# Usage

The basic usage of pocl should be as easy as any other OpenCL implementation.

While it is possible to link against pocl directly, the recommended way is to use the ICD interface.

## Linking your program with pocl through an icd loader

You can link your OpenCL program against an ICD loader. If your ICD loader is correctly configured to load pocl, your program will be able to use pocl. See the section below for more information about ICD and ICD loaders.

Example of compiling an OpenCL host program using the free ocl-icd loader:

    gcc example1.c -o example `pkg-config --libs --cflags OpenCL`

Example of compiling an OpenCL host program using the AMD ICD loader (no pkg-config support):

    gcc example1.c -o example -lOpenCL

## Installable client driver (ICD)

pocl is built with the ICD extensions of OpenCL by default. This allows you to have several OpenCL implementations concurrently on your computer, and select the one to use at runtime by selecting the corresponding cl_platform. ICD support can be disabled by adding the flag:

    -DENABLE_ICD=OFF

to the CMake invocation.

The ocl-icd ICD loader allows to use the OCL_ICD_VENDORS environment variable to specify a (non-standard) replacement for the /etc/OpenCL/vendors directory.

An ICD loader is an OpenCL library acting as a “proxy” to one of the various OpenCL implementations installed in the system. pocl does not provide an ICD loader itself, but NVidia, AMD, Intel, Khronos, and the free ocl-icd project each provides one.

- ocl-icd
- Khronos

## Linking your program directly with pocl

Passing the appropriate linker flags is enough to use pocl in your program. However, please bear in mind that:

The pkg-config tool is used to locate the libraries and headers in the installation directory.

Example of compiling an OpenCL host program against pocl using the pkg-config:

    gcc example1.c -o example `pkg-config --libs --cflags pocl`

In this link mode, your program will always require the pocl OpenCL library. It wont be able to run with another OpenCL implementation without recompilation.

## Using pocl on MacOSX

On MacOSX, you can either link your program directly with pocl or link through an ICD loader. If you use an ICD loader, Apple OpenCL implementation will be invisible, unless you use a wrapper library to expose the Apple OpenCL implementation as an ICD.

## Tuning pocl behavior with ENV variables

The behavior of pocl can be controlled with multiple environment variables listed below. The variables are helpful both when using and when developing pocl.

- POCL_AFFINITY

> Linux-only, specific to pthread driver. If set to 1, each thread of the pthread CPU driver sets its affinity to its index. This may be useful with very long running kernels, or when using subdevices (lets any idle cores enter deeper sleep). Defaults to 0 (most people don’t need this).

- POCL_BUILDING

> If set, the pocl helper scripts, kernel library and headers are searched first from the pocl build directory. Only has effect if ENABLE_POCL_BUILDING was enabled at build (by default it is).

- POCL_CACHE_DIR

> If this is set to an existing directory, pocl uses it as the cache directory for all compilation results. This allows reusing compilation results between pocl invocations. If this env is not set, then the default cache directory will be used, which is `$XDG_CACHE_HOME/pocl/kcache` (if set) or `$HOME/.cache/pocl/kcache/` on Unix-like systems.

- POCL_DEBUG

> Enables debug messages to stderr. This will be mostly messages from error condition checks in OpenCL API calls and Event/API timing information. Useful to e.g. distinguish between various reasons a call could return CL_INVALID_VALUE. If clock_gettime is available, messages will include a timestamp.

> The old way (setting POCL_DEBUG to 1) has been updated to support categories. Using this limits the amount of debug messages produced. Current options are: error,warning,general,memory,llvm,events,cache,locking,refcounts,timing,hsa,tce,all. Note: setting POCL_DEBUG to 1 still works and equals error+warning+general.

- POCL_DEBUG_LLVM_PASSES

> When set to 1, enables debug output from LLVM passes during optimization.

- POCL_DEVICES and POCL_x_PARAMETERS

> POCL_DEVICES is a space separated list of the device instances to be enabled. This environment variable is used for the following devices:

> - basic A minimalistic example device driver for executing
>     kernels on the host CPU. No multithreading.
> - pthread Native kernel execution on the host CPU with
>     threaded execution of work groups using pthreads.
> - ttasim Device that simulates a TTA device using the
>     TCE’s ttasim library. Enabled only if TCE libraries installed.
> - hsa* Uses HSA Runtime API to control HSA-compliant
>     kernel agents that support HSAIL finalization.

> If POCL_DEVICES is not set, one pthread device will be used. To specify parameters for drivers, the POCL_<drivername><instance>_PARAMETERS environment variable can be specified (where drivername is in uppercase). Example:

>     export POCL_DEVICES="pthread ttasim ttasim"
>     export POCL_TTASIM0_PARAMETERS="/path/to/my/machine0.adf"
>     export POCL_TTASIM1_PARAMETERS="/path/to/my/machine1.adf"

> Creates three devices, one CPU device with pthread multithreading and two TTA device simulated with the ttasim. The ttasim devices gets a path to the architecture description file of the tta to simulate as a parameter. POCL_TTASIM0_PARAMETERS will be passed to the first ttasim driver instantiated and POCL_TTASIM1_PARAMETERS to the second one.

