set_directive_disaggregate - 2022.2 English

Vitis High-Level Synthesis User Guide (UG1399)

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2022.2 English


The set_directive_disaggregate command lets you deconstruct a struct variable into its individual elements. The number and type of elements created are determined by the contents of the struct itself.

Important: Structs used as arguments to the top-level function are aggregated by default, but can be disaggregated with this directive or pragma. Refer to AXI4-Stream Interfaces for important information about disaggregating structs associated with streams.


set_directive_disaggregate <location> <variable>
  • <location> is the location (in the format function[/label]) where the variable to disaggregate is found.
  • <variable> specifies the struct variable name.


This command has no options.

Example 1

The following example shows the struct variable a in function top will be disaggregated:

set_directive_disaggregate top a

Example 2

Disaggregated structs can be addressed in your code by the using standard C/C++ coding style as shown below. Notice the different methods for accessing the pointer element (a) versus the reference element (c);

struct SS
  int x[N];
  int y[N];
int top(SS *a, int b[4][6], SS &c) {

set_directive_disaggregate top a
set_directive_interface -mode s_axilite top a->x
set_directive_interface -mode s_axilite top a->y

set_directive_disaggregate top c
set_directive_interface -mode ap_memory top c.x
set_directive_interface -mode ap_memory top c.y

Example 3

The following example shows the Dot struct containing the RGB struct as an element. If you apply set_directive_disaggregate to variable Arr, then only the top-level Dot struct is disaggregated.

struct Pixel { 
char R; 
char G; 
char B; 

struct Dot { 
Pixel RGB; 
unsigned Size; 

#define N 1086 
void DUT(Dot Arr[N]) {

set_directive_disaggregate DUT Arr

If you want to disaggregate the whole struct, Dot and RGB, then you can assign the set_directive_disaggregate as shown below.

void DUT(Dot Arr[N]) { 
#pragma HLS disaggregate variable=Arr->RGB 

set_directive_disaggregate DUT Arr->RGB
The results in this case will be:
void DUT(char Arr_RGB_R[N], char Arr_RGB_G[N], char Arr_RGB_B[N], unsigned Arr_Size[N]) {