On 20/02/2023 12:59, Xi Ruoyao wrote: > On Mon, 2023-02-20 at 11:30 +0000, Jonny Grant wrote: > >> Thank you for the suggestion, I gave that nonnull attribute a try, but >> it doesn't appear to warn for this example. >> >> https://godbolt.org/z/boqTj6oWE > > Ouch... The optimizer inlined make_std_string so both -Wnonnull and - > fanalyzer fails to catch the issue here. > > Adding noipa attribute for make_std_string will work, but will also > cause the generated code stupidly slow. Maybe: > > #ifdef WANT_DIAGNOSTIC > #define MAKE_STD_STRING_ATTR __attribute__ ((noipa, nonnull)) > #else > #define MAKE_STD_STRING_ATTR > #endif > > std::string make_std_string(const char * const str) MAKE_STD_STRING_ATTR; > > It still looks very stupid though. > >> Feels useful to get build warnings if compiler knows nullptr is going >> to be dereferenced, as clang does. > > The problem is in this case nullptr is not dereferenced, at all. So if > we want a warning here we'll have to invent some new __builtin or > __attribute__ to give the compiler a hint. AFAIK there is no such > facility now. I see, yes it is tricky, as many functions take NULL, like time(NULL) from <time.h>, need to actually dereference it to get the build warning! that basic_string() constructor doesn't dereference the nullptr as it throws logic_error("construction from null is not valid") So then I am back to putting my actual dereference in, just in that build where it is enabled TRIGGER_NULLPTR_WARNING https://godbolt.org/z/4h3MqGh1s I added myassert() to catch it otherwise so I can at least get the file and line info rather than just the basic_string exception throw logic_error message > And you cannot simply justifying to add a new facility because "I feel > it useful". Generally you need to show the benefit will be at least > equally great than the maintenance burden introduced into the GCC code > base. And unfortunately usually we can only measure the burden after > really writing all the code... So it's not easy to convince someone to > develop such a new feature. > >> Personally I feel runtime should equally handle possible nullptr by >> constructing strings in a try catch block so any exceptions are >> handled or logged at least... > > A portable runtime should not assume std::string(NULL) will raise an > exception because other C++ standard libraries may behave differently. > The portable solution is to make a wrapper around std::string > constructor and check if the parameter is NULL. You're right, I shouldn't rely on this logic_error which maybe specific to GNU. I made another example that wraps, and also catches exceptions https://godbolt.org/z/ez3q76cj4 There was an issue as char_traits does __builtin_strlen() which doesn't check for nullptr so crashes, so I couldn't just do out_string = str; >> Personally I would be pleased if GCC had a warning I could enable to >> report any logic_error exceptions it knew would execute. > > Or maybe when a program will definitely raise an uncatched exception. > But is the feature really useful? This will not happen for anything > other than simple toy programs. That's a good point. I've seen C++ programs sometimes don't have good exception handling, some don't have any at all, it goes right back to the top and calls abort() when there's a std::out_of_range exception etc. It would be good to have the opportunity to see a build warning when there would be an uncaught exception the optimizer knows about. Kind regards Jonny
