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(gcc-295.info)Bit Fields

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Bit Fields
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   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 (Note: 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.

`(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.