Static Variables - 2021.2 English

Vitis High-Level Synthesis User Guide (UG1399)

Document ID
UG1399
ft:locale
English (United States)
Release Date
2021-12-15
Version
2021.2 English

Static variables are used to keep data between loop iterations, often resulting in registers in the final implementation. If this is encountered in pipelined functions, Vitis HLS might not be able to optimize the design sufficiently, which would result in initiation intervals longer than required.

The following is a typical example of this situation:

function_foo()
{
	static bool change = 0
	if (condition_xyz){ 
		change = x; // store 
	}
	y = change; // load
}

If Vitis HLS cannot optimize this code, the stored operation requires a cycle and the load operation requires an additional cycle. If this function is part of a pipeline, the pipeline has to be implemented with a minimum initiation interval of 2 as the static change variable creates a loop-carried dependency.

One way the user can avoid this is to rewrite the code, as shown in the following example. It ensures that only a read or a write operation is present in each iteration of the loop, which enables the design to be scheduled with II=1.

function_readstream()
{
	static bool change = 0
	bool change_temp = 0;
	if (condition_xyz)
	{ 
		change = x; // store 
		change_temp = x; 
	}
	else
	{ 
	change_temp = change; // load 
	}
	y = change_temp;
}