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Subject: Re: Reinterpreting pointers of character types
From: Corentin (corentin.jabot_at_[hidden])
Date: 2021-01-29 01:46:51


On Fri, Jan 29, 2021, 08:20 Tom Honermann <tom_at_[hidden]> wrote:

> On 1/28/21 1:57 PM, Jens Maurer via SG16 wrote:
>
> On 28/01/2021 19.37, Corentin via SG16 wrote:
>
> On Thu, Jan 28, 2021 at 7:22 PM Peter Brett <pbrett_at_[hidden] <mailto:pbrett_at_[hidden]> <pbrett_at_[hidden]>> wrote:
>
> I think the big problem here is trying to make it a template.____
>
> __ __
>
> Make it named. It’s literally not possible to use this correctly in generic code.
>
>
> Question then is do we want to solve the issue for wchar_t?
> Because having the name of the encoding in the function kinda precludes that - the sizeof(wchar_t) being platform dependant
>
> You only get away with char* -> char8_t* because "char" has special
> aliasing exceptions.
>
> You'll get the full set of aliasing concerns for
> wchar_t* -> char16_t* or char32_t*
>
> I think what we're looking for is a portable solution for this ICU hack
> <https://github.com/unicode-org/icu/blob/master/icu4c/source/common/unicode/char16ptr.h#L30-L36>
> (generalized to make it work for [unsigned] char* conversion to char8_t*);
> the goal being to enable some form of explicit restricted pointer
> interconvertibility between same sized/aligned types.
>
> I don't understand the ICU hack sufficiently well to relate it to a memory
> or object model. I'm also not sure that it actually works (though it may
> suffice for the scenarios that are encountered in practice).
>
> Perhaps something like this would suffice.
>
> template<typename To, typename From>
> requires requires {
> requires std::is_trivial_v<To>;
> requires std::is_trivial_v<From>;
> requires sizeof(To) == sizeof(From);
> requires alignof(To) == alignof(From);
> }
> To* alias_barrier_cast(From *p) {
> asm volatile("" : : "rm"(p) : "memory");
> return reinterpret_cast<To*>(p);
> }
>
> I don't think we want to generalize to all trivial types which are not
characters types.
i would like to find out is whether the problem we know exists with
char/char8_t also exists for wchar_t/char16_t (or char32_t, although
wchar_t is of limited uses on platforms where its 32 bits).

> Tom.
>



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