Pointers - 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

Pointers are used extensively in C/C++ code and are supported for synthesis, but it is generally recommended to avoid the use of pointers in your code. This is especially true when using pointers in the following cases:

  • When pointers are accessed (read or written) multiple times in the same function.
  • When using arrays of pointers, each pointer must point to a scalar or a scalar array (not another pointer).
  • Pointer casting is supported only when casting between standard C/C++ types, as shown.
Note: Pointer to pointer is not supported.

The following code example shows synthesis support for pointers that point to multiple objects.

#include "pointer_multi.h"

dout_t pointer_multi (sel_t sel, din_t pos) {
 static const dout_t a[8] = {1, 2, 3, 4, 5, 6, 7, 8};
 static const dout_t b[8] = {8, 7, 6, 5, 4, 3, 2, 1};

 dout_t* ptr;
 if (sel) 
 ptr = a; 
 else 
 ptr = b;

 return ptr[pos];
} 

Vitis HLS supports pointers to pointers for synthesis but does not support them on the top-level interface, that is, as argument to the top-level function. If you use a pointer to pointer in multiple functions, Vitis HLS inlines all functions that use the pointer to pointer. Inlining multiple functions can increase runtime.

#include "pointer_double.h"

data_t sub(data_t ptr[10], data_t size, data_t**flagPtr)
{
 data_t x, i;

 x = 0;
 // Sum x if AND of local index and pointer to pointer index is true
 for(i=0; i<size; ++i)
   if (**flagPtr & i)
        x += *(ptr+i);
 return x;
}

data_t pointer_double(data_t pos, data_t x, data_t* flag)
{
 data_t array[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
 data_t* ptrFlag;
 data_t i;

 ptrFlag = flag;

 // Write x into index position pos
 if (pos >=0 & pos < 10) 
 *(array+pos) = x;

 // Pass same index (as pos) as pointer to another function
 return sub(array, 10, &ptrFlag);
}

Arrays of pointers can also be synthesized. See the following code example in which an array of pointers is used to store the start location of the second dimension of a global array. The pointers in an array of pointers can point only to a scalar or to an array of scalars. They cannot point to other pointers.

#include "pointer_array.h"

data_t A[N][10];

data_t pointer_array(data_t B[N*10]) {
 data_t i,j;
 data_t sum1;

 // Array of pointers
 data_t* PtrA[N];

 // Store global array locations in temp pointer array
 for (i=0; i<N; ++i) 
    PtrA[i] = &(A[i][0]);

 // Copy input array using pointers
 for(i=0; i<N; ++i) 
    for(j=0; j<10; ++j) 
       *(PtrA[i]+j) = B[i*10 + j];

  // Sum input array
 sum1 = 0;
 for(i=0; i<N; ++i)
    for(j=0; j<10; ++j) 
       sum1 += *(PtrA[i] + j);

 return sum1;
}

Pointer casting is supported for synthesis if native C/C++ types are used. In the following code example, type int is cast to type char.

#define N 1024

typedef int data_t;
typedef char dint_t;

data_t pointer_cast_native (data_t index, data_t A[N]) {
 dint_t* ptr;
 data_t i =0, result = 0;
 ptr = (dint_t*)(&A[index]);

 // Sum from the indexed value as a different type
 for (i = 0; i < 4*(N/10); ++i) {
   result += *ptr;
   ptr+=1;
 }
 return result;
} 

Vitis HLS does not support pointer casting between general types. For example, if a struct composite type of signed values is created, the pointer cannot be cast to assign unsigned values.

struct { 
 short first;  
 short second;  
} pair;

// Not supported for synthesis
*(unsigned*)(&pair) = -1U;

In such cases, the values must be assigned using the native types.

struct { 
 short first;  
 short second;  
} pair;

// Assigned value
pair.first = -1U; 
pair.second = -1U;