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

ID 683846
Date 6/21/2022
Public

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5.12.1. Programming Strategies for Inferring the Accumulator

To take advantage of the single cycle floating-point accumulator feature, you can modify the accumulator description in your kernel code to improve efficiency or work around programming restrictions.

Describing an Accumulator Using Multiple Loops

Consider a case where you want to describe an accumulator using multiple loops, with some of the loops being unrolled:

float acc = 0.0f;
for (i = 0; i < k; i++) {
    #pragma unroll
    for(j=0;j < 16; j++)
        acc += (x[i+j]*y[i+j]);
}

In this situation, it is important to compile the kernel with the -ffp-reassociate Intel® FPGA SDK for OpenCL™ Offline Compiler command option to enable the offline compiler to rearrange the operations in a way that exposes the accumulation. If you do not compile the kernel with -ffp-reassociate, the resulting accumulator structure will have a high initiation interval (II). II is the number of cycles between launching successive loop iterations. The higher the II value, the longer the accumulator structure must wait before it can process the next loop iteration.

Modifying a Multi-Loop Accumulator Description

In cases where you cannot compile an accumulator description using the -ffp-reassociate offline compiler command option rewrite the code to expose the accumulation.

For the code example above, rewrite it in the following manner:

float acc = 0.0f;
for (i = 0; i < k; i++) {
    float my_dot = 0.0f;
    #pragma unroll
    for(j=0;j < 16; j++)
        my_dot += (x[i+j]*y[i+j]);
    acc += my_dot;
}

Modifying an Accumulator Description Containing a Variable or Non-Zero Initial Value

Consider a situation where you might want to apply an offset to a description of an accumulator that begins with a non-zero value:

float acc = array[0];
for (i = 0; i < k; i++) {
    acc += x[i];
}

Because the accumulator hardware does not support variable or non-zero initial values in a description, you must rewrite the description.

float acc = 0.0f;
for (i = 0; i < k; i++) {
    acc += x[i];
}
acc += array[0];

Rewriting the description in the above manner enables the kernel to use an accumulator in a loop. The loop structure is then followed by an increment of array[0].