# Copyright (C) 2012-2020  (see AUTHORS file for a list of contributors)
#
# GNSS-SDR is a software-defined Global Navigation Satellite Systems receiver
#
# This file is part of GNSS-SDR.
#
# SPDX-License-Identifier: GPL-3.0-or-later
#
/*! \page concepts_terms_and_techniques Concepts, Terms, and Techniques

This page is primarily a list of definitions and brief overview of successful
techniques previously used to develop VOLK_GNSSSDR protokernels.

## Definitions and Concepts

### SIMD

SIMD stands for Single Instruction Multiple Data. Leveraging SIMD instructions
is the primary optimization in VOLK_GNSSSDR.

### Architecture

A VOLK_GNSSSDR architecture is normally called an Instruction Set Architecture (ISA).
The architectures we target in VOLK_GNSSSDR usually have SIMD instructions.

### Vector

A vector in VOLK_GNSSSDR is the same as a C array. It sometimes, but not always
coincides with the mathematical definition of a vector.

### Kernel

The 'kernel' part of the VOLK_GNSSSDR name comes from the high performance computing
use of the word. In this context it is the inner loop of a vector operation.
Since we don't use the word vector in the math sense a vector operation is an
operation that is performed on a C array.

### Protokernel

A protokernel is an implementation of a kernel. Every kernel has a 'generic'
protokernel that is implemented in C. Other protokernels are optimized for a
particular architecture.


## Techniques

### New Kernels

Add new kernels to the list in lib/kernel_tests.h. This adds the kernel to
VOLK_GNSSSDR's QA tool as well as the volk profiler. Many kernels are able to
share test parameters, but new kernels might need new ones.

If the VOLK_GNSSSDR kernel does not 'fit' the the standard set of function parameters
expected by the volk_profile structure, you need to create a VOLK_GNSSSDR puppet
function to help the profiler call the kernel. This is essentially due to the
function run_volk_gnsssdr_tests which has a limited number of function prototypes that
it can test.

### Protokernels

Adding new proto-kernels (implementations of VOLK_GNSSSDR kernels for specific
architectures) is relatively easy. In the relevant <kernel>.h file in
the volk_gnsssdr/include/volk_gnsssdr/volk_gnsssdr<input-fingerprint_function-name_output-fingerprint>.h
file, add a new #ifdef/#endif block for the LV_HAVE_<arch> corresponding
to the <arch> you a working on (e.g. SSE, AVX, NEON, etc.).

For example, for volk_gnsssdr_16ic_x2_multiply_16ic_neon:

\code
#ifdef LV_HAVE_NEON
#include <arm_neon.h>

static inline void volk_gnsssdr_16ic_x2_multiply_16ic_neon(lv_16sc_t* out, const lv_16sc_t* in_a, const lv_16sc_t* in_b, unsigned int num_points)
{
    lv_16sc_t *a_ptr = (lv_16sc_t*) in_a;
    lv_16sc_t *b_ptr = (lv_16sc_t*) in_b;
    unsigned int quarter_points = num_points / 4;
    int16x4x2_t a_val, b_val, c_val;
    int16x4x2_t tmp_real, tmp_imag;
    unsigned int number = 0;

    for(; number < quarter_points; ++number)
        {
            a_val = vld2_s16((int16_t*)a_ptr);
            b_val = vld2_s16((int16_t*)b_ptr);

            tmp_real.val[0] = vmul_s16(a_val.val[0], b_val.val[0]);
            tmp_real.val[1] = vmul_s16(a_val.val[1], b_val.val[1]);

            tmp_imag.val[0] = vmul_s16(a_val.val[0], b_val.val[1]);
            tmp_imag.val[1] = vmul_s16(a_val.val[1], b_val.val[0]);

            c_val.val[0] = vsub_s16(tmp_real.val[0], tmp_real.val[1]);
            c_val.val[1] = vadd_s16(tmp_imag.val[0], tmp_imag.val[1]);
            vst2_s16((int16_t*)out, c_val);

            a_ptr += 4;
            b_ptr += 4;
            out += 4;
        }

    for(number = quarter_points * 4; number < num_points; number++)
        {
            *out++ = (*a_ptr++) * (*b_ptr++);
        }
}
#endif /* LV_HAVE_NEON */
\endcode

### Allocating Memory

SIMD code can be very sensitive to the alignment of the vectors, which is
generally something like a 16-byte or 32-byte alignment requirement. The
VOLK_GNSSSDR dispatcher functions, which is what we will normally call as users of
VOLK_GNSSSDR, makes sure that the correct aligned or unaligned version is called
depending on the state of the vectors passed to it. However, things typically
work faster and more efficiently when the vectors are aligned. As such, VOLK_GNSSSDR
has memory allocate and free methods to provide us with properly aligned
vectors. We can also ask VOLK_GNSSSDR to give us the current machine's alignment
requirement, which makes our job even easier when porting code.

To get the machine's alignment, simply call the size_t volk_gnsssdr_get_alignment().

Allocate memory using void* volk_gnsssdr_malloc(size_t size, size_t alignment).

Make sure that any memory allocated by VOLK_GNSSSDR is also freed by VOLK_GNSSSDR with volk_gnsssdr_free(void *p).


*/
