[libc] [llvm] