[libc-commits] [libc] [libc][math] Qualify log with constant evaluation support (PR #184745)

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lntue wrote:

> So, the rational for `LIBC_CONSTEXPR` is not to replace `constexpr` qualifier completely, but rather:
> 
> * to indicate the function may or may not have constant evaluation code
> * work hand in hand with `LIBC_ENABLE_CONSTEXPR` to force constant evaluation code in function body
> * mark these functions as `constexpr` when the constant evaluation path is forced
> 
> Any function that can be `constexpr` all the way with no violation in its function body shall be marked as `constexpr`. Any function that is needed in constant evaluation context but it has compliant and non-compliant code section in it shall be marked as `LIBC_CONSTEXPR`.
> 
> `LIBC_CONSTEXPR` does not mark or qualify the function as `constexpr` if it's not enforced manually.
> 
> The constraints it poses is the following:
> 
> * Any function that uses `constexpr`-qualified function can be qualified either as `constexpr`, `LIBC_CONSTEXPR` or none.
> * Any function that uses `LIBC_CONSTEXPR`-qualified function can only be qualified as `LIBC_CONSTEXPR` (because of the nature of `LIBC_CONSTEXPR`) or none
> 
> We can get rid of `LIBC_CONSTEXPR` and only mark as `constexpr` all the way but this will cause trouble with GCC as noted before in the refactoring effort when we are not producing `constexpr` expressions, so that's why we need a way to dynamically adjust the qualifier and the underlying code according to the evaluation mode.
> 
> It shall be noted that the constant evaluation code route used to implement internal utils as `constexpr` compatible depends on a specific compiler features that's not available in all compilers/versions, thus the dependent targets (`LLVM::APFloat` in this case) should upgrade the minimum compiler version used to compiler this specific portion of the code.

Can you add a TODO to the comment above `LIBC_CONSTEXPR` definition to remove the macro once clang/LLVM bump their min GCC version later than 9 (?)

https://github.com/llvm/llvm-project/pull/184745