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

[Jason Merrill <jason@redhat.com>]

On 2/9/22 03:30, Richard Biener wrote:
> On Tue, Feb 8, 2022 at 11:38 PM Jason Merrill via Gcc-patches
> <gcc-patches@gcc.gnu.org> 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.
>>
>>> +       tree var = SSA_NAME_VAR (lhs_ref.ref);
>>> +       if (DECL_BY_REFERENCE (var))
> 
> I think you need to test var && TREE_CODE (var) == PARM_DECL here since
> for DECL_BY_REFERENCE RESULT_DECL we _do_ escape to the caller.  Also
> SSA_NAME_VAR var might be NULL.
> 
>>> +         /* 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 think points-to analysis got this correct when the reference was marked
> restrict but now it also fails at this, making DSE fail to eliminate the
> store in
> 
> struct A { A(); ~A(); int *p; };
> 
> void foo (struct A a, int *p)
> {
>    a.p = p;
> }

Well, that's conservatively correct; since we don't know the definition 
of ~A, we don't know whether it copies p somewhere, e.g.

int *global_p;
A::~A() { global_p = p; }

in which case eliminating the store would be an invalid optimization, 
just as it would be if 'a' were a local variable.

>> 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).