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.

Turning on Debug Printf in Validation Layer

We suggest to use Vulkan Configurator (VkConfig) to enable Debug Printf

For those who "just need to quick use it" there is a VK_LAYER_PRINTF_ONLY_PRESET environment variable that will turn on Debug Printf and turn off all of the other validation logic.

# Windows
set VK_LAYER_PRINTF_ONLY_PRESET=1

# Linux
export VK_LAYER_PRINTF_ONLY_PRESET=1

# Android
adb setprop debug.vulkan.khronos_validation.printf_only_preset=1

Settings

The following are the various Debug Printf settings (listed as environment variables, but work like all other settings).

All settings also found in VkConfig

• VK_LAYER_PRINTF_ENABLE will turn on Debug Printf alongside the other validation
  • VK_LAYER_PRINTF_ENABLE=1 turn on
• 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_VERBOSE will print extra information (pipeline, shader, command, etc)
  • VK_LAYER_PRINTF_VERBOSE=1 turn on
• 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);
}

glslang will automatically add the Debug Printf instructions

Using Debug Printf in HLSL and Slang Shaders

In HLSL and Slang, Debug Printf can be invoked as follows (Try Online):

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

Both dxc and slangc will automatically add the Debug Printf instructions

Recommendations

If you print every time a shader is executed you can easily get millions of things trying to print. It is recommended to use built-ins to limit what is printed

// Vertex Shader
if (gl_VertexIndex == 0) {
    debugPrintfEXT("Only print for a single vertex shader invocation\n");
}

// Fragment Shader
if (gl_FragCoord.x > 0.0 && gl_FragCoord.x < 0.1 &&
    gl_FragCoord.y > 0.0 && gl_FragCoord.y < 0.1) {
    debugPrintfEXT("Only print for a few fragment shader invocation\n");
}

// Compute Shader
if (gl_LocalInvocationIndex == 0) {
    debugPrintfEXT("Only print for a single compute invocation\n");
}

Debug Printf Output

Debug Printf error message are returned as VK_DEBUG_UTILS_MESSAGE_SEVERITY_INFO_BIT_EXT

For your custom callback, you can look for 0x4fe1fef9 in VkDebugUtilsMessengerCallbackDataEXT::messageIdNumber as the magic hash if it is a Debug Printf message

The Validation Layers will try to turn on info level messages when using Debug Printf so the message is found

The VkDebugUtilsMessengerCallbackDataEXT::pMessage will contain the location and on a newline print out the error message such as:

vkQueueSubmit(): pSubmits[0] Debug Printf:

x == 100

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"

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:

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

The vkCmdDrawIndexed in question now has 51 messages.

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

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 Printf 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 Debug Printf 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 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

2. 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

3. See the Debug Printf output in Launcher window:

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.