Re: [PATCH #2/4] c++: mark short-enums as packed

[Jason Merrill <jason@redhat.com>]

On 11/22/23 03:17, Alexandre Oliva wrote:
> On Nov 20, 2023, Jason Merrill <jason@redhat.com> wrote:
> 
>> I think the warning is wrong here.
> 
> Interesting...  Yeah, your analysis makes perfect sense.
> 
> Still, we're left with a divergence WRT the TYPE_PACKED status of enum
> types between C and C++.
> 
> It sort of kind of makes sense to mark short enums as packed, because,
> well, they are.

The enum is conceptually packed into a smaller integer type, sure.

> Even enum types with explicit attribute packed, that IIUC uses the same
> underlying type selection as -fshort-enums, IIRC are not be marked with
> TYPE_PACKED in C++, at least not at the place where I proposed to set
> it.  Do you consider that behavior correct?

Since attribute ((packed)) has this meaning, it seems reasonable to set 
TYPE_PACKED to express it.

> Even if the warning happens to be buggy in this regard, it is at best
> (or worst) accessory to this patch, in that it makes that difference
> between languages apparent, and I worry that there might be other middle
> end tests involving TYPE_PACKED that would get things different in C vs
> C++.  (admittedly, I haven't searched for occurrences of TYPE_PACKED in
> the tree, but I could, to alleviate my concerns, in case there's a
> decision to keep them different)

The middle-end doesn't seem to use TYPE_PACKED for anything other than 
structure layout.

>> In the analyzer testcase, we have a cast from an
>> enum pointer that we don't know what it points to, and even if it did
>> point to the obj_type member of struct connection, that wouldn't be a
>> problem because it's at offset 0.
> 
> Maybe I misunderstand the point of the warning, but ISTM that the
> circumstance it's warning about is real: the member is not as aligned as
> the enclosing struct, so the cast is risky.  Now, I suppose the idiom of
> finding the enclosing struct given a member is common enough that we
> don't want to warn about it in general.  I'm not sure what makes packed
> structs special in this regard, though.  I don't really see much
> difference, more laxly-aligned fields seem equally warn-worthy, whether
> the enclosing struct is packed or not, but what do I know?

Exactly.  If we want to warn about casting from pointer to less-aligned 
type to pointer to more-aligned type, that's already 
-Wcast-align=strict; whether the lower alignment is due to TYPE_PACKED 
seems irrelevant.

The observation that the type-based warning is a subset of 
-Wcast-align=strict was previously made in the discussion of the patch 
for PR88928.

And the motivating testcase for the warning was about converting from 
unaligned int* to aligned int*, not to a different type at all.  And 
that warning doesn't involve TYPE_PACKED.

The clang -Waddress-of-packed-member doesn't seem to include the 
type-based warning.

>> Also, -fshort-enums has nothing to do with structure packing
> 
> *nod*, it's about packing of the enum type itself.  It is some sort of a
> degenerated aggregate type ;-) But yeah, I guess it doesn't fit the
> circumstance the warning was meant to catch, and the fact that in C is
> does is a consequence of marking C short enums as TYPE_PACKED.
> 
> Which might be a bug in C.
> 
> But wouldn't it be a bug in C++ if an enum with attribute packed weren't
> markd as TYPE_PACKED?  Or is TYPE_PACKED really meant to say something
> about the enclosing struct rather than about the enclosed type itself?
> (am I getting too philosophical here? :-)

I'm coming to the conclusion that your C++ patch is fine but we should 
remove the TYPE_PACKED warning from 
check_address_or_pointer_of_packed_member.  And maybe add 
-Wcast-align=strict to -Wextra.

Jason