This optimization removes invariant subexpressions from the body of a loop.
此优化将不变量子表达式从循环体中移除。
And the subexpressions can be combined using the and, or, and not logical operators.
而子表达式又可使用and、or和not逻辑运算符组合起来。
When all parenthetical subexpressions have been evaluated, the rest of the expression is evaluated.
所有括号内的子表达式都求值完毕后，表达式的其它部分再求值。
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