GNU Compiler Collection (GCC) Internals: Bit-Fields

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### 13.11 Bit-Fields

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.