Gallium LLVMpipe Driver

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 8 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 3.4 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 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.

  • scons (optional)

Building

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

Using

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.

For performance evaluation pass build=release to scons, and use the corresponding lib directory without the -debug suffix.

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. 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):

  • copy build/windows-x86-debug/gallium/targets/libgl-gdi/opengl32.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

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.

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.