On Wed, Mar 9, 2022 at 1:25 PM David Malcolm via Gcc <gcc@gcc.gnu.org> wrote: > > We gained __attribute__ ((access, ...)) in GCC 10: > https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html > which identifies one of the pointer/reference arguments of a function > as being accessed according to an access-mode: read_only, read_write, > write_only, or none. > > We also have __attribute__ ((nonnull)) to indicate that a function > argument (or all of them) must be non-NULL. > > There doesn't seem to be a relationship between these in the > implementation, but it strikes me that almost anywhere that a user > might use the "access" attribute, that parameter is probably going to > be required to be nonnull - though perhaps there are cases where APIs > check for NULL and reject them gracefully?
No, I think they are separate. The access just says these access attributes are read only, write only, read-write or don't access what the pointer points to; it does not say they have to be read or written to. I think it is a bad idea to connect the two ideas because you could have some cases where an argument is optional but is only read from; or is only written to (there are many in GCC sources even). Thanks, Andrew Pinski > > Might we want to somehow make __attribute__ ((access, ...)) imply > __attribute__ ((nonnull))? (for non "none" access modes, perhaps?) > > If so, one place to implement this might be in tree.cc's > get_nonnull_args, and have it add to the bitmap any arguments that > have an appropriate access attribute. > > get_nonnull_args is used in various places: > - validating builtins > - in ranger_cache::block_apply_nonnull > - by -Wnonnull (in pass_post_ipa_warn::execute) > - by -Wanalyzer-possible-null-argument and -Wanalyzer-null-argument; > I'm tracking the failure of these last two to make use of __attribute__ > ((access)) in PR analyzer/104860. > > So do we: > > (a) leave it up to the user, requiring them to specify __attribute__ > ((nonnull)) in addition to __attribute__ ((access, ...)) > > (b) leave it up to the individual sites in GCC that currently make use > of get_nonnull_args to add logic for handling __attribute__ ((access, > ...)) > > (c) extend get_nonnull_args > > ? > > Thoughts? > Dave >
