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:
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
If you want development snapshot builds of LLVM for Fedora, you can use the Copr repository at 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:
mkdir build
cd build
meson -D glx=xlib -D gallium-drivers=swrast
ninja
To build on Android requires the additional step of building LLVM
for Android using the NDK. Before following the steps in
Android’s documentation you must build a version
of LLVM that targets the NDK with all the required libraries for
llvmpipe, and then create a wrap file so the meson knows where to
find the LLVM libraries. It can be a bit tricky to get LLVM to build
properly using the Android NDK, so the below cmake command can be
used as a reference to configure LLVM to build with the NDK for x86.
You need to set the ANDROID_NDK_ROOT
and INSTALL_PREFIX
environment variable appropriately.
cmake ../llvm-project-18.1.1.src/llvm \
-DCMAKE_TOOLCHAIN_FILE=${ANDROID_NDK_ROOT}/build/cmake/android.toolchain.cmake \
-DANDROID_ABI=x86_64 \
-DANDROID_PLATFORM=android-23 \
-DANDROID_NDK=${ANDROID_NDK_ROOT} \
-DCMAKE_ANDROID_ARCH_ABI=x86_64 \
-DCMAKE_ANDROID_NDK=${ANDROID_NDK_ROOT} \
-DCMAKE_BUILD_TYPE=MinSizeRel \
-DCMAKE_SYSTEM_NAME=Android \
-DCMAKE_SYSTEM_VERSION=23 \
-DCMAKE_INSTALL_PREFIX=${INSTALL_PREFIX} \
-DCMAKE_CXX_FLAGS='-march=x86-64 --target=x86_64-linux-android23 -fno-rtti' \
-DLLVM_HOST_TRIPLE=x86_64-linux-android23 \
-DLLVM_TARGETS_TO_BUILD=X86 \
-DLLVM_BUILD_LLVM_DYLIB=OFF \
-DLLVM_BUILD_TESTS=OFF \
-DLLVM_BUILD_EXAMPLES=OFF \
-DLLVM_BUILD_DOCS=OFF \
-DLLVM_BUILD_TOOLS=OFF \
-DLLVM_ENABLE_RTTI=OFF \
-DLLVM_BUILD_INSTRUMENTED_COVERAGE=OFF \
-DLLVM_NATIVE_TOOL_DIR=${ANDROID_NDK_ROOT}toolchains/llvm/prebuilt/linux-x86_64/bin \
-DLLVM_ENABLE_PIC=False
make -j$(nproc) install
You will also need to create a wrap file, so that meson is able
to find the LLVM libraries built with the NDK. The process for this
is described in meson documentation. For example a
file like this would work subprojects/llvm/meson.build
where
INSTALL_PREFIX
is replaced with the path LLVM was installed to.
project('llvm', ['cpp'])
cpp = meson.get_compiler('cpp')
_deps = []
_search = join_paths('$INSTALL_PREFIX', 'lib')
foreach d: ['libLLVMAggressiveInstCombine', 'libLLVMAnalysis', 'libLLVMAsmParser', 'libLLVMAsmPrinter', 'libLLVMBinaryFormat', 'libLLVMBitReader', 'libLLVMBitstreamReader', 'libLLVMBitWriter', 'libLLVMCFGuard', 'libLLVMCFIVerify', 'libLLVMCodeGen', 'libLLVMCodeGenTypes', 'libLLVMCore', 'libLLVMCoroutines', 'libLLVMCoverage', 'libLLVMDebugInfoBTF', 'libLLVMDebugInfoCodeView', 'libLLVMDebuginfod', 'libLLVMDebugInfoDWARF', 'libLLVMDebugInfoGSYM', 'libLLVMDebugInfoLogicalView', 'libLLVMDebugInfoMSF', 'libLLVMDebugInfoPDB', 'libLLVMDemangle', 'libLLVMDiff', 'libLLVMDlltoolDriver', 'libLLVMDWARFLinker', 'libLLVMDWARFLinkerClassic', 'libLLVMDWARFLinkerParallel', 'libLLVMDWP', 'libLLVMExecutionEngine', 'libLLVMExegesis', 'libLLVMExegesisX86', 'libLLVMExtensions', 'libLLVMFileCheck', 'libLLVMFrontendDriver', 'libLLVMFrontendHLSL', 'libLLVMFrontendOffloading', 'libLLVMFrontendOpenACC', 'libLLVMFrontendOpenMP', 'libLLVMFuzzerCLI', 'libLLVMFuzzMutate', 