Re: [PATCH] Add a new type attribute always_alias (PR79671)

[Bernd Edlinger <bernd dot edlinger at hotmail dot de>]

On 04/05/17 22:17, Jason Merrill wrote:
> On Wed, Apr 5, 2017 at 1:41 PM, Bernd Edlinger
> <bernd.edlinger@hotmail.de> wrote:
>> On 04/05/17 17:20, Richard Biener wrote:
>>>> Also, wonder if you need to mark all types containing such arrays,
>>>> if you couldn't just set that flag in C++ on unsigned char/std::byte
>>>> arrays (and on anything with that attribute), have that imply alias set
>>>> 0 on it and then let the rest of alias machinery handle aggregate types
>>>> containing such fields.
>>>
>>> Yes, I expected it to work like this (didn't look at the patch yet).
>
> My impression is that this is how GCC 6 worked, but GCC 7 decides to
> ignore alias set 0 members.  Is that right?
>

This is how I always thought it worked, until Jakub pointed out that
it is documented differently:

@item may_alias
@cindex @code{may_alias} type attribute
Accesses through pointers to types with this attribute are not subject
to type-based alias analysis, but are instead assumed to be able to 
alias any other type of objects.
In the context of section 6.5 paragraph 7 of the C99 standard,
an lvalue expression dereferencing such a pointer is treated like
having a character type.

=> So nobody said anything about accesses without pointers.



>> I want to allow *only* what the C++ standard requires or what Jason says
>> of course :), and not a single bit more, because it suppresses otherwise
>> correct optimizations.
>>
>> So a struct with a std::byte member is not alias_set 0,
>> only the std::byte itself is, but an array of std::byte
>> is itself typeless_storage, and makes the whole structure
>> also typeless_storage
>
> Well, only the array member, not the whole structure, but it may make
> sense for GCC to treat the whole structure as such internally.
>


Yes, that is only as close as I can get in the moment.

It does the same as may_alias but additionally objects
declared with that type have alias set 0, I just don't
know if I have yet found the right words so that it can
be understood.  Based on the feedback I have now written:

+@item typeless_storage
+@cindex @code{typeless_storage} type attribute
+An object declared with a type with this attribute behaves like a
+character type with respect to aliasing semantics.



Bernd.