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

ID 683846
Date 12/13/2021

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Document Table of Contents Work-Item Serial Execution of Channels

Work-item serial execution refers to an ordered execution behavior where work-item sequential IDs determine their execution order in the compute unit.

When you implement channels in a kernel, the Intel® FPGA SDK for OpenCL™ Offline Compiler enforces that kernel behavior is equivalent to having at most one work-group in flight within the compute unit at a time. The compiler also ensures that the kernel executes channels in work-item serial execution, where the kernel executes work-items with smaller IDs first. A work-item has the identifier (x, y, z, group), where x, y, z are the local 3D identifiers, and group is the work-group identifier.

The work-item ID (x0, y0, z0, group0) is considered to be smaller than the ID (x1, y1, z1, group1) if one of the following conditions is true:

  • group0 < group1
  • group0 = group1 and z0 < z1
  • group0 = group1 and z0 = z1 and y0 < y1
  • group0 = group1 and z0 = z1 and y0 = y1 and x0 < x1

Work-items with incremental IDs execute in a sequential order. For example, the work-item with an ID (x0, y0, z0, group0) executes the write channel call first. Then, the work-item with an ID (x1, y0, z0, group0) executes the call, and so on. Defining this order ensures that the system is verifiable with external models.

Channel Execution in Loop with Multiple Work-Items

When channels exist in the body of a loop with multiple work-items, as shown below, each loop iteration executes prior to subsequent iterations. This implies that loop iteration 0 of each work-item in a work-group executes before iteration 1 of each work-item in a work-group, and so on.

__kernel void ordering (__global int * data, int X) {
  int n = 0;
  while (n < X)
    write_channel_intel (req, data[get_global_id(0)]);

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