SPRU514Z July 2001 – October 2023 SM320F28335-EP
When the target has hardware floating-point support, some loops can be converted to hardware loops called repeat blocks (RPTB). Normally, a loop looks like this:
Label:
...loop body...
SUB loop_count
CMP
B LabelThe same loop, when converted to a RPTB loop, looks like this:
RPTB end_label, loop_count
...loop body...
end_label:A repeat block loop is loaded into a hardware buffer and executed for the specified number of iterations. This kind of loop has minimal or zero branching overhead, and can improve performance. The loop count is stored in a special register RB (repeat block register), and the hardware seamlessly decrements the count without any explicit subtractions. Thus, there is no overhead due to the subtract, the compare, and the branch. The only overhead is due to the RPTB instruction that executes once before the loop. The RPTB instruction takes one cycle if the number of iterations is a constant, and 4 cycles otherwise. This overhead is incurred once per loop.
There are limitations on the minimum and maximum loop size for a loop to qualify for becoming a repeat block, due to the presence of the buffer. Also, the loop cannot contain any inner loops or function calls.