Description
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.
Syntax
Place the attribute in the OpenCL source before the loop definition:
__attribute__((opencl_unroll_hint(<n>)))
Where:
- <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.
Examples
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.
__attribute__((opencl_unroll_hint(2)))
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];
}