Subzero - Fast code generator for PNaCl bitcode =============================================== Design ------ See the accompanying DESIGN.rst file for a more detailed technical overview of Subzero. Building -------- Subzero is set up to be built within the Native Client tree. Follow the `Developing PNaCl `_ instructions, in particular the section on building PNaCl sources. This will prepare the necessary external headers and libraries that Subzero needs. Checking out the Native Client project also gets the pre-built clang and LLVM tools in ``native_client/../third_party/llvm-build/Release+Asserts/bin`` which are used for building Subzero. The Subzero source is in ``native_client/toolchain_build/src/subzero``. From within that directory, ``git checkout master && git pull`` to get the latest version of Subzero source code. The Makefile is designed to be used as part of the higher level LLVM build system. To build manually, use the ``Makefile.standalone``. There are several build configurations from the command line:: make -f Makefile.standalone make -f Makefile.standalone DEBUG=1 make -f Makefile.standalone NOASSERT=1 make -f Makefile.standalone DEBUG=1 NOASSERT=1 make -f Makefile.standalone MINIMAL=1 make -f Makefile.standalone ASAN=1 make -f Makefile.standalone TSAN=1 ``DEBUG=1`` builds without optimizations and is good when running the translator inside a debugger. ``NOASSERT=1`` disables assertions and is the preferred configuration for performance testing the translator. ``MINIMAL=1`` attempts to minimize the size of the translator by compiling out everything unnecessary. ``ASAN=1`` enables AddressSanitizer, and ``TSAN=1`` enables ThreadSanitizer. The result of the ``make`` command is the target ``pnacl-sz`` in the current directory. Building within LLVM trunk -------------------------- Subzero can also be built from within a standard LLVM trunk checkout. Here is an example of how it can be checked out and built:: mkdir llvm-git cd llvm-git git clone http://llvm.org/git/llvm.git cd llvm/projects/ git clone https://chromium.googlesource.com/native_client/pnacl-subzero cd ../.. mkdir build cd build cmake -G Ninja ../llvm/ ninja ./bin/pnacl-sz -version This creates a default build of ``pnacl-sz``; currently any options such as ``DEBUG=1`` or ``MINIMAL=1`` have to be added manually. ``pnacl-sz`` ------------ The ``pnacl-sz`` program parses a pexe or an LLVM bitcode file and translates it into ICE (Subzero's intermediate representation). It then invokes the ICE translate method to lower it to target-specific machine code, optionally dumping the intermediate representation at various stages of the translation. The program can be run as follows:: ../pnacl-sz ./path/to/.pexe ../pnacl-sz ./tests_lit/pnacl-sz_tests/.ll At this time, ``pnacl-sz`` accepts a number of arguments, including the following: ``-help`` -- Show available arguments and possible values. (Note: this unfortunately also pulls in some LLVM-specific options that are reported but that Subzero doesn't use.) ``-notranslate`` -- Suppress the ICE translation phase, which is useful if ICE is missing some support. ``-target=`` -- Set the target architecture. The default is x8632. Future targets include x8664, arm32, and arm64. ``-filetype=obj|asm|iasm`` -- Select the output file type. ``obj`` is a native ELF file, ``asm`` is a textual assembly file, and ``iasm`` is a low-level textual assembly file demonstrating the integrated assembler. ``-O`` -- Set the optimization level. Valid levels are ``2``, ``1``, ``0``, ``-1``, and ``m1``. Levels ``-1`` and ``m1`` are synonyms, and represent the minimum optimization and worst code quality, but fastest code generation. ``-verbose=`` -- Set verbosity flags. This argument allows a comma-separated list of values. The default is ``none``, and the value ``inst,pred`` will roughly match the .ll bitcode file. Of particular use are ``all``, ``most``, and ``none``. ``-o `` -- Set the assembly output file name. Default is stdout. ``-log `` -- Set the file name for diagnostic output (whose level is controlled by ``-verbose``). Default is stdout. ``-timing`` -- Dump some pass timing information after translating the input file. Running the test suite ---------------------- Subzero uses the LLVM ``lit`` testing tool for part of its test suite, which lives in ``tests_lit``. To execute the test suite, first build Subzero, and then run:: make -f Makefile.standalone check-lit There is also a suite of cross tests in the ``crosstest`` directory. A cross test takes a test bitcode file implementing some unit tests, and translates it twice, once with Subzero and once with LLVM's known-good ``llc`` translator. The Subzero-translated symbols are specially mangled to avoid multiple definition errors from the linker. Both translated versions are linked together with a driver program that calls each version of each unit test with a variety of interesting inputs and compares the results for equality. The cross tests are currently invoked by running:: make -f Makefile.standalone check-xtest Similar, there is a suite of unit tests:: make -f Makefile.standalone check-unit A convenient way to run the lit, cross, and unit tests is:: make -f Makefile.standalone check Assembling ``pnacl-sz`` output as needed ---------------------------------------- ``pnacl-sz`` can now produce a native ELF binary using ``-filetype=obj``. ``pnacl-sz`` can also produce textual assembly code in a structure suitable for input to ``llvm-mc``, using ``-filetype=asm`` or ``-filetype=iasm``. An object file can then be produced using the command:: llvm-mc -triple=i686 -filetype=obj -o=MyObj.o Building a translated binary ---------------------------- There is a helper script, ``pydir/szbuild.py``, that translates a finalized pexe into a fully linked executable. Run it with ``-help`` for extensive documentation. By default, ``szbuild.py`` builds an executable using only Subzero translation, but it can also be used to produce hybrid Subzero/``llc`` binaries (``llc`` is the name of the LLVM translator) for bisection-based debugging. In bisection debugging mode, the pexe is translated using both Subzero and ``llc``, and the resulting object files are combined into a single executable using symbol weakening and other linker tricks to control which Subzero symbols and which ``llc`` symbols take precedence. This is controlled by the ``-include`` and ``-exclude`` arguments. These can be used to rapidly find a single function that Subzero translates incorrectly leading to incorrect output. There is another helper script, ``pydir/szbuild_spec2k.py``, that runs ``szbuild.py`` on one or more components of the Spec2K suite. This assumes that Spec2K is set up in the usual place in the Native Client tree, and the finalized pexe files have been built. (Note: for working with Spec2K and other pexes, it's helpful to finalize the pexe using ``--no-strip-syms``, to preserve the original function and global variable names.) Status ------ Subzero currently fully supports the x86-32 architecture, for both native and Native Client sandboxing modes. The x86-64 architecture is also supported in native mode only, and only for the x32 flavor due to the fact that pointers and 32-bit integers are indistinguishable in PNaCl bitcode. Sandboxing support for x86-64 is in progress. ARM and MIPS support is in progress. Two optimization levels, ``-Om1`` and ``-O2``, are implemented. The ``-Om1`` configuration is designed to be the simplest and fastest possible, with a minimal set of passes and transformations. * Simple Phi lowering before target lowering, by generating temporaries and adding assignments to the end of predecessor blocks. * Simple register allocation limited to pre-colored or infinite-weight Variables. The ``-O2`` configuration is designed to use all optimizations available and produce the best code. * Address mode inference to leverage the complex x86 addressing modes. * Compare/branch fusing based on liveness/last-use analysis. * Global, linear-scan register allocation. * Advanced phi lowering after target lowering and global register allocation, via edge splitting, topological sorting of the parallel moves, and final local register allocation. * Stack slot coalescing to reduce frame size. * Branch optimization to reduce the number of branches to the following block.