Re: [PATCH] handle "invisible" reference in -Wdangling-pointer (PR104436)

[Martin Sebor <msebor@gmail.com>]

On 3/9/22 06:17, Richard Biener wrote:
> On Fri, Feb 11, 2022 at 12:05 AM Martin Sebor via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
>>
>> On 2/8/22 15:37, Jason Merrill wrote:
>>> On 2/8/22 16:59, Martin Sebor wrote:
>>>> Transforming a by-value arguments to by-reference as GCC does for some
>>>> class types can trigger -Wdangling-pointer when the argument is used
>>>> to store the address of a local variable.  Since the stored value is
>>>> not accessible in the caller the warning is a false positive.
>>>>
>>>> The attached patch handles this case by excluding PARM_DECLs with
>>>> the DECL_BY_REFERENCE bit set from consideration.
>>>>
>>>> While testing the patch I noticed some instances of the warning are
>>>> uninitentionally duplicated as the pass runs more than once.  To avoid
>>>> that, I also introduce warning suppression into the handler for this
>>>> instance of the warning.  (There might still be others.)
>>>
>>> The second test should verify that we do warn about returning 't' from a
>>> function; we don't want to ignore the DECL_BY_REFERENCE RESULT_DECL.
>>
>> The indirect aggregate case isn't handled and needs more work but
>> since you brought it up I thought I should look into finishing it.
>> The attached patch #2 adds support for it.  It also incorporates
>> Richard's suggestion to test PARM_DECL.  Patch #2 applies on top
>> of patch #1 which is unchanged from the first revision.
> 
> patch #1 would be OK if you'd do the PARM_DECL check there - I'd
> rather defer #2 to stage1.
> 
> Richard.
> 
>>
>> I have retested it on x86_64-linux and by building Glibc and
>> Binutils + GDB.
>>
>> If now is too late for the aggregate enhancement I'm okay with
>> deferring it until stage 1.

I committed the adjusted patch in r12-7650.  I'll try to remember to
ping the second patch in stage 1.

Martin

>>
>>>
>>>> +      tree var = SSA_NAME_VAR (lhs_ref.ref);
>>>> +      if (DECL_BY_REFERENCE (var))
>>>> +        /* Avoid by-value arguments transformed into by-reference.  */
>>>> +        continue;
>>>
>>> I wonder if we can we express this property of invisiref parms somewhere
>>> more general?  I imagine optimizations would find it useful as well.
>>> Could pointer_query somehow treat the reference as pointing to a
>>> function-local object?
>>
>> I don't quite see where in the pointer_query class this would be
>> useful (the class also isn't used for optimization).  I could add
>> a helper to the access_ref class to query this property in warning
>> code but as this is the only potential client I'm also not sure
>> that's quite what you had in mind.  I'd need some guidance as to
>> what you're thinking of here.
>>
>> Martin
>>
>>
>>>
>>> I previously tried to express this by marking the reference as
>>> 'restrict', but that was wrong
>>> (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97474).
>>>
>>> Jason
>>>