[Bug c/7652] -Wswitch-break : Warn if a switch case falls through

["arthur.j.odwyer at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=7652

Arthur O'Dwyer <arthur.j.odwyer at gmail dot com> changed:

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                 CC|                            |arthur.j.odwyer at gmail
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--- Comment #15 from Arthur O'Dwyer <arthur.j.odwyer at gmail dot com> 2012-11-26 22:49:02 UTC ---
TL;DR — I would like to see GCC and Clang both implement
__builtin_fallthrough().

I believe Lint recognizes the "magic comment" /*FALLTHROUGH*/

    case 1:
        foo();
        /*FALLTHROUGH*/
    case 2:

as a hint to suppress the warning. I think EDG's front-end has similar logic;
certainly Green Hills' compiler recognizes /*FALLTHROUGH*/. (My memory is fuzzy
because I no longer work there, but I know that Green Hills recognized a couple
kinds of magic comment before I got there, which would have been six years
ago.)  I admit that the "magic comment" approach has problems: for example, you
can't #define a macro to expand to a comment. Also it complicates the parser.

Clang currently suppresses the warning only if the C++11 attribute
[[clang::fallthrough]] is applied to a null statement immediately preceding
"case 2:", but this doesn't work outside of C++11 mode, and it's ridiculously
inappropriate as an industrywide solution (as it contains the word "clang" in
the name of the attribute).

    case 1:
        foo();
        [[clang::fallthrough]];
    case 2:

Alternatively, someone in this clang-dev thread has proposed adding a
__builtin_fallthrough() intrinsic that would suppress the warning, which is not
a bad idea at all.
http://clang-developers.42468.n3.nabble.com/should-Wimplicit-fallthrough-require-C-11-td4028144.html
Notice that __builtin_fallthrough() could be #defined away on compilers that
don't support it, and unlike a "magic comment", you can #define something to
expand *to* it as well.

    case 1:
        foo();
        __builtin_fallthrough();
    case 2:

I'd like to see __builtin_fallthrough() added to all major compilers. :)