On Mon, Aug 25, 2025 at 5:54 PM Adrian Johnston <adrian3_at_[hidden]> wrote:


> I am not suggesting the standard somehow defined constexpr to do
> nothing at all. What I am saying is there is a distinction between
> "it can be evaluated at compile time" and "the code generator will
> more thoroughly evaluate it at compile time than any other code if it
> doesn't have to." Can you show me an example of constexpr resulting in
> better code generation at runtime (and not compile time?)
>

That is not what constexpr is for when applied to function. constexpr does
not mean: optimizer try to generate better codegen for this function.
It means (for functions): this function can be called at compile time. Now
that in itself may generate better code,
as we saw with the sort example I provided, it forced clang to execute sort
at runtime so there was no runtime call to sort.
Not because std::sort is constexpr, but because variable was constexpr. Now
obviously to initialize variable that required std::sort to be constexpr,
what I am saying is that important part of constexpr is what I wrote
before:
when I see
 constexpr auto var = ...

I know var is computed at compile time or compilation will fail.


>
> Ok, but I have been writing C++ for 20 years and I still don't have
> any idea what people are doing running all this code C++ at compile
> time. I know of examples from numerical computing that are
> compelling. My only use case failed horribly and silently. Yeah, sure
> constexpr is better than template meta programming, but that wasn't a
> problem we had all day long.
>

If you problems do not require constexpr do not use constexpr.
If you are not sure if your problems benefit from constexpr check out some
respected open source libraries related to your problem and see
if they use constexpr or google some blog articles/cppcon talks, e.g High
Performance Message Dispatch - Luke Valenty
<https://www.youtube.com/watch?v=DLgM570cujU>

But there is no requirement for you to use every feature of the language.