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mesa/docs/drivers/llvmpipe.rst
Erik Faye-Lund dacaaa0b90 docs: keep up with intels ever-moving documentation 
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Reviewed-by: Corentin Noël <corentin.noel@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25585>
2023-10-09 11:54:12 +00:00

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LLVMpipe
========
Introduction
------------
The Gallium LLVMpipe driver is a software rasterizer that uses LLVM to
do runtime code generation. Shaders, point/line/triangle rasterization
and vertex processing are implemented with LLVM IR which is translated
to x86, x86-64, or ppc64le machine code. Also, the driver is
multithreaded to take advantage of multiple CPU cores (up to 32 at this
time). It's the fastest software rasterizer for Mesa.
Requirements
------------
- For x86 or amd64 processors, 64-bit mode is recommended. Support for
SSE2 is strongly encouraged. Support for SSE3 and SSE4.1 will yield
the most efficient code. The fewer features the CPU has the more
likely it is that you will run into underperforming, buggy, or
incomplete code.
For ppc64le processors, use of the Altivec feature (the Vector
Facility) is recommended if supported; use of the VSX feature (the
Vector-Scalar Facility) is recommended if supported AND Mesa is built
with LLVM version 4.0 or later.
See ``/proc/cpuinfo`` to know what your CPU supports.
- Unless otherwise stated, LLVM version 3.9 or later is required.
For Linux, on a recent Debian based distribution do:
.. code-block:: console
aptitude install llvm-dev
If you want development snapshot builds of LLVM for Debian and
derived distributions like Ubuntu, you can use the APT repository at
`apt.llvm.org <https://apt.llvm.org/>`__, which are maintained by
Debian's LLVM maintainer.
For a RPM-based distribution do:
.. code-block:: console
yum install llvm-devel
If you want development snapshot builds of LLVM for Fedora, you can
use the Copr repository at `fedora-llvm-team/llvm-snapshots <https://copr.fedorainfracloud.org/coprs/g/fedora-llvm-team/llvm-snapshots/>`__,
which is maintained by Red Hat's LLVM team.
For Windows you will need to build LLVM from source with MSVC or
MINGW (either natively or through cross compilers) and CMake, and set
the ``LLVM`` environment variable to the directory you installed it
to. LLVM will be statically linked, so when building on MSVC it needs
to be built with a matching CRT as Mesa, and you'll need to pass
``-DLLVM_USE_CRT_xxx=yyy`` as described below.
+-----------------+----------------------------------------------------------------+
| LLVM build-type | Mesa build-type |
| +--------------------------------+-------------------------------+
| | debug,checked | release,profile |
+=================+================================+===============================+
| Debug | ``-DLLVM_USE_CRT_DEBUG=MTd`` | ``-DLLVM_USE_CRT_DEBUG=MT`` |
+-----------------+--------------------------------+-------------------------------+
| Release | ``-DLLVM_USE_CRT_RELEASE=MTd`` | ``-DLLVM_USE_CRT_RELEASE=MT`` |
+-----------------+--------------------------------+-------------------------------+
You can build only the x86 target by passing
``-DLLVM_TARGETS_TO_BUILD=X86`` to CMake.
Building
--------
To build everything on Linux invoke meson as:
.. code-block:: console
mkdir build
cd build
meson -D glx=xlib -D gallium-drivers=swrast
ninja
Using
-----
Environment variables
~~~~~~~~~~~~~~~~~~~~~
.. envvar:: LP_NATIVE_VECTOR_WIDTH
We can use it to override vector bits. Because sometimes it turns
out LLVMpipe can be fastest by using 128 bit vectors,
yet use AVX instructions.
.. envvar:: GALLIUM_NOSSE
Deprecated in favor of ``GALLIUM_OVERRIDE_CPU_CAPS``,
use ``GALLIUM_OVERRIDE_CPU_CAPS=nosse`` instead.
.. envvar:: LP_FORCE_SSE2
Deprecated in favor of ``GALLIUM_OVERRIDE_CPU_CAPS``
use ``GALLIUM_OVERRIDE_CPU_CAPS=sse2`` instead.
Linux
~~~~~
On Linux, building will create a drop-in alternative for ``libGL.so``
into
::
build/foo/gallium/targets/libgl-xlib/libGL.so
or
::
lib/gallium/libGL.so
To use it set the ``LD_LIBRARY_PATH`` environment variable accordingly.
Windows
~~~~~~~
On Windows, building will create
``build/windows-x86-debug/gallium/targets/libgl-gdi/opengl32.dll`` which
is a drop-in alternative for system's ``opengl32.dll``, which will use
the Mesa ICD, ``build/windows-x86-debug/gallium/targets/wgl/libgallium_wgl.dll``.
To use it put both DLLs in the same directory as your application. It can also
be used by replacing the native ICD driver, but it's quite an advanced usage, so if
you need to ask, don't even try it.
There is however an easy way to replace the OpenGL software renderer
that comes with Microsoft Windows 7 (or later) with LLVMpipe (that is,
on systems without any OpenGL drivers):
- copy
``build/windows-x86-debug/gallium/targets/wgl/libgallium_wgl.dll`` to
``C:\Windows\SysWOW64\mesadrv.dll``
- load this registry settings:
::
REGEDIT4
; https://technet.microsoft.com/en-us/library/cc749368.aspx
; https://www.msfn.org/board/topic/143241-portable-windows-7-build-from-winpe-30/page-5#entry942596
[HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Windows NT\CurrentVersion\OpenGLDrivers\MSOGL]
"DLL"="mesadrv.dll"
"DriverVersion"=dword:00000001
"Flags"=dword:00000001
"Version"=dword:00000002
- Ditto for 64 bits drivers if you need them.