'libLLVMGlobalISel', 'libLLVMHipStdPar', 'libLLVMInstCombine', 'libLLVMInstrumentation', 'libLLVMInterfaceStub', 'libLLVMInterpreter', 'libLLVMipo', 'libLLVMIRPrinter', 'libLLVMIRReader', 'libLLVMJITLink', 'libLLVMLibDriver', 'libLLVMLineEditor', 'libLLVMLinker', 'libLLVMLTO', 'libLLVMMC', 'libLLVMMCA', 'libLLVMMCDisassembler', 'libLLVMMCJIT', 'libLLVMMCParser', 'libLLVMMIRParser', 'libLLVMObjCARCOpts', 'libLLVMObjCopy', 'libLLVMObject', 'libLLVMObjectYAML', 'libLLVMOption', 'libLLVMOrcDebugging', 'libLLVMOrcJIT', 'libLLVMOrcShared', 'libLLVMOrcTargetProcess', 'libLLVMPasses', 'libLLVMProfileData', 'libLLVMRemarks', 'libLLVMRuntimeDyld', 'libLLVMScalarOpts', 'libLLVMSelectionDAG', 'libLLVMSupport', 'libLLVMSymbolize', 'libLLVMTableGen', 'libLLVMTableGenCommon', 'libLLVMTableGenGlobalISel', 'libLLVMTarget', 'libLLVMTargetParser', 'libLLVMTextAPI', 'libLLVMTextAPIBinaryReader', 'libLLVMTransformUtils', 'libLLVMVectorize', 'libLLVMWindowsDriver', 'libLLVMWindowsManifest', 'libLLVMX86AsmParser', 'libLLVMX86CodeGen', 'libLLVMX86Desc', 'libLLVMX86Disassembler', 'libLLVMX86Info', 'libLLVMX86TargetMCA', 'libLLVMXRay']
_deps += cpp.find_library(d, dirs : _search)
endforeach
dep_llvm = declare_dependency(
include_directories : include_directories('$INSTALL_PREFIX/include'),
dependencies : _deps,
version : '6.0.0',
)
has_rtti = false
irbuilder_h = files('$INSTALL_PREFIX/include/llvm/IR/IRBuilder.h')
Afterwards you can continue following the instructors to build mesa on Android and follow the steps to add the driver directly to an Android OS image.
Using¶
Environment variables¶
- 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.
- GALLIUM_NOSSE¶
Deprecated in favor of
GALLIUM_OVERRIDE_CPU_CAPS
, useGALLIUM_OVERRIDE_CPU_CAPS=nosse
instead.
- LP_FORCE_SSE2¶
Deprecated in favor of
GALLIUM_OVERRIDE_CPU_CAPS
useGALLIUM_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
toC:\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:
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.
FlameGraph support¶
Outside Linux, it is possible to generate a FlameGraph with resolved JIT symbols.
Set the environment variable JIT_SYMBOL_MAP_DIR
to a directory path,
and run your LLVMpipe program. Follow the FlameGraph instructions:
capture traces using a supported tool (for example DTrace),
and fold the stacks using the associated script
(stackcollapse.pl
for DTrace stacks).
LLVMpipe will create a jit-symbols-XXXXX.map
file containing the symbol
address table inside the chosen directory. It will also dump the JIT
disassemblies to jit-symbols-XXXXX.map.asm
. Run your folded traces and
both output files through the bin/flamegraph_map_lp_jit.py
script to map
addresses to JIT symbols, and annotate the disassembly with the sample counts.
Unit testing¶
Building will also create several unit tests in
build/linux-???-debug/gallium/drivers/llvmpipe
:
lp_test_blend
: blendinglp_test_conv
: SIMD vector conversionlp_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 thelp_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 fromlp_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.h
file for reference.
Recommended Reading¶
Rasterization
Texture sampling
SIMD
Optimization
LLVM
General