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Re: [std-proposals] Slow bulky integer types (128-bit)

From: Jonathan Wakely <cxx_at_[hidden]>
Date: Wed, 29 Mar 2023 10:55:48 +0100
On Wed, 29 Mar 2023 at 10:50, Giuseppe D'Angelo via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> Il 29/03/23 11:15, Timur Doumler via Std-Proposals ha scritto:
> >
> > Do I understand it correctly that the primary motivation for this change
> > in C23 and C++23 was that changing uintmax_t would be an ABI break,
> > which is deemed unacceptable for the major compiler vendors?
> Changing uintmax_t would be an ABI break. The change in C++23 is to
> actually allow implementations to actually offer __int128 as an extended
> integer type, while keeping e.g. a 64 bit intmax_t. Otherwise, they
> couldn't do that: the moment __int128 exists as an integer type,
> intmax_t needs to widen, and break ABI.
> It still sounds like a terrible idea to me. What's the point of intmax_t
> if it's not capable of faithfully representing values of *all* integer
> types? I'd rather see intmax_t deprecated than breaking completely
> reasonable code:
> > void f(std::signed_integral auto x)
> > {
> > std::intmax_t v{x}; // this should *always* be well-formed and never
> lose data
> > }
> The solution is simple:
intmx_t is not the maximum width integer type, it's the maximum width
integer type that has C APIs for working with it.

So it's the widest type that has an overload of std::div. Wider types might
be available, but there is no guarantee you can use them with std::div.

Just stop thinking the "max" means "maximum width, period" and see it as
"maximum width with libc support".

> > Because if the ABI issue didn't exist, it would indeed seem
> > *conceptually* more correct to have __uint128_t be an integer type and
> > uintmax_t to be 128 bit, would it not?
> Right, but this is now a question for compiler vendors -- why do they
> not consider __int128 an extended integer type at all (at least, in
> strict mode)?
> E.g. GCC https://gcc.gnu.org/onlinedocs/gcc/Integers-implementation.html
> Was it just for the intmax incompatibility?


It is impossible to meet all three of these constraints and conform to the

- intmax_t is the widest integral type
- __int128 is an integral type
- stable ABI

The choice is to be non-conforming (which libc++ does) or break one of the
three constraints.

In C23 and C++23 the first constraint has been weakened, so that libc++ is
now conforming, and a future release of GCC will define
is_integral_v<__int128> as true in strict mode.

Received on 2023-03-29 09:56:02