Expressions Involving ap_[u]<> types - 2021.2 English

Vitis High-Level Synthesis User Guide (UG1399)

Document ID
English (United States)
Release Date
2021.2 English

Variables of ap_[u]<> types may generally be used freely in expressions involving C/C++ operators. Some behaviors may be unexpected. These are discussed in detail below.

Zero- and Sign-Extension on Assignment From Narrower to Wider Variables

When assigning the value of a narrower bit-width signed (ap_int<>) variable to a wider one, the value is sign-extended to the width of the destination variable, regardless of its signedness.

Similarly, an unsigned source variable is zero-extended before assignment.

Explicit casting of the source variable may be necessary to ensure expected behavior on assignment. See the following example:

ap_uint<10> Result;

ap_int<7> Val1 = 0x7f;
ap_uint<6> Val2 = 0x3f;

Result = Val1; // Yields: 0x3ff (sign-extended)
Result = Val2; // Yields: 0x03f (zero-padded)

Result = ap_uint<7>(Val1); // Yields: 0x07f (zero-padded)
Result = ap_int<6>(Val2); // Yields: 0x3ff (sign-extended)

Truncation on Assignment of Wider to Narrower Variables

Assigning the value of a wider source variable to a narrower one leads to truncation of the value. All bits beyond the most significant bit (MSB) position of the destination variable are lost.

There is no special handling of the sign information during truncation. This may lead to unexpected behavior. Explicit casting may help avoid this unexpected behavior.