When you use an array as an argument to the top-level function, Vitis HLS assumes one of the following:
- Memory is off-chip and Vitis HLS synthesizes M_AXI ports on the interface to access the memory as the default for the Vitis Kernel flow.
- Memory is standard block RAM with a latency of 1 as the default behavior in the Vivado IP flow. The data is ready one clock cycle after the address is supplied.
To configure how Vitis HLS creates these ports:
- Specify the interface as a M_AXI, BRAM, or FIFO interface using the INTERFACE pragma or directive.
- Specify the RAM as a single or dual-port RAM using the
storage_typeoption of the INTERFACE pragma or directive. - Specify the RAM latency using the
latencyoption of the INTERFACE pragma or directive. - Use array optimization directives, ARRAY_PARTITION, or ARRAY_RESHAPE, to reconfigure the structure of the array and therefore, the number of I/O ports.
Tip: Because access to
the data is limited through a memory (RAM or FIFO) port, arrays on the interface can
create a performance bottleneck. Typically, you can overcome these bottlenecks using
optimization directives.
Arrays must be sized when used in synthesizable code. If, for
example, the declaration
d_i[4] in Array Interfaces is changed to d_i[], Vitis HLS
issues a message that the design cannot be synthesized:
@E [SYNCHK-61] array_RAM.c:52: unsupported memory access on variable 'd_i' which is (or contains) an array with unknown size at compile time.