From: Martin Sebor <msebor@gmail.com>
To: Jason Merrill <jason@redhat.com>, gcc-patches <gcc-patches@gcc.gnu.org>
Subject: PING [PATCH] handle "invisible" reference in -Wdangling-pointer (PR104436)
Date: Tue, 1 Mar 2022 16:14:48 -0700 [thread overview]
Message-ID: <90625bdb-a2dc-ac21-e013-7607ba42c0a7@gmail.com> (raw)
In-Reply-To: <b52a2cd4-58ec-e0ef-2e9c-751cebd44fc2@gmail.com>
Pinging the two patches here:
https://gcc.gnu.org/pipermail/gcc-patches/2022-February/590230.html
On 2/10/22 16:04, Martin Sebor 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.
>
> 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.
>
>>
>>> + 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
>>
next prev parent reply other threads:[~2022-03-01 23:14 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-08 21:59 Martin Sebor
2022-02-08 22:37 ` Jason Merrill
2022-02-09 8:30 ` Richard Biener
2022-02-09 15:00 ` Jason Merrill
2022-02-10 23:04 ` Martin Sebor
2022-03-01 23:14 ` Martin Sebor [this message]
2022-03-09 13:17 ` Richard Biener
2022-03-16 15:47 ` Martin Sebor
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