compiler: document SPIR-V capture + replace

Reviewed-by: Natalie Vock <natalie.vock@gmx.de>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36472>

docs/envvars.rst

@@ -309,6 +309,14 @@ Core Mesa environment variables
 
    see :ref:`Experimenting with Shader Replacements <replacement>`
 
+.. envvar:: MESA_SPIRV_DUMP_PATH
+
+   see :ref:`SPIR-V Shader Capture <spirv_capture>`
+
+.. envvar:: MESA_SPIRV_READ_PATH
+
+   see :ref:`SPIR-V Shader Replacement <spirv_replacement>`
+
 .. envvar:: MESA_VK_VERSION_OVERRIDE
 
    changes the Vulkan physical device version as returned in

docs/index.rst

@@ -115,6 +115,7 @@ Linux, FreeBSD, and other operating systems.
    gallium/index
    vulkan/index
    nir/index
+   spirv/index
    isl/index
    isaspec
    rusticl

docs/spirv/index.rst

@@ -0,0 +1,73 @@
+SPIR-V Debugging
+================
+
+SPIR-V is a binary shader intermediate representation used extensively by
+Vulkan applications. Mesa translates SPIR-V shaders into its internal NIR
+representation using the `spirv_to_nir` function.
+
+.. _spirv_capture:
+
+SPIR-V Shader Capture
+~~~~~~~~~~~~~~~~~~~~~
+Set the environment variable `MESA_SPIRV_DUMP_PATH` to a directory where
+the SPIR-V shaders will be captured. This is useful for debugging purposes,
+allowing developers to inspect the SPIR-V shaders provided by applications.
+
+Shader capture filenames are a hexadecimal representation of the shader's
+BLAKE3 hash. The file extension corresponds to the shader stage, e.g.
+`.vert` for vertex shaders or `.frag` for fragment shaders.
+
+It is usually necessary to disable the shader cache to activate the dump logic.
+This can be done by setting the `MESA_SHADER_CACHE_DISABLE` environment variable to `1`.
+
+.. code-block:: sh
+
+   MESA_SPIRV_DUMP_PATH=$(pwd) MESA_SHADER_CACHE_DISABLE=1 vkcube
+
+.. _spirv_replacement:
+
+SPIR-V Shader Replacement
+~~~~~~~~~~~~~~~~~~~~~~~~~
+SPIR-V shaders can be replaced with alternative SPIR-V shaders
+during runtime. This is useful for debugging or testing purposes, allowing
+developers to swap out shaders without modifying the application.
+
+To enable shader replacement, set the environment variable
+`MESA_SPIRV_READ_PATH` to the directory containing the replacement SPIR-V shaders.
+The filenames of the replacement shaders must match the original shader's BLAKE3
+hash and the extensions must match the original shader stages (e.g., `.vert`, `.frag`).
+
+It is usually necessary to disable the shader cache to ensure that
+the replacement shaders are used. This can be done by setting the
+`MESA_SHADER_CACHE_DISABLE` environment variable to `1`.
+
+When a shader is replaced, Mesa will log the replacement action, including
+the original shader's BLAKE3 hash and the filename of the replacement shader.
+Set `MESA_SPIRV_LOG_LEVEL=info` to see these log messages printed to `stderr`.
+
+.. code-block:: sh
+
+   MESA_SPIRV_READ_PATH=$(pwd)/replace MESA_SPIRV_LOG_LEVEL=info MESA_SHADER_CACHE_DISABLE=1 vkcube
+
+To replace a shader with a modified version of a captured shader, you can
+convert the captured SPIR-V shader to GLSL, edit as needed, and then
+recompile it back to SPIR-V.
+
+.. code-block:: sh
+
+   # Capture a SPIR-V shader
+   MESA_SPIRV_DUMP_PATH=$(pwd) MESA_SHADER_CACHE_DISABLE=1 vkcube
+
+   # Convert a captured SPIR-V shader to Vulkan-flavored GLSL
+   # spriv-cross is bundled with the Vulkan SDK and also generally available via package managers
+   spirv-cross -V 0x36c56e0f736238.frag --output 0x36c56e0f736238.frag.glsl
+
+   # <Edit the GLSL file as needed>
+
+   # Recompile the GLSL shader to SPIR-V
+   # Some shaders may require additional flags, such as "--auto-map-locations"
+   glslang -V 0x36c56e0f736238.frag.glsl -o replace/0x36c56e0f736238.frag
+
+   # run the application with the replacement shader
+   MESA_SPIRV_READ_PATH=$(pwd)/replace MESA_SPIRV_LOG_LEVEL=info MESA_SHADER_CACHE_DISABLE=1 vkcube
+