Intel® FPGA SDK for OpenCL™ Pro Edition: Programming Guide

ID 683846
Date 3/28/2022

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5.12. Single-Cycle Floating-Point Accumulator for Single Work-Item Kernels

Single work-item kernels that perform accumulation in a loop can take advantage of the single-cycle floating-point accumulator feature of the Intel® FPGA SDK for OpenCL™ Offline Compiler. The offline compiler searches for these kernel instances and attempts to map an accumulation that executes in a loop into the accumulator structure.

The offline compiler supports an accumulator that adds or subtracts a value. To use this feature, describe the accumulation in a way that allows the offline compiler to infer the accumulator.

  • The accumulator is only available on Intel® Arria® 10 devices.
  • The accumulator must be part of a loop.
  • The accumulator must have an initial value of 0.
  • The accumulator cannot be conditional.

Below are examples of a description that results in the correct inference of the accumulator by the offline compiler.

channel float4 RANDOM_STREAM;

__kernel void acc_test(__global float *a, int k) {
    // Simplest example of an accumulator.
    // In this loop, the accumulator acc is incremented by 5.
    int i;
    float acc = 0.0f;
    for (i = 0; i < k; i++) {
    a[0] = acc;

__kernel void acc_test2(__global float *a, int k) {
    // Extended example showing that an accumulator can be
    // conditionally incremented. The key here is to describe the increment
    // as conditional, not the accumulation itself.
    int i;
    float acc = 0.0f;
    for (i = 0; i < k; i++) {
        acc += ((i < 30) ? 5 : 0);
    a[0] = acc;

__kernel void acc_test3(__global float *a, int k) {
    // A more complex case where the accumulator is fed
    // by a dot product.
    int i;
    float acc = 0.0f;
    for (i = 0; i < k; i++ ){
        float4 v = read_channel_intel(RANDOM_STREAM);
        float x1 = v.x;
        float x2 = v.y;
        float y1 = v.z;
        float y2 = v.w;
        acc += (x1*y1+x2*y2);
    a[0] = acc;

__kernel void loader(__global float *a, int k) {
    int i;
    float4 my_val = 0;
    for(i = 0; i < k; i++) {
        if ((i%4) == 0)
            write_channel_intel(RANDOM_STREAM, my_val);
        if ((i%4) == 0) my_val.x = a[i];
        if ((i%4) == 1) my_val.y = a[i];
        if ((i%4) == 2) my_val.z = a[i];
        if ((i%4) == 3) my_val.w = a[i];

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