hls::stream<> objects, include
the header file hls_stream.h. Streaming data objects
are defined by specifying the type and variable name. In this example, a 128-bit unsigned
integer type is defined and used to create a stream variable called
#include "ap_int.h" #include "hls_stream.h" typedef ap_uint<128> uint128_t; // 128-bit user defined type hls::stream<uint128_t> my_wide_stream; // A stream declaration
Streams must use scoped naming. Xilinx
recommends using the scoped
hls:: naming shown in the
example above. However, if you want to use the
namespace, you can rewrite the preceding example as:
#include <ap_int.h> #include <hls_stream.h> using namespace hls; typedef ap_uint<128> uint128_t; // 128-bit user defined type stream<uint128_t> my_wide_stream; // hls:: no longer required
Given a stream specified as
hls::stream<T>, the type T can be:
- Any C++ native data type
- A Vitis HLS arbitrary precision type
- A user-defined struct containing either of the above types
A stream can also be specified as
hls::stream<Type, Depth>, where Depth indicates the depth of the FIFO
needed in the verification adapter that the HLS tool creates for RTL co-simulation.
Streams can be optionally named. Providing a name for the stream allows the name to be used in reporting. For example, Vitis HLS automatically checks to ensure all elements from an input stream are read during simulation. Given the following two streams:
stream<uint8_t> bytestr_in1; stream<uint8_t> bytestr_in2("input_stream2");
Any warning on elements of the streams are reported as follows, where it is
input_stream2 refers to
WARNING: Hls::stream 'hls::stream<unsigned char>.1' contains leftover data, which may result in RTL simulation hanging. WARNING: Hls::stream 'input_stream2' contains leftover data, which may result in RTL simulation hanging.
When streams are passed into and out of functions, they must be passed-by-reference as in the following example:
void stream_function ( hls::stream<uint8_t> &strm_out, hls::stream<uint8_t> &strm_in, uint16_t strm_len )
Vitis HLS supports both blocking and non-blocking access methods.
A complete design example using streams is provided in the Vitis HLS examples. Refer to the
hls_stream example in the design examples available from the Vitis IDE welcome screen.