- POCL_EXTRA_BUILD_FLAGS

> Adds the contents of the environment variable to all clBuildProgram() calls. E.g. `POCL_EXTRA_BUILD_FLAGS="-g -cl-opt-disable"` can be useful for force adding debug data all the built kernels to help debugging kernel issues with tools such as gdb or valgrind.

- POCL_IMPLICIT_FINISH

> Add an implicit call to clFinish afer every clEnqueue* call. Useful mostly for pocl internal development, and is enabled only if pocl is configured with `--enable-debug`.

- POCL_KERNEL_CACHE

> If this is set to 0 at runtime, kernel compilation files will be deleted at clReleaseProgram(). Note that it’s currently not possible for pocl to avoid interacting with LLVM via on-disk files, so pocl requires some disk space at least temporarily (at runtime).

- POCL_LEAVE_KERNEL_COMPILER_TEMP_FILES

> If this is set to 1, the kernel compiler cache/temporary directory that contains all the intermediate compiler files are left as it is. This will be handy for debugging

- POCL_MAX_PTHREAD_COUNT

> The maximum number of threads created for work group execution in the pthread device driver. The default is to determine this from the number of hardware threads available in the CPU.

- POCL_MAX_WORK_GROUP_SIZE

> Forces the maximum WG size returned by the device or kernel work group queries to be at most this number. For certain devices, this is can only be lower than their hardware limits.

- POCL_MEMORY_LIMIT

> Integer option, unit: gigabytes. Limits the total global memory size reported by pocl for the pthread/basic devices (this will also affect local/constant/max-alloc-size numbers, since these are derived from global mem size).

- POCL_OFFLINE_COMPILE

> Bool. When enabled(==1), some drivers will create virtual devices which are only good for creating pocl binaries. Requires those drivers to be compiled with support for compilation for those devices.

- POCL_VECTORIZER_REMARKS

> When set to 1, prints out remarks produced by the loop vectorizer of LLVM during kernel compilation.

- POCL_WORK_GROUP_METHOD

> The kernel compiler method to produce the work group functions from multiple work items. Legal values:

> > auto – Choose the best available method depending on the
> >   kernel and the work group size. Use POCL_FULL_REPLICATION_THRESHOLD=N to set the maximum local size for a work group to be replicated fully with ‘repl’. Otherwise, ‘loops’ is used.
> > loops – Create for-loops that execute the work items

> >   (under stabilization). The drawback is the need to save the thread contexts in arrays.

> >   The loops will be unrolled a certain number of times of which maximum can be controlled with POCL_WILOOPS_MAX_UNROLL_COUNT=N environment variable (default is to not perform unrolling).

> > loopvec – Create work-item for-loops (see ‘loops’) and execute
> >   the LLVM LoopVectorizer. The loops are not unrolled but the unrolling decision is left to the generic LLVM passes (the default).
> > repl – Replicate and chain all work items. This results
> >   in more easily scalarizable private variables, thus might avoid storing work-item context to memory. However, the code bloat is increased with larger WG sizes.

- POCL_SIGFPE_HANDLER

> Defaults to 1. If set to 0, pocl will not install the SIGFPE handler. See Known issues in pocl / things to be aware of

- POCL_STARTUP_DELAY

  Default 0. If set to an integer N > 0, libpocl will make a pause of N seconds once, when it’s loading. Useful e.g. to set up a LTTNG tracing session.

- POCL_TRACE_EVENT, POCL_TRACE_EVENT_OPT and POCL_TRACE_EVENT_FILTER

> If POCL_TRACE_EVENT is set to some tracer name, then all events will be traced automatically. Depending on the backend, traces may be output in different formats. POCL_TRACE_EVENT_FILTER is a comma separated list of string to indicate which event status should be filtered. For instance to trace complete and running events POCL_TRACE_EVENT_FILTER should be set to “complete,running”. Default behavior is to trace all events.

> > text – Basic text logger for each events state
> >   Use POCL_TRACE_EVENT_OPT=<file> to set the output file. If not specified, it defaults to pocl_trace_event.log
> > lttng – LTTNG tracepoint support. When activated, a lttng session
> >   must be started. The following tracepoints are available:
> >     - pocl_trace:ndrange_kernel -> Kernel execution
> >     - pocl_trace:read_buffer -> Read buffer
> >     - pocl_trace:write_buffer -> Write buffer
> >     - pocl_trace:copy_buffer -> Copy buffer
> >     - pocl_trace:map -> Map image/buffer
> >     - pocl_trace:command -> other commands

> >   For more information, please see lttng documentation: http://lttng.org/docs/#doc-tracing-your-own-user-application