On Mon, Nov 14, 2022, 10:49 David Brown <da...@westcontrol.com> wrote:
> > > On 14/11/2022 16:10, NightStrike wrote: > > > > > > On Mon, Nov 14, 2022, 04:42 David Brown via Gcc <gcc@gcc.gnu.org > > > > > Warnings are not perfect - there is always the risk of false > positives > > and false negatives. And different people will have different ideas > > about what code is perfectly reasonable, and what code is risky and > > should trigger a warning. Thus gcc has warning flag groups (-Wall, > > -Wextra) that try to match common consensus, and individual flags for > > personal fine-tuning. > > > > Sometimes it is useful to have a simple way to override a warning in > > code, without going through "#pragma GCC diagnostic" lines (which are > > powerful, but not pretty). > > > > So if you have : > > > > int i; > > if (a == 1) i = 1; > > if (b == 1) i = 2; > > if (c == 1) i = 3; > > return i; > > > > the compiler will warn that "i" may not be initialised. But if you > > /know/ that one of the three conditions will match (or you don't care > > what "i" is if it does not match), then you know your code is fine > and > > don't want the warning. Writing "int i = i;" is a way of telling the > > compiler "I know what I am doing, even though this code looks dodgy, > > because I know more than you do". > > > > It's just like writing "while ((*p++ = *q++));", or using a cast to > > void > > to turn off an "unused parameter" warning. > > > > > > Wouldn't it be easier, faster, and more obvious to the reader to just > > use "int i = 0"? I'm curious what a real world use case is where you > > can't do the more common thing if =0. > > > > You can write "int i = 0;" if you prefer. I would not, because IMHO > doing so would be wrong, unclear to the reader, less efficient, and > harder to debug. > > In the code above, the value returned should never be 0. So why should > "i" be set to 0 at any point? That's just an extra instruction the > compiler must generate (in my line of work, my code often needs to be > efficient). More importantly, perhaps, it means that if you use > diagnostic tools such as sanitizers you are hiding bugs from them > instead of catching them - a sanitizer could catch the case of "return > i;" when "i" is not set. > > (I don't know if current sanitizers will do that or not, and haven't > tested it, but they /could/.) > > But I'm quite happy with : > > int i = i; // Self-initialise to silence warning > > I don't think there is a "perfect" solution to cases like this, and > opinions will always differ, but self-initialisation seems a good choice > to me. Regardless of the pros and cons in this particular example, the > handling of self-initialisation warnings in gcc is, AFAIUI, to allow > such code for those that want to use it. Thanks for the extended explanation, insight, and detail!
