Re: [PATCH] c++: End lifetime of objects in constexpr after destructor call [PR71093]

[Richard Biener <rguenther@suse.de>]

> Am 11.12.2023 um 20:12 schrieb Jason Merrill <jason@redhat.com>:
> 
> On 12/11/23 03:02, Richard Biener wrote:
>>> On Sun, 10 Dec 2023, Jason Merrill wrote:
>>> On 12/10/23 05:22, Richard Biener wrote:
>>>>> Am 09.12.2023 um 21:13 schrieb Jason Merrill <jason@redhat.com>:
>>>>> 
>>>>> On 11/2/23 21:18, Nathaniel Shead wrote:
>>>>>> Bootstrapped and regtested on x86-64_pc_linux_gnu.
>>>>>> I'm not entirely sure if the change I made to have destructors clobber
>>>>>> with
>>>>>> CLOBBER_EOL instead of CLOBBER_UNDEF is appropriate, but nothing seemed to
>>>>>> have
>>>>>> broken by doing this and I wasn't able to find anything else that really
>>>>>> depended on this distinction other than a warning pass. Otherwise I could
>>>>>> experiment with a new clobber kind for destructor calls.
>>>>> 
>>>>> It seems wrong to me: CLOBBER_EOL is documented to mean that the storage is
>>>>> expiring at that point as well, which a (pseudo-)destructor does not imply;
>>>>> it's perfectly valid to destroy an object and then create another in the
>>>>> same storage.
>>>>> 
>>>>> We probably do want another clobber kind for end of object lifetime. And/or
>>>>> one for beginning of object lifetime.
>>>> 
>>>> There?s not much semantically different between UNDEF and end of object but
>>>> not storage lifetime?  At least for what middle-end optimizations do.
>>> 
>>> That's fine for the middle-end, but Nathaniel's patch wants to distinguish
>>> between the clobbers at beginning and end of object lifetime in order to
>>> diagnose stores to an out-of-lifetime object in constexpr evaluation.
>> Ah, I see.  I did want to add CLOBBER_SOL (start-of-life) when working
>> on PR90348, but I always fail to finish working on that stack-slot sharing
>> issue.  But it would be for the storage life, not object life, also
>> added by gimplification.
>>> One option might be to remove the clobber at the beginning of the constructor;
>>> are there any useful optimizations enabled by that, or is it just pedantically
>>> breaking people's code?
>> It's allowing DSE to the object that was live before the new one.  Not
>> all objects require explicit destruction (which would get you a clobber)
>> before storage can be re-used.
>>>> EOL is used by stack slot sharing and that operates on the underlying
>>>> storage, not individual objects live in it.
>>> 
>>> I wonder about changing the name to EOS (end of storage [duration]) to avoid
>>> similar confusion with object lifetime?
>> EOS{L,D}?  But sure, better names (and documentation) are appreciated.
> 
> Maybe something like this?  Or shall we write out the names like CLOBBER_OBJECT_START, CLOBBER_STORAGE_END, etc?

Yeah, the abbreviations look a bit confusing so spelling it out would be better

Richard 

> 
> <0001-tree-add-to-clobber_kind.patch>