[PATCH v2] Document that the 'access' and 'nonnull' attributes are independent

[David Malcolm <dmalcolm@redhat.com>]

On Mon, 2022-03-14 at 16:18 -0600, Martin Sebor wrote:
> On 3/9/22 14:57, David Malcolm via Gcc wrote:
> > On Wed, 2022-03-09 at 13:30 -0800, Andrew Pinski wrote:
> > > 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 for the clarification...
> > 
> > > 
> > > 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, ...))
> > 
> > ...so that's (a) then.
> > 
> > I think it might be more user-friendly to be explicit about this in
> > the
> > documentation, maybe something like the attached?
> 
> I agree it's worth clarifying the manual.
> 
> But I don't think there's a way to annotate a function to indicate
> that it will definitely access an object (or dereference a pointer).
> Attribute access just implies that it might dereference it (unless
> the size is zero), and attribute nonnull that the pointer must not
> be null, not that it will be dereferenced (or even that it must be
> valid, although that's implied by the language and should probably
> be enforced in all contexts by some other warning).
> 
> The combination of access with nonzero size and nonnull only means
> that the pointer must be nonnull and point to an object with at least
> size elements.

Here's an updated version of the patch which expresses that also.

OK for trunk?

Dave

> 
> Martin
> 
> > 
> > (not yet fully tested, but seems to build)
> > 
> > Dave
> > 
> > 
> > 
> > 
> > > > 
> > > > (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
> > > > 
> > > 
> > 
> 

gcc/ChangeLog:
	* doc/extend.texi (Common Function Attributes): Document that
	'access' does not imply 'nonnull'.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>
---
 gcc/doc/extend.texi | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index a4a25e86928..790014f3da5 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -2652,6 +2652,15 @@ The mode is intended to be used as a means to help validate the expected
 object size, for example in functions that call @code{__builtin_object_size}.
 @xref{Object Size Checking}.
 
+Note that the @code{access} attribute merely specifies how an object
+referenced by the pointer argument can be accessed; it does not imply that
+an access @strong{will} happen.  If the pointer will definitely be
+dereferenced, you may want to also add
+@code{__attribute__((nonnull (@var{arg-index})))} to the function for the
+pointer parameter in question (though the presence of the @code{nonnull}
+attribute does not imply that the parameter will be dereferenced, merely
+that it must be non-null).
+
 @item alias ("@var{target}")
 @cindex @code{alias} function attribute
 The @code{alias} attribute causes the declaration to be emitted as an alias
-- 
2.26.3