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

[Carlos Sánchez de La Lama]

Hi Ralf,

> 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).

Quite interesting. We were planning to add "vectorization" to our passes also, but if I understood the paper correctly your approach uses full speculation, which is all right for SIMD architectures but might not be so for multi-issue processors. The pocl project comes from a generalization of our work in using OpenCL as entry language for static ILP architectures (http://dx.doi.org/10.1109/ICSAMOS.2010.5642061), so we do not only focus on DLP but also ILP (ultimate goal of pocl is portable performance of OpenCL code among different platforms).

Our idea was to use an hybrid method, with vectorized code in the unconditional sections and replicated/looped as we do now on the conditional parts of the kernels.

> 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).

That is wrong, at least for pocl. We do not (by far) support the whole API, the main new point on pocl is the LLVM passes to statically create the different work items in a workgroup, and the barrier handling. Our kernel runtime library is currently in fact fairly small, including just a little more then the implementation-dependent functions. We are considering merging efforts with liblcl in that point.

BR,

Carlos

> 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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