On Thu, 30 Jan 2025 at 09:41, Breno Guimarães via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> But in your paper you copy the contents to unaligned by copy the object
> contents into byte representation. That is fine. It's not UB.
>

It uses memcpy on a non-trivially copyable type, that's UB.


> And everytime you access It, you put It on aligned memory first, so its ok
> too.
> I don't see how this paper requires any change
>
> Em qui., 30 de jan. de 2025, 06:18, Frederick Virchanza Gotham via
> Std-Proposals <std-proposals_at_[hidden]> escreveu:
>
>> On Thu, Jan 30, 2025 at 12:14 AM Thiago Macieira wrote:
>> >
>> > On Wednesday 29 January 2025 Frederick wrote:
>> >>
>> > > All I'm asking for is that pointer arithmetic will be well-defined for
>> > > all pointer types even when the address is unaligned -- and I think
>> > > this won't affect any optimisers. In fact it won't even require a
>> > > change to any modern compilers.
>> >
>> > You haven't answered the most important question: why?
>> >
>> > What does this allow you to do that it didn't before?
>>
>>
>> I've mentioned the reason a few times but I'll spell it out here. The
>> libstdc++ implementation of 'std::string' contains a pointer into its
>> own buffer, sort of like:
>>
>> class string {
>> char buf[16];
>> char *p;
>> string(void) { p = buf; }
>> };
>>
>> If I relocate a string from aligned memory into unaligned memory, then
>> everything will be okay because the pointer 'p' points to something
>> that doesn't have any alignment requirements.
>>
>> But now let's consider basic_string<char32_t>:
>>
>> class u32string {
>> char32_t buf[16];
>> char32_t *p;
>> u32string(void) { p = buf; }
>> };
>>
>> The pointer 'p' points to something that has an alignment requirement
>> of 4. There are two consequences to this:
>> (1) Strictly speaking, sizeof(void*) might be bigger than
>> sizeof(char32_t*). Of course this is just a fairytale and no such
>> computer exists nowadays, but nonetheless the Standard needlessly
>> accommodates it.
>> (2) If I set 'p' to an unaligned address, maybe the lower 2 bits
>> won't retain their value because of UB.
>>
>> On Page 5 of this paper, I relocate a basic_string into unaligned
>> memory: http://www.virjacode.com/papers/unaligned_latest.pdf
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