Gallium LLVMpipe Driver¶
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 8 at this time). It’s the fastest software rasterizer for Mesa.
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.
/proc/cpuinfoto know what your CPU supports.
Unless otherwise stated, LLVM version 3.4 is recommended; 3.3 or later is required.
For Linux, on a recent Debian based distribution do:
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, which are maintained by Debian’s LLVM maintainer.
For a RPM-based distribution do:
yum install llvm-devel
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
LLVMenvironment 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=yyyas described below.
You can build only the x86 target by passing
To build everything on Linux invoke scons as:
scons build=debug libgl-xlib
Alternatively, you can build it with meson with:
mkdir build cd build meson -D glx=gallium-xlib -D gallium-drivers=swrast ninja
but the rest of these instructions assume that scons is used. For Windows the procedure is similar except the target:
scons platform=windows build=debug libgl-gdi
On Linux, building will create a drop-in alternative for
To use it set the
LD_LIBRARY_PATH environment variable accordingly.
For performance evaluation pass
build=release to scons, and use the
corresponding lib directory without the
On Windows, building will create
is a drop-in alternative for system’s
opengl32.dll. To use it put it
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):
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.
To profile llvmpipe you should build as
scons build=profile <same-as-before>
This will ensure that frame pointers are used both in C and JIT functions, and that no tail call optimizations are done by gcc.
Linux perf integration¶
On Linux, it is possible to have symbol resolution of JIT code with Linux perf:
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.
Building will also create several unit tests in
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
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
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. See the
llvm-c/Core.hfile for reference.