Declaration expression codes do not represent arithmetic operations but rather state assertions about their operands.
(strict_low_part (subreg:m (reg:n r) 0))
setexpression. In addition, the operand of this expression must be a non-paradoxical
subregexpression. The presence of
strict_low_partsays that the part of the register which is meaningful in mode n, but is not part of mode m, is not to be altered. Normally, an assignment to such a subreg is allowed to have undefined effects on the rest of the register when m is less than a word.