[cfe-dev] False positive for -Wunreachable-code

[Abramo Bagnara]

Il 28/10/2012 11:47, Matthieu Monrocq ha scritto:
> 
> 
> On Sun, Oct 28, 2012 at 8:12 AM, Abramo Bagnara
> <abramo.bagnara at bugseng.com <mailto:abramo.bagnara at bugseng.com>> wrote:
> 
>     $ cat p.c
>     #include <stdio.h>
> 
>     enum e { a, b = 4 } x = 3;
> 
>     void g(int v) {
>       printf("%d\n", v);
>     }
> 
>     int main(int argc, char **argv) {
>       switch (x) {
>       case a:
>         g(0);
>         break;
>       case b:
>         g(1);
>         break;
>       default:
>         g(2);
>         break;
>       }
>     }
>     $ _clang -Wunreachable-code -Wcovered-switch-default -O2 p.c
>     p.c:17:3: warning: default label in switch which covers all enumeration
>     values
>           [-Wcovered-switch-default]
>       default:
>       ^
>     p.c:18:7: warning: will never be executed [-Wunreachable-code]
>         g(2);
>           ^
>     $ ./a.out
>     2
> 
>     Of course -Wcovered-switch-default warning is a perfectly true positive.
> 
>     My reading of the standard is that nothing prevent an enum to have a
>     value different from listed enum constants if this value is compatible
>     with enum range (and code generation seems to agree on that).
> 
>     Are there any objection to file a bug report?
> 
> 
> Well, I would object on the basis that for all "regular" uses of the
> switch/enum combination: ie, the enum values are only the enumerators
> and the switch does cover all enumerators; then this warning is
> perfectly valid.
> 
> Therefore I would rather say that people who use enums for bit-masks and
> thus have variables of the enum type with values other than the
> enumerators (which is fine) should either not be using a switch on this
> enum OR simply disable this warning. Maybe casting `x` to `int` such as
> `switch(int(x))` would also work.
> 
> How do you propose to distinguish the two widely different usages of enum ?
> 
> I personally would be rather disappointed to see
> -Wcovered-switch-default disappear since at both the company I work on
> and in my pet programs we only ever use enum for its enumerators and
> thus this warning is very useful.

There is a misunderstanding here:

- I think that -Wcovered-switch-default is very useful and the warning
show above is a *TRUE* positive, so there is nothing to fix there

- my point is that -Wunreachable-code is showing a false negative for
the example above and should be fixed

-- 
Abramo Bagnara

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mailto:abramo.bagnara at bugseng.com