Declaration expression codes do not represent arithmetic operations but rather state assertions about their operands.
(strict_low_part (subreg:m (reg:n r) 0))
This expression code is used in only one context: as the destination operand of a
set expression. In addition, the operand of this expression
must be a non-paradoxical
The presence of
strict_low_part says 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.