Re: warning: conversion from ‘int’ to ‘char’ may change value

["Liu Hao" <lh_mouse at 126 dot com>]

在 2018/9/20 22:08, Vincent Lefevre 写道:
>> In C++, declaring n1 const avoids the warning regardless of
>> optimization levels.
> 
> If the constant propagation is done at -O0, this could explain
> the behavior.
> 
> Or do you mean that GCC remembers the type the data come from,
> i.e. assuming char is signed, if n1 is of type char, then ~n1
> is necessarily representable in a char, thus can be regarded
> as of being of type char in its analysis?
> 

In C++ adding the `const` qualifier makes `~n1` a constant expression. 
In C it never is, regardless of qualifiers.

BTW, I am quite disappointed with such 'false' warnings, because by 
performing a compound AND-and-ASSIGN operation on a `char` object I have 
no interest in bits that don't fit into a `char`, be they ones or 
zeroes. Perhaps there are scenarios where they shouldn't be ignored, but 
I can't think of any.


-- 
Best regards,
LH_Mouse