[LLVMdev] ANN: libclc (OpenCL C library implementation)

Ralf Karrenberg Chareos at gmx.de
Wed Oct 19 14:13:16 PDT 2011

Hi everybody,

the compiler design lab at Saarland University (chair of Sebastian Hack) 
is also working on an LLVM-based OpenCL driver.

The project started as a use-case for our "Whole-Function Vectorization" 
library, which allows to transform a function to compute the same as W 
executions of the original code by using SIMD instructions (W = 4 for 
SSE/AltiVec, 8 for AVX).

The algorithm, a few details on the OpenCL driver, and some results were 
published at CGO'11:

In contrast to Clover and pocl, we aimed at maximum performance before 
full support of the API (which simply requires more manpower than one 
PhD student).
The driver was evaluated using benchmarks from the AMD APP SDK, e.g. 
Mandelbrot, NBody, FastWalshTransform, Histogram, BlackScholes, DCT, ...
Our measurements show that our driver outperforms both Intel's and AMD's 
CPU driver for almost all of the benchmarks.

We plan to release both the vectorization library as well as the OpenCL 
driver under a BSD-style license, but there is still significant work to 
be done in terms of stability and feature completeness (e.g. the ICD 
mechanism only works as expected under Windows).
Also, due to simplicity we currently employ AMD's OpenCL-to-LLVM 
frontend and OpenCL-library from an earlier SDK. libclc sounds like the 
perfect fit for us to get rid of that.

I agree on the fact that we should try to merge the different 
open-source OpenCL projects, but at least at first glance they do not 
seem to share too many design decisions.

More information on Whole-Function Vectorization and our driver can be 
found on the project web page:

Best regards,

Am 19.10.2011 17:38, schrieb Hal Finkel:
> Do we have a list of these open-source LLVM-based OpenCL projects
> somewhere? Off the top of my head, we have:
> libclc: http://www.pcc.me.uk/~peter/libclc/
> pocl: https://launchpad.net/pocl
> clover: http://cgit.freedesktop.org/~steckdenis/clover/
> (I think that all of these have BSD- or MIT-style licenses).
> Are there any others?
>   -Hal
> On Wed, 2011-10-19 at 14:47 +0100, Peter Collingbourne wrote:
>> Hi,
>> This is to announce the availability of libclc, an open source, BSD
>> licensed implementation of the library requirements of the OpenCL C
>> programming language, as specified by the OpenCL 1.1 Specification.
>> libclc is intended to be used with Clang's OpenCL frontend.
>> libclc website: http://www.pcc.me.uk/~peter/libclc/
>> libclc is designed to be portable and extensible. To this end,
>> it provides generic implementations of most library requirements,
>> allowing the target to override the generic implementation at the
>> granularity of individual functions.
>> libclc currently only supports the PTX target, but support for more
>> targets is welcome.
>> How does this project relate to the recently announced Portable OpenCL
>> (POCL) project?  Unlike POCL, this project is not intended to provide
>> an OpenCL host library (i.e. the OpenCL Platform Layer and OpenCL
>> Runtime specified in sections 4-5 of the OpenCL specification).
>> Instead, it provides only the requirements for the OpenCL C
>> Programming Language (section 6 et seq).  It is intended to be used
>> with an existing host library implementation, and comply with its
>> ABI requirements.
>> An example of such a host library is NVIDIA's OpenCL host library
>> for PTX -- the intention is to at some point provide a mechanism
>> for using the NVIDIA implementation of OpenCL with Clang, libclc
>> and LLVM's PTX backend instead of NVIDIA's own OpenCL compiler.
>> Another example would be POCL's host library, and the POCL developers
>> have expressed an interest in using libclc as their OpenCL C library
>> instead of developing their own.
>> I will hope to find time over the next few weeks to add libclc support
>> to the Clang driver.  The intention is that compiling OpenCL C programs
>> to PTX would be as easy as (something like this):
>> clang -target ptx32 -S file.cl
>> such that the driver would automatically locate the libclc headers,
>> add them to the include path and pre-include the main header file.
>> (The libclc support will of course be optional, and a -cl-stdlib=
>> flag will be provided to allow for switching between OpenCL standard
>> library implementations.)
>> Thanks,

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