Am Freitag, dem 05.07.2024 um 17:23 +0200 schrieb Alejandro Colomar: > Hi Martin, > > On Fri, Jul 05, 2024 at 05:02:15PM GMT, Martin Uecker wrote: > > > But when the thing gets non-trivial, as in strtol(3), GCC misses the > > > -Wrestrict diagnostic, as reported in > > > <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112833>. > > > > > > Let's write a reproducer by altering the dumb.c program from above, with > > > just another reference: > > > > > > int > > > dumb2(int *restrict a, int *restrict *restrict ap) > > > { > > > // We don't access the objects > > > return a == *ap; > > > } > > > > > > int > > > main(void) > > > { > > > int x = 3; > > > int *xp = &x; > > > > > > return dumb2(&x, &xp); > > > } > > > > > > GCC doesn't report anything bad here, even though it's basically the > > > same as the program from above: > > > > > > $ cc -Wall -Wextra dumb2.c > > > $ > > > > strtol does have a "char * restrict * restrict" though, so the > > situation is different. A "char **" and a "const char *" > > shouldn't alias anyway. > > Pedantically, it is actually declared as 'char **restrict' (the inner > one is not declared as restrict, even though it will be restricted, > since there are no other unrestricted pointers). > > I've written functions that more closely resemble strtol(3), to show > that in the end they all share the same issue regarding const-ness: > > $ cat d.c > int d(const char *restrict ca, char *restrict a) > { > return ca > a; > } > > int main(void) > { > char x = 3; > char *xp = &x; > d(xp, xp); > } > $ cc -Wall -Wextra d.c > d.c: In function ‘main’: > d.c:10:9: warning: passing argument 2 to ‘restrict’-qualified parameter > aliases with argument 1 [-Wrestrict] > 10 | d(xp, xp); > | ^ > > This trivial program causes a diagnostic. (Although I think the '>' > should also cause a diagnostic!!) > > Let's add a reference, to resemble strtol(3): > > $ cat d2.c > int d2(const char *restrict ca, char *restrict *restrict ap) > { > return ca > *ap; > } > > int main(void) > { > char x = 3; > char *xp = &x; > d2(xp, &xp); > } > $ cc -Wall -Wextra d2.c > $ > > Why does this not cause a -Wrestrict diagnostic, while d.c does? How > are these programs any different regarding pointer restrict-ness?
It would require data flow anaylsis to produce the diagnostic while the first can simply be diagnosed by comparing arguments. Martin > > > > Well, I don't know how to report that defect to WG14. If you help me, > > > I'll be pleased to do so. Do they have a public mailing list or > > > anything like that? > > > > One can submit clarification or change requests: > > > > https://www.open-std.org/jtc1/sc22/wg14/www/contributing.html > > Thanks! Will do. Anyway, I think this should be discussed in glibc/gcc > in parallel, since it's clearly a missed diagnostic, and possibly a > dangerous use of restrict if the compiler does any assumptions that > shouldn't be done. > > Have a lovely day! > Alex >
