Interface Data Types - 2023.2 English

Vitis Unified Software Platform Documentation: Application Acceleration Development (UG1393)

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2023.2 English

All the basic C++ data types are supported for compute() arguments: bool, char, int, short, long, float, double. These C++ type modifiers are supported as well: signed, unsigned, short and long.

In addition, arbitrary precision data types are also supported, ap_int<N> and ap_uint<N>, as described in Arbitrary Precision (AP) Data Types in the Vitis HLS tool. The ap_int and ap_uint are pre-defined data types in Vitis HLS, where N is a integer with a max limit of 1024. This allows finer bit-width control, especially to handle data packing or to match a global memory data bus width.

The compute() interface arguments can be classified as Scalar types or Buffer types of arguments as described below.

Scalar Type

A scalar argument is a basic data-type that represents a single word passed to the compute() call. VSC will infer a scalar argument type when any of the basic data types described earlier are used as a pass-by-value argument of compute(). For example size and value are scalars here: compute(int size, float value);

Tip: The transfer of a scalar word is done using the AXI4-Lite hardware interface.

A user-defined C-struct can be used as a pass-by-value scalar argument, with the following rules:

  • Struct cannot have a field with C++ bit specifiers, for example: int field_x:4;
  • Struct cannot have a pointer field, for example: int* ptr_field;
  • The total byte-size of the struct must be a strict power of two. If not, the user is required to declare a dummy field to fill in the remaining bytes (e.g. char pad[remainder]).
Scalar arguments are always passed by value, and are typically inputs to the accelerator. A standard C++ reference argument is not allowed. For example:
compute(int size, float& result_value) // reference argument is not allowed

However, a C++ pointer type can be used in place of a reference when a PE function argument is still a reference. For example:

my_acc::compute(int inp, data_t *out_p) { // reference not allowed
  my_PE(inp, *out_p); // deref the ptr for my_PE out_ref
  my_PE_two(inp ... ); // passing inp to multiple PEs is allowed
my_acc::my_PE(int in, data_t &out_ref) { // reference is allowed
// application code
my_acc::send_while ... { ...
  out_p = my_acc::alloc_buf(bp, 1);  // required
  my_acc::compute(inp, out_p);

VSC will infer a scalar for out_p, when this output argument when all these conditions apply:

  • It must be pointing to only one element.
    Important: The application code must allocate memory of size 1 using alloc_buf()as described in VPP_ACC Class API.
  • The kernel code writes exactly one value to it, for example: *out_p = result;
  • No VSC guidance macros are specified on this argument.

A scalar argument of compute() can be passed to multiple PEs. In hardware a single AXI4-Lite port will drive individual scalar registers of every PE function that takes this scalar argument.

Buffer Type

A buffer is a pointer or array argument to the compute() function, which can hold one or more elements. The contents of the buffer is transferred to/from the device based on the corresponding platform interface (SYS_PORT connection). The base (element) type of the pointer or array argument can be any basic data types described in the earlier section. In addition, a user-defined C-struct data type can be also be used as a base element type.

Note: A buffer type argument requires global memory space both on the device and in the CPU host. Therefore, when using a pointer type the application code is required to call memory allocation (alloc_buf described in VPP_ACC Class API.

When using a C-struct as a base type for an pointer or an array, the following rules apply:

  • Struct cannot have field with C++ bit specifiers, for example: int field_x:4;
  • Struct cannot have a pointer field, for example: int* ptr_field;
Important: The C++ data modifiers (like const, register, volatile) are not-allowed at the compute() interface.

The following example shows coding styles, both allowed and not-allowed, for buffer type arguments:

// accelerator interface
my_acc::compute(int A, int *B, int C[10]) {
// application code
int S, *BB, *CC;
my_acc::send_while ... { ...
  BB = my_acc::alloc_buf( ... ); // required
  CC = my_acc::alloc_buf( ... ); // required
  compute (S, AA, BB);

In the example above, the compute() arguments A and B are treated the same way in the application code so that it is required to allocate memory for these buffer arguments.

Tip: A buffer argument of compute() can only be passed to a single PE function within.

In the example, A is a scalar input argument. The caller only passes an integer argument to the compute() call, the value is transferred to the device using AXI4-Lite. Whereas, B and C are buffer arguments. They can be either input, output, bi-directional or remote. The caller has to allocate memory for this buffer using alloc_buf().