Tomasz Kaminski [Monday, 20 October 2025, 10:47:12 CEST]:
> […] so what I suggest instead is to introduce:
>
> template<typename... _Args>
> [[__gnu__::__always_inline__]]
> constexpr __should_constprop(const _Args&... __args)
> {
> if conteval { // of if (__builtin_is_constant_evaluated())
> return true;
> }
> return __is_constprop(__args) && ...;
> }
>
> The _M_isconstprop member on simd, should become __is_constprop hidden
> friend.
> […]
BTW, I re-checked why I needed __builtin_is_constant_evaluated() even though
__builtin_constant_p(arg) should always be true in constant evaluated code. It
seems this was only necessary for Clang <= 18. I'd still keep it as a shortcut
in the multi-arg overload to avoid asking all these __is_constprop questions
in constant expressions.
Also, the difference in naming between __should_constprop and __is_constprop
is too subtle. I now named both functions __is_constprop. With Clang >= 19
that should also be fine.
However, I'm not really happy with how the code reads now, because that "is
constprop" includes is_constant_evaluated isn't that obvious in the name.
How about __is_value_known(...) instead of __is_constprop(...)?
Motivation: e.g.
if (!__is_constprop(__x))
This is saying "if not consteval and the value of __x is not constant
propagated". But that first part is not obvious IMHO.
if (!__is_value_known(__x))
The value clearly is known in a constant expression. Maybe it's less clear
that the value can also be known via constant propagation. But that shouldn't
really matter.
- Matthias
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Dr. Matthias Kretz https://mattkretz.github.io
GSI Helmholtz Center for Heavy Ion Research https://gsi.de
std::simd
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