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12.14 Declarations

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 subreg expression.

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.