Greetings VOLK enthusiasts, The last remaining frontier for VOLK development: Dealing automatically with buffer alignment, head cases, and tail cases. Here is my attempt... All work can be found on the volk_work branch of my github https://github.com/guruofquality/gnuradio/tree/volk_work
--------------------------------------------------------------------------- -- Combining unaligned and aligned kernels --------------------------------------------------------------------------- So, A VOLK kernel is a computational function with one or more implementations of this function. Each implementation has a unique name and requirements for SIMD instructions sets and alignment. Currently, most VOLK kernels are actually split into two kernels, one called aligned and the other called unaligned... based on the ability for an implementation to handle randomly aligned pointers, or not. One of the things this branch does is combine those two separate kernels into one. What was bar_u.h and bar_a.h becomes bar.h. Here is the break down: 1) A kernel is now completely in one file, not two. After-all, its conceptually really just *one* kernel. Doing so lets us take advantage of potential common code. For example, we were copying the generic C implementation of the kernel and copying it twice or bar_a and bar_u. 2) The API need not be broken. Since the build system has a concept of implementations that do and do not require alignment, it can still provide separate function pointers for specifically aligned and unaligned implementations. So kernel bar.h provides bar() which is new; and the familiar bar_a(), and bar_u() 3) The build system can automatically dispatch based on alignment. If all of the pointers are on alignment boundaries, bar() will call bar_a(), otherwise, bar() will call bar_u(). Doing this, I was able to remove all of the conditional dispatching based on alignment inside of the various gnuradio work functions that were calling into volk. In other words, VOLK itself does the dirty work. 4) No tedious code shuffling required. This handy python script converts and combines all the old implementations automatically! https://github.com/guruofquality/gnuradio/blob/volk_work/volk/convert_kernels.py --------------------------------------------------------------------------- -- Custom dispatchers --------------------------------------------------------------------------- Now that unaligned and aligned kernels are conceptually together, the user can also implement custom dispatchers for a kernel within the same space as the kernel implementations. Why write a dispatcher? Well, the automatic behaviour has limitations. For example, suppose that one of the pointers is an accumulator and you dont care about its alignment constraints. A custom dispatcher also means that the user can now implement head and tail cases. Head cases mean that we can attempt to recover alignment when starting with unaligned pointers. And tail cases means that the kernel implementations themselves dont have to copy-paste the generic implementation to deal with remainder num_points. The following README contains two working examples of custom dispatchers: https://github.com/guruofquality/gnuradio/blob/volk_work/volk/kernels/README.txt --------------------------------------------------------------------------- -- GNU Radio considerations --------------------------------------------------------------------------- The VOLK changes should be API compatible, so we could change nothing. Usage of volk in work functions - with these changes its no longer necessary to perform the dispatching inside of the work function. This should be conceptually appealing at the very least. Usage of set_alignment_multiple - I suspect that we would still want the re-alignment aspect of this setting. That is, after handling an edge case, the scheduler tries to get back to a state of buffer alignment. On the other hand, if the author of the kernel writes a custom dispatcher that handles the head case, then there is nothing to gain by manipulating the scheduler in this way. Note: ignore the changes to gnuradio (outside of volk) on the volk_work branch. Those changes should be obviously, left to be decided. --------------------------------------------------------------------------- As always, feedback is welcome! -josh _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
