[PATCH 2/2] add -Wdangling-pointer [PR #63272]

Eric Gallager egall@gwmail.gwu.edu
Tue Nov 2 07:40:44 GMT 2021

On Mon, Nov 1, 2021 at 6:20 PM Martin Sebor via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
> Patch 2 in this series adds support for detecting the uses of
> dangling pointers: those to auto objects that have gone out of
> scope.  Like patch 1, to minimize false positives this detection
> is very simplistic.  However, thanks to the more deterministic
> nature of the problem (all local objects go out of scope) is able
> to detect more instances of it.  The approach I used is to simply
> search the IL for clobbers that dominate uses of pointers to
> the clobbered objects.  If such a use is found that's not
> followed by a clobber of the same object the warning triggers.
> Similar to -Wuse-after-free, the new -Wdangling-pointer option
> has multiple levels: level 1 to detect unconditional uses and
> level 2 to flag conditional ones.  Unlike with -Wuse-after-free
> there is no use case for testing dangling pointers for
> equality, so there is no level 3.
> Tested on x86_64-linux and  by building Glibc and Binutils/GDB.
> It found no problems outside of the GCC test suite.
> As with the first patch in this series, the tests contain a number
> of xfails due to known limitations marked with pr??????.  I'll
> open bugs for them before committing the patch if I don't resolve
> them first in a followup.
> Martin

So, I'd just like to take this chance to re-state my preference (as a
user) for having separate named options for warnings instead of having
a single option with multiple levels, so that users can toggle just
one but not the other. With the numerical levels, one can detect only
unconditional uses, and not conditional ones, by using level one, but
they can't do it the other way around (i.e. detecting conditional
uses, but not unconditional ones), though. I think having a split like
the one that exists between -Wuninitialized and -Wmaybe-uninitialized
would make sense here.


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