Profiling
---------
Linux perf integration
~~~~~~~~~~~~~~~~~~~~~~
On Linux, it is possible to have symbol resolution of JIT code with
`Linux perf <https://perf.wiki.kernel.org/>`__:
::
perf record -g /my/application
perf report
When run inside Linux perf, LLVMpipe will create a
``/tmp/perf-XXXXX.map`` file with symbol address table. It also dumps
assembly code to ``/tmp/perf-XXXXX.map.asm``, which can be used by the
``bin/perf-annotate-jit.py`` script to produce disassembly of the
generated code annotated with the samples.
You can obtain a call graph via
`Gprof2Dot <https://github.com/jrfonseca/gprof2dot#linux-perf>`__.
Unit testing
------------
Building will also create several unit tests in
``build/linux-???-debug/gallium/drivers/llvmpipe``:
- ``lp_test_blend``: blending
- ``lp_test_conv``: SIMD vector conversion
- ``lp_test_format``: pixel unpacking/packing
Some of these tests can output results and benchmarks to a tab-separated
file for later analysis, e.g.:
::
build/linux-x86_64-debug/gallium/drivers/llvmpipe/lp_test_blend -o blend.tsv
Development Notes
-----------------
- When looking at this code for the first time, start in lp_state_fs.c,
and then skim through the ``lp_bld_*`` functions called there, and
the comments at the top of the ``lp_bld_*.c`` functions.
- The driver-independent parts of the LLVM / Gallium code are found in
``src/gallium/auxiliary/gallivm/``. The filenames and function
prefixes need to be renamed from ``lp_bld_`` to something else
though.
- We use LLVM-C bindings for now. They are not documented, but follow
the C++ interfaces very closely, and appear to be complete enough for
code generation. See `this stand-alone
example <https://npcontemplation.blogspot.com/2008/06/secret-of-llvm-c-bindings.html>`__.
See the ``llvm-c/Core.h`` file for reference.
.. _recommended_reading:
Recommended Reading
-------------------
- Rasterization
- `Triangle Scan Conversion using 2D Homogeneous
Coordinates <https://redirect.cs.umbc.edu/~olano/papers/2dh-tri/>`__
- `Rasterization on
Larrabee <https://www.drdobbs.com/parallel/rasterization-on-larrabee/217200602>`__
- `Rasterization using half-space
functions <http://web.archive.org/web/20110820052005/http://www.devmaster.net/codespotlight/show.php?id=17>`__
- `Advanced
Rasterization <http://web.archive.org/web/20140514220546/http://devmaster.net/posts/6145/advanced-rasterization>`__
- `Optimizing Software Occlusion
Culling <https://fgiesen.wordpress.com/2013/02/17/optimizing-sw-occlusion-culling-index/>`__
- Texture sampling
- `Perspective Texture
Mapping <https://chrishecker.com/Miscellaneous_Technical_Articles#Perspective_Texture_Mapping>`__
- `Texturing As In
Unreal <https://www.flipcode.com/archives/Texturing_As_In_Unreal.shtml>`__
- `Run-Time MIP-Map
Filtering <http://web.archive.org/web/20220709145555/http://www.gamasutra.com/view/feature/3301/runtime_mipmap_filtering.php>`__
- `Will "brilinear" filtering
persist? <https://alt.3dcenter.org/artikel/2003/10-26_a_english.php>`__
- `Trilinear
filtering <http://ixbtlabs.com/articles2/gffx/nv40-rx800-3.html>`__
- `Texture tiling and
swizzling <https://fgiesen.wordpress.com/2011/01/17/texture-tiling-and-swizzling/>`__
- SIMD
- `Whole-Function
Vectorization <https://compilers.cs.uni-saarland.de/projects/wfv/#pubs>`__
- Optimization
- `Optimizing Pixomatic For Modern x86
Processors <https://www.drdobbs.com/optimizing-pixomatic-for-modern-x86-proc/184405807>`__
- `Intel 64 and IA-32 Architectures Optimization Reference
Manual <https://www.intel.com/content/www/us/en/content-details/779559/intel-64-and-ia-32-architectures-optimization-reference-manual.html>`__
- `Software optimization
resources <https://www.agner.org/optimize/>`__
- `Intel Intrinsics
Guide <https://www.intel.com/content/www/us/en/docs/intrinsics-guide/index.html>`__
- LLVM
- `LLVM Language Reference
Manual <https://llvm.org/docs/LangRef.html>`__
- `The secret of LLVM C
bindings <https://npcontemplation.blogspot.com/2008/06/secret-of-llvm-c-bindings.html>`__
- General
- `A trip through the Graphics
Pipeline <https://fgiesen.wordpress.com/2011/07/09/a-trip-through-the-graphics-pipeline-2011-index/>`__
- `WARP Architecture and
Performance <https://learn.microsoft.com/en-us/windows/win32/direct3darticles/directx-warp#warp-architecture-and-performance>`__
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