Impact of Coding Style - 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

The primary impact of a coding style on functions is on the function arguments and interface.

If the arguments to a function are sized accurately, Vitis HLS can propagate this information through the design. There is no need to create arbitrary precision types for every variable. In the following example, two integers are multiplied, but only the bottom 24 bits are used for the result.

#include "ap_int.h"

ap_int<24> foo(int x, int y) {  
 int tmp;

 tmp = (x * y);
 return tmp
} 

When this code is synthesized, the result is a 32-bit multiplier with the output truncated to 24-bit.

If the inputs are correctly sized to 12-bit types (int12) as shown in the following code example, the final RTL uses a 24-bit multiplier.

#include "ap_int.h"
typedef ap_int<12> din_t;
typedef ap_int<24> dout_t;

dout_t func_sized(din_t x, din_t y) {  
 int tmp;

 tmp = (x * y);
 return tmp
}

Using arbitrary precision types for the two function inputs is enough to ensure Vitis HLS creates a design using a 24-bit multiplier. The 12-bit types are propagated through the design. Xilinx recommends that you correctly size the arguments of all functions in the hierarchy.

In general, when variables are driven directly from the function interface, especially from the top-level function interface, they can prevent some optimizations from taking place. A typical case of this is when an input is used as the upper limit for a loop index.