The open-source x264 program does support OpenCL acceleration -- when building x264 it will check for the presence of OpenCL development support and then at runtime the --opencl switch must be passed for exploiting the potential of any OpenCL hardware. The x264 test profile part of the Phoronix Test Suite is strictly intended for CPU-based testing so this weekend I added a x264-opencl test profile that uses the same revision of x264 and the same media file, but the only difference is that it forces OpenCL support. So now with the Phoronix Test Suite it's as easy as running phoronix-test-suite benchmark x264 x264-opencl to run the CPU-bound x264 and the OpenCL version for easy comparison purposes.
Using the Intel Core i7 4790K with an Ubuntu 14.10 development build I compiled the latest Beignet code and checked to see if the x264 OpenCL support was of any benefit to this high-end Haswell processor with HD Graphics 4600...
Long story short, the OpenCL-enabled version was slower for Intel Beignet on the i7-4790K. That result file with more details is 1408310-LI-X264BENCH50.
From the Core i7 4790K I also uploaded some other Intel Beignet OpenCL results to 1408317-LI-DEVILCANY02. For this Intel Devil's Canyon Haswell system, LuxMark was failing to run in this configuration but the test did run on the Ivy Bridge Core i7 system tested in the earlier article, but at least other Phoronix Test Suite OpenCL tests were executing successfully.