Using and Porting GNU CC - Bit Fields

Bit Fields

Special expression codes exist to represent bitfield instructions. These types of expressions are lvalues in RTL; they may appear on the left side of an assignment, indicating insertion of a value into the specified bit field.

(sign_extract:m loc size pos)

This represents a reference to a sign-extended bit field contained or starting in loc (a memory or register reference). The bit field is size bits wide and starts at bit pos. The compilation option BITS_BIG_ENDIAN says which end of the memory unit pos counts from.

If loc is in memory, its mode must be a single-byte integer mode. If loc is in a register, the mode to use is specified by the operand of the insv or extv pattern (see Standard Names) and is usually a full-word integer mode.

The mode of pos is machine-specific and is also specified in the insv or extv pattern.

The mode m is the same as the mode that would be used for loc if it were a register.

(zero_extract:m loc size pos)

Like sign_extract but refers to an unsigned or zero-extended bit field. The same sequence of bits are extracted, but they are filled to an entire word with zeros instead of by sign-extension.