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

Building for Android

To build for 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 that 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 script below can be used as a reference to configure LLVM to build with the NDK for x86. You need to set the ANDROID_NDK_ROOT, ANDROID_SDK_VERSION and LLVML_INSTALL_PREFIX environment variables appropriately.

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 the following script will create the subprojects/llvm/meson.build wrap file, after setting LLVM_INSTALL_PREFIX to the path where LLVM was installed to.

The list of libraries passed in dep_llvm below should match what it was produced by the LLVM build from above.

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, use GALLIUM_OVERRIDE_CPU_CAPS=nosse instead.

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:

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: 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. See the llvm-c/Core.h file for reference.