Dynamic Memory Usage - 2021.2 English

Vitis High-Level Synthesis User Guide (UG1399)

Document ID
English (United States)
Release Date
2021.2 English

Any system calls that manage memory allocation within the system, for example, malloc(), alloc(), and free(), are using resources that exist in the memory of the operating system and are created and released during runtime. To be able to synthesize a hardware implementation the design must be fully self-contained, specifying all required resources.

Memory allocation system calls must be removed from the design code before synthesis. Because dynamic memory operations are used to define the functionality of the design, they must be transformed into equivalent bounded representations. The following code example shows how a design using malloc() can be transformed into a synthesizable version and highlights two useful coding style techniques:

  • The design does not use the __SYNTHESIS__ macro.

    The user-defined macro NO_SYNTH is used to select between the synthesizable and non-synthesizable versions. This ensures that the same code is simulated in C/C++ and synthesized in Vitis HLS.

  • The pointers in the original design using malloc() do not need to be rewritten to work with fixed sized elements.

    Fixed sized resources can be created and the existing pointer can simply be made to point to the fixed sized resource. This technique can prevent manual recoding of the existing design.

#include "malloc_removed.h"
#include <stdlib.h>
//#define NO_SYNTH

dout_t malloc_removed(din_t din[N], dsel_t width) {  

#ifdef NO_SYNTH
 long long *out_accum = malloc (sizeof(long long));
 int* array_local = malloc (64 * sizeof(int));
 long long _out_accum;
 long long *out_accum = &_out_accum;
 int _array_local[64];
 int* array_local = &_array_local[0];
 int i,j;
 LOOP_SHIFT:for (i=0;i<N-1; i++) {
 if (i<width) 

 LOOP_ACCUM:for (j=0;j<N-1; j++) {
 *out_accum += *(array_local+j);

 return *out_accum;

Because the coding changes here impact the functionality of the design, Xilinx does not recommend using the __SYNTHESIS__ macro. Xilinx recommends that you perform the following steps:

  1. Add the user-defined macro NO_SYNTH to the code and modify the code.
  2. Enable macro NO_SYNTH, execute the C/C++ simulation, and save the results.
  3. Disable the macro NO_SYNTH, and execute the C/C++ simulation to verify that the results are identical.
  4. Perform synthesis with the user-defined macro disabled.

This methodology ensures that the updated code is validated with C/C++ simulation and that the identical code is then synthesized. As with restrictions on dynamic memory usage in C/C++, Vitis HLS does not support (for synthesis) C/C++ objects that are dynamically created or destroyed.