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Extending VOLK

There are two primary routes for extending VOLK for your own use. The preferred route is by writing kernels and proto-kernels as part of this repository and sending patches upstream. The alternative is creating your own VOLK module. The information for modifying this repository will be useful in both cases.

## Modifying this repository

### Adding kernels

Adding kernels refers to introducing a new function to the VOLK API that is presumably a useful math function/operation. The first step is to create the file in volk/kernels/volk. Follow the naming scheme provided in the VOLK terms and techniques page. First create the generic protokernel.

The generic protokernel should be written in plain C using explicitly sized types from stdint.h or volk_complex.h when appropriate. volk_complex.h includes explicitly sized complex types for floats and ints. The name of the generic kernel should be volk_signature_from_file_generic. If multiple versions of the generic kernel exist then a description can be appended to generic_, but it is not required to use alignment flags in the generic protokernel name. It is required to surround the entire generic function with preprocessor ifdef fences on the symbol LV_HAVE_GENERIC.

Finally, add the kernel to the list of test cases in volk/lib/kernel_tests.h. Many kernels should be able to use the default test parameters, but if yours requires a lower tolerance, specific vector length, or other test parameters just create a new instance of volk_test_params_t for your kernel.

### Adding protokernels

The primary purpose of VOLK is to have multiple implementations of an operation tuned for a specific CPU architecture. Ideally there is at least one protokernel of each kernel for every architecture that VOLK supports. The pattern for protokernel naming is volk_kernel_signature_architecture_nick. The architecture should be one of the supported VOLK architectures. The nick is an optional name to distinguish between multiple implementations for a particular architecture.

Architecture specific protokernels can be written in one of three ways. The first approach should always be to use compiler intrinsic functions. The second and third approaches are using either in-line assembly or assembly with .S files. Both methods of writing assembly exist in VOLK and should yield equivalent performance; which method you might choose is a matter of opinion. Regardless of the actual method the public function should be declared in the kernel header surrounded by ifdef fences on the symbol that fits the architecture implementation.

#### Compiler Intrinsics

Compiler intrinsics should be treated as functions that map to a specific assembly instruction. Most VOLK kernels take the form of a loop that iterates through a vector. Form a loop that iterates on a number of items that is natural for the architecture and then use compiler intrinsics to do the math for your operation or algorithm. Include the appropriate header inside the ifdef fences, but before your protokernel declaration.

#### In-line Assembly

In-line assembly uses a compiler macro to include verbatim assembly with C code. The process of in-line assembly protokernels is very similar to protokernels based on intrinsics.

#### Assembly with .S files

To write pure assembly protokernels, first declare the function name in the kernel header file the same way as any other protokernel, but include the extern keyword. Second, create a file (one for each protokernel) in volk/kernels/volk/asm/$arch. Disassemble another protokernel and copy the disassembled code in to this file to bootstrap a working implementation. Often the disassembled code can be hand-tuned to improve performance.

## VOLK Modules

VOLK has a concept of modules. Each module is an independent VOLK tree. Modules can be managed with the volk_modtool application. At a high level the module is a clone of all of the VOLK machinery without kernels. volk_modtool also makes it easy to copy kernels to a module.

Kernels and protokernels are added to your own VOLK module the same way they are added to this repository, which was described in the previous section.