3.6.1.1 Introduction to Pragmas

SmartHLS™ accepts pragma directives in the source code to guide the hardware generation. This reference section explains all of the pragmas available for SmartHLS.

The pragmas follow the following syntax:
#pragma HLS <category> <feature> <parameter>(<value>)
The category refers to the general usage class of the pragma. Each pragma has one of the following categories:
  • function: configure a hardware function.
  • loop: configure loop optimizations.
  • interface: configure hardware interfaces (arguments / global variables).
  • memory: configure hardware memory implementation.

Each category can have different configurable features. Some categories / features have parameters to passed to the pragma. A parameter can be optional with a default behaviour if not specified. The value of a parameter can be either integer, boolean (true|false), name (variable / argument), or a set of pre-specified values.

Note

Important:

For integer parameters, the user is allowed to use constants (or expressions of constants) defined using #define directive. For example, this is allowed:

#define N 10
void fun() {
  #pragma HLS loop unroll factor(N+1)
  for (int i = 0; i < 100; i++)
    ...
}
The pragma position is not arbitrary and placing the pragma in an incorrect position will cause an error. Each pragma can has one of the following positions:
  • At the beginning of function definition block before any other statements.
  • Before global / local variable declaration.
  • Before loop block.