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如何使用按位运算符实现逻辑运算符

[英]How to implement logical operators using bitwise operators

原文 2019-09-23 15:36:11 0 1 c/ bitwise-operators/ logical-operators

How can the logical operators ||逻辑运算符如何|| and && be implemented using only the bitwise operators & , ^ , |和&&仅使用按位运算符& 、 ^ 、 |来实现, ~ , >> , << the logical operator ! , ~ , >> , <<逻辑运算符! , and + ? , 和+ ? I've looked around on stack overflow and google for someone's previous answer or some assembly implementations but haven't found anything yet.我在堆栈溢出和谷歌上四处寻找某人以前的答案或一些汇编实现，但还没有找到任何东西。 I figure if no one has any solution I might just turn to an HDL and see what it synthesizes.我想如果没有人有任何解决方案，我可能会转向 HDL 并看看它合成了什么。

1 个解决方案

They can't be, although it would be tempting to suggest that a && b can be written as !!a & !!b and a || b不可能，尽管建议a && b可以写成!!a & !!b和a || b是很有诱惑力的。 a || b as !!a | !!b a || b作为!!a | !!b !!a | !!b . !!a | !!b 。

This is because ||这是因为|| and && have a property where the evaluation of the second argument does not happen if the result of the expression is known from the result of the first argument.和&&有一个属性，如果从第一个参数的结果中知道表达式的结果，则不会发生对第二个参数的评估。 Eg for true || A例如为true || A true || A , A is not evaluated, and for false && B , B is not evaluated. true || A , A未评估，对于false && B ， B未评估。 So if you attempted to replicate either ||因此，如果您尝试复制|| or && with bitwise operators, then you could well introduce side effects into your program.或&&与位运算符，那么你可以很好地在你的程序中引入副作用。

Also ||还有|| and && are sequencing points , whereas the bitwise operators are not.和&&是排序点，而按位运算符不是。 So a++ && a++ is defined for example, but a++ & a++ is undefined.因此，例如定义了a++ && a++ ，但未定义a++ & a++ 。