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Comment: Added things to the "Building" section
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Editor: BerndSchmidt
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 * In-tree testing is not supported yet with the scheme described above. I (Bernd) think that what should happen is that the host compiler configure step copies the already installed accel/ subdirectory from the install location to its build directory.

OpenMP 4.0 and OpenACC offloading support in GCC

Terminology

Host compiler — a regular compiler. Not to be confused with build/host/target configure terms.

Accel compiler — a compiler that reads intermediate representation from the special LTO sections, and generates code for the accelerator device. Also called the "offload compiler".

Building host and accel compilers

The host and offload compilers need to be able to find each other. This is achieved by installing the offload compiler into special locations, and informing each about the presence of the other. All available offload compilers must first be configured with "--enable-as-accelerator-for=host-triplet", and installed into the same prefix as the host compiler. Then the host compiler is built with "--enable-offload-targets=target1,target2,..." which identifies the offload compilers that have already been built and installed.

The install locations for the offload compilers differ from those of a normal cross toolchain, by the following mapping:

bin/$target-gcc

->

bin/$host-accel-$target-gcc

lib/gcc/$target/$ver/

->

lib/gcc/$host/$ver/accel/$target

It may be necessary to compile offload compilers with a sysroot, since otherwise install locations for libgomp could clash (maybe that library needs to move into lib/gcc/..?)

A target needs to provide a mkoffload tool if it wishes to be usable as an accelerator. It is installed as one of EXTRA_PROGRAMS, and the host lto-wrapper knows how to find it from the paths described above. mkoffload will invoke the offload compiler in LTO mode to produce an offload binary from the host object files, then post-process this to produce a new object file that can be linked in with the host executable. It can find the host compiler by examining the COLLECT_GCC environment variable, and it must take care to clear this and certain other environment variables when executing the offload compiler so as to not confuse it.

The object file produced with mkoffload should contain a constructor that calls GOMP_offload_register to identify itself at run-time. Arguments to that function are a symbol called __OPENMP_TARGET__ (provided by libgcc and unique per shared object), a table with information about mappings between host and offload functions and variables, and a target identifier.

Compilation process

Currently offloading works only with -flto.

Host compiler performs the following actions:

  1. After #pragma omp target lowering and expansion, a new outlined function with the attribute "omp declare target" emerges — it will be later compiled both by host and accel compilers to produce two versions (or N+1 versions in case of N different accel targets).

  2. The expansion phase replaces pragmas with corresponding calls to the runtime library libgomp. These calls are preceded by initialization of special structures, containing arguments for outlined functions.

  3. During the ipa_write_summaries pass the intermediate representation of outlined functions is streamed out into the .gnu.target_lto_* sections of the "fat" object file. This object file also may contain .gnu.lto_* sections for the regular link-time optimizations.

  4. When all source files are compiled, pre-linker driver collect2 is invoked. If linker plugin is available, collect2 runs the linker, which loads liblto_plugin.so, which runs lto-wrapper. In case if linker plugin is not available, collect2 runs lto-wrapper directly.

  5. lto-wrapper runs mkoffload for each accel target, specified during the configuration.

  6. mkoffload runs accel compiler, which reads IR from the .gnu.target_lto_* sections and compiles it for the accel target. Then mkoffload packs this target code (image) into the special section of a new host's object file. Also this object file has a constructor, which calls GOMP_offload_register from libgomp. The arguments are: type of the accelerator, pointers to the tables with addresses, and some other data needed by this target, like a pointer to the image.

  7. Linker adds new object files, produced by mkoffloads, to the list of host's input object files.

Compilation with offloading using linker plugin:

  • gcc

    • cc1 # Compile first source file into plain asm + intermediate representation + IR for accel

    • as # Assemble this asm + IR into temporary object file

    • ... # Compile and assemble all remaining source files

    • collect2 # Pre-linker driver

      • collect-ld # Simple wrapper over ld

        • ld with liblto_plugin.so # Perform linking

          • lto-wrapper # Is called from liblto_plugin.so

            • gcc

              • lto1 # Perform whole program analysis and split into new partitions

            • gcc

              • lto1 # Perform local transformations in the first object file

              • as # Assemble into final object code

            • ... # Perform local transformations in each partitioned object file

            • intelmic/mkoffload # Prepare offload image for Intel MIC devices

              • accel_gcc # Read target IR from all partitions and produce target DSO

              • objcopy # Save target DSO in a special section in a new host's object file

            • .../mkoffload # Prepare images for other targets

              • ...

Compilation with offloading without linker plugin:

  • gcc

    • cc1 # Compile first source file into plain asm + intermediate representation + IR for accel

    • as # Assemble this asm + IR into temporary object file

    • ... # Compile and assemble all remaining source files

    • collect2 # Pre-linker driver

      • lto-wrapper # Run lto-wrapper directly from collect2

        • gcc

          • lto1 # Perform whole program analysis and split into new partitions

        • gcc

          • lto1 # Perform local transformations in the first object file

          • as # Assemble into final object code

        • ... # Perform local transformations in each partitioned object file

        • intelmic/mkoffload # Prepare offload image for Intel MIC devices

          • accel_gcc # Read target IR from all partitions and produce target DSO

          • objcopy # Save target DSO in a special section in a new host's object file

        • .../mkoffload # Prepare images for other targets

          • ...

      • collect-ld # Simple wrapper over ld

        • ld # Perform final linking

Runtime support in libgomp

libgomp plugins

libgomp is designed to be independent of accelerator type it work with. In order to make it possible, plugins are used, while the libgomp itself contains only a generic interface and callbacks to the plugin for invoking target-dependent functionality. Plugins are shared object, implementing a set of routines:

get_type
get_num_devices
offload_register
device_init
device_get_table
device_alloc
device_free
device_dev2host
device_host2dev
device_run

When required, libgomp performs a search in the plugins directory for the plugins named libgomp-plugin-*.so.1 and for each found file checks whether it is a proper plugin. The plugins use target-dependent libraries and perform low-level interaction with the accel devices. E.g., the plugin for Intel MIC devices uses liboffloadmic for implementing libgomp callbacks.

Address translation

TODO: Describe address translation.

Execution process

When an executable or dynamic shared object is loaded, it calls GOMP_offload_register N times, where N is number of accel images, embedded into this exec/dso. This function stores the pointers to the images and other data needed by accel plugin into offload_images.

The first call to GOMP_target, GOMP_target_data or GOMP_target_update performs corresponding device initialization: it calls device_init from the plugin, and then stores address mapping table in the splay tree.

In case of Intel MIC, device_init creates a new process on the device, and then offloads the accel images with the type == TARGET_TYPE_INTEL_MIC. All accel images, even inside the executable, represent dynamic shared objects, which are loaded into the newly created process.

How to try offloading enabled GCC

Currently all necessary changes could be found in a git branch named kyukhin/gomp4-offload. It contains set of patches from gomp-4_0-branch, some non-committed patches, libgomp plugin, Intel MIC runtime offload library liboffloadmic and an emulator. This emulator lies under liboffloadmic and reproduces MIC's HW and SW stack behavior allowing to run offloaded code in a separate address space using the host machine.

1. Building accel compiler:

/gcc_src/configure --build=x86_64-intelmic-linux-gnu --host=x86_64-intelmic-linux-gnu --target=x86_64-intelmic-linux-gnu --enable-as-accelerator-for=x86_64-pc-linux-gnu --enable-liboffloadmic=target --prefix=/install target_configargs="--enable-version-specific-runtime-libs"
make
make install

2. Building host compiler:

export LIBOFFLOAD_TARGET_PATH="/install/lib/gcc/x86_64-intelmic-linux-gnu/4.10.0/"
/gcc_src/configure --build=x86_64-pc-linux-gnu --host=x86_64-pc-linux-gnu --target=x86_64-pc-linux-gnu --enable-offload-targets=x86_64-intelmic-linux-gnu --enable-liboffloadmic=host --prefix=/install
make
make install

3. Building an application:

/install/bin/gcc -fopenmp -flto test.c

4. Running an application using the emulator:

export LIBGOMP_PLUGIN_PATH="/gcc_build_host/x86_64-pc-linux-gnu/libgomp/plugins/intelmic"
export LD_LIBRARY_PATH="/install/lib64/"
export MIC_LD_LIBRARY_PATH="/install/lib/gcc/x86_64-intelmic-linux-gnu/4.10.0/"
./a.out

Running 'make check' (run tests using the emulator):

configure, make and *install* accel compiler (see #1)
configure and make host compiler (see #2)
export COMPILER_PATH="/install/libexec/gcc/x86_64-pc-linux-gnu/4.10.0"
export LIBGOMP_PLUGIN_PATH="/gcc_build_host/x86_64-pc-linux-gnu/libgomp/plugins/intelmic"
export LD_LIBRARY_PATH="/gcc_build_host/x86_64-pc-linux-gnu/liboffloadmic/.libs"
export MIC_LD_LIBRARY_PATH="/install/lib/gcc/x86_64-intelmic-linux-gnu/4.10.0/"
cd gcc_build_host
make check-target-libgomp

Known issues

  • Do not delete build dir of the host compiler, since make install for libgomp plugin is not working yet.

  • Also there might be an issue with building libgomp plugin when using relative path to the configure.

  • In-tree testing is not supported yet with the scheme described above. I (Bernd) think that what should happen is that the host compiler configure step copies the already installed accel/ subdirectory from the install location to its build directory.

None: Offloading (last edited 2019-02-08 18:42:03 by CatherineMoore)