Special expression codes exist to represent bit-field instructions.
(sign_extract:m loc size pos
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
(see Standard Names) and is usually a full-word integer mode,
which is the default if none is specified.
The mode of pos is machine-specific and is also specified
The mode m is the same as the mode that would be used for loc if it were a register.
sign_extract can not appear as an lvalue, or part thereof,
(zero_extract:m loc size pos
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.
sign_extract, this type of expressions can be lvalues
in RTL; they may appear on the left side of an assignment, indicating
insertion of a value into the specified bit-field.