[Bug c++/85747] suboptimal code without constexpr

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85747

--- Comment #2 from Marc Glisse <glisse at gcc dot gnu.org> ---
(In reply to Antony Polukhin from comment #0)
> Could the compiler detect that `a[7]` holds values known at compile time and
> force the constexpr on `sort(a + 0, a + 7);`?

There has to be a limit. If I write a program that computes the trillion's
decimal of pi, this is a constant, do you expect the compiler to evaluate the
whole program and compile it to just return cst? We are moving into a realm
where we would want to mix compilation and execution, sort of JIT.
For smaller functions, some heuristics could be used to try compile-time
evaluation, but sorting an array of size 7 already seems large to me.

> Could the compiler detect that
> the function `generate()` is an `__attribute__((const))` function without
> arguments and fully evaluate it's body?

If you call the function twice in a row, the compiler removes the second call
and reuses the result from the first, so at least it notices that generate is
pure.