Function Inlining - 2024.2 English

Vitis High-Level Synthesis User Guide (UG1399)

Document ID
UG1399
Release Date
2024-11-13
Version
2024.2 English

Function inlining removes the function hierarchy by disolving the function logic into the calling function. A function is inlined using the INLINE directive. Inlining a function can improve the area by allowing the components within the function to be better shared or optimized with the logic in the calling function. This type of function inlining is also performed automatically by Vitis HLS for small functions.

Inlining allows function sharing to be better controlled. For functions to be shared they must be used within the same level of hierarchy. In this code example, function foo_top calls foo twice and function foo_sub.

foo_sub (p, q) {
 int q1 = q + 10;
 foo(p1,q); // foo_3
 ...
}
void foo_top { a, b, c, d} {
 ...
 foo(a,b); //foo_1
 foo(a,c); //foo_2
 foo_sub(a,d);
 ...
}

Inlining function foo_sub and using the ALLOCATION directive to specify that only one instance of the function foo is used, results in a design that only has one instance of function foo: one-third the area of the example above. Using this strategy allows for more fine-grained control of what user-defined resources can be shared and is a useful feature to when area utilization is a consideration.

foo_sub (p, q) {
#pragma HLS INLINE
 int q1 = q + 10;
 foo(p1,q); // foo_3
 ...
}
void foo_top { a, b, c, d} {
#pragma HLS ALLOCATION instances=foo limit=1 function
 ...
 foo(a,b); //foo_1
 foo(a,c); //foo_2
 foo_sub(a,d);
 ...
}

The INLINE directive optionally allows all functions below the specified function to be recursively inlined by using the recursive option. If the recursive option is used on the top-level function, all function hierarchy in the design is removed.

The INLINE off option can optionally be applied to functions to prevent them from being inlined. This option may be used to prevent Vitis HLS from automatically inlining a function.

The INLINE directive is a powerful way to substantially modify the structure of the code without actually performing any modifications to the source code and provides a very powerful method for architectural exploration.