opencl_unroll_hint - 2021.2 English

Vitis Unified Software Platform Documentation: Application Acceleration Development (UG1393)

Document ID
English (United States)
Release Date
2021.2 English


Important: This is a compiler hint which the compiler can ignore.

Loop unrolling is an optimization technique available in the Vitis compiler. The purpose of the loop unroll optimization is to expose concurrency to the compiler. This newly exposed concurrency reduces latency and improves performance, but also consumes more FPGA fabric resources.

The OPENCL_UNROLL_HINT attribute is part of the OpenCL Specification, and specifies that loops (for, while, do) can be unrolled by the Vitis compiler. See Loop Unrolling for more information.

The OPENCL_UNROLL_HINT attribute qualifier must appear immediately before the loop to be affected. You can use this attribute to specify full unrolling of the loop, partial unrolling by a specified amount, or to disable unrolling of the loop.


Place the attribute in the OpenCL source before the loop definition:



  • <n> is an optional loop unrolling factor and must be a positive integer, or compile time constant expression. An unroll factor of 1 disables unrolling.
    Tip: If <n> is not specified, the compiler automatically determines the unrolling factor for the loop.


The following example unrolls the for loop by a factor of 2. This results in two parallel loop iterations instead of four sequential iterations for the compute unit to complete the operation.

for(int i = 0; i < LENGTH; i++) {
bufc[i] = bufa[i] * bufb[i];

Conceptually the compiler transforms the loop above to the following code.

for(int i = 0; i < LENGTH; i+=2) {
bufc[i] = bufa[i] * bufb[i];
bufc[i+1] = bufa[i+1] * bufb[i+1];