GNU Compiler Collection (GCC) Internals: Bit-Fields |
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Special expression codes exist to represent bit-field instructions.
(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,
which is the default if none is specified.
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.
A sign_extract
can not appear as an lvalue, or part thereof,
in RTL.
(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.
Unlike 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.