https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61236
--- Comment #9 from Mukund Sivaraman <muks at banu dot com> --- Hi Jakub, Markus We discussed this during our daily standup call today, and there are two points we'd like to make: 1. The qsort() defintion in C99 doesn't explicitly state that base must not be NULL, though it seems you are deducing that from "the initial element of which is pointed to by base." The POSIX definition of qsort() adds this: "If the nel argument has the value zero, the comparison function pointed to by compar shall not be called and no rearrangement shall take place." 2. From our perpective as users of GCC, this kind of agressive optimization seems counter-intuitive. We'd like code to compile to correct object code first before performance. When the compiler knows at that point that base (=x) is NULL as an argument to qsort(), why isn't it warning when the attribute expects it to be non-NULL, esp. as it is using this inferred decision to optimize code down below? The compiler knows x is NULL at this point in this codepath regardless of what qsort()'s attributes say. Why is it using the attribute then? qsort() also does not assert (at runtime) that base is non-NULL. There is no way to detect this for code which used to run correctly before, but doesn't anymore (without it _hopefully_ crashing somewhere). Other similar functions such as memcpy(), etc. also have this annotation in glibc, whereas there is no definition of n=0 case in C99. This example of qsort() is in libc, but imagine a case where a program uses a 3rd party system installed utility shared library. If the library, in a new version, adds a nonnull annotation for a function, but the library function itself continues to work for NULL input, see what happens to the program: The library is not affected, but the pointer in the calling program is affected if the compiler infers that the pointer is non-NULL due to the attribute. The calling program is now buggy due to a change in the library. How do we discover it? It makes sense to just avoid the qsort() in our case and we will update our code to do so, but please consider the arguments above.
