[cfp-interest 3550] Fwd: [SC22WG14.32508] The [[reproducible]] Example 3, §6.7.13.8.3

[Jim Thomas]

> Begin forwarded message:
> 
> From: JeanHeyd Meneide <phdofthehouse at gmail.com>
> Subject: [SC22WG14.32508] The [[reproducible]] Example 3, §6.7.13.8.3
> Date: July 18, 2025 at 3:33:01 PM PDT
> To: wg14 <sc22wg14 at open-std.org>
> 
> Hi Everybody,
> 
>     The C Floating Point group met during its May meeting and
> reviewed the use of [[reproducible]] in conjunction with this code
> snippet in the standard N3550 in EXAMPLE 3, §6.7.13.8.3:
> 
>    inline double distance (double const x[static 2]) [[reproducible]] {
>        #pragma STDC FP_CONTRACT OFF
>        #pragma STDC FENV_ROUND  FE_TONEAREST
>        // We assert that sqrt will not be called with invalid arguments
>        // and the result only depends on the argument value.
>        extern typeof(sqrt) [[unsequenced]] sqrt;
>        return sqrt(x[0]*x[0] + x[1]*x[1]);
>    }
> 
> The problem, unfortunately, is that this is not enough to make it
> impossible to observe the effect of calling `sqrt` due to certain
> floating point exceptions / floating point flags. While there is a way
> in TS 18661-5 to turn off those flags from occurring with
> 
> #pragma STDC FENV_EXCEPT FE_ALL_EXCEPT NO_FLAG
> 
>     That is not in the main text of the standard, and therefore is
> unusable. CFP's recommendation is to remove this snippet since the
> code is incorrect and cannot be made correct without invalidating the
> purpose of the example, and so I've done so.
> 
>     We are going to need a new example for a [[reproducible]]
> attribute, here. Preferably, something that shows its ability to be
> applied to functions; it could perhaps be used with a fixed-point
> bounding box collision test function, or something similar
> lightweight.
> 
>     Let me know if you have an idea or example for what could replace it.
> 
> Sincerely,
> JeanHeyd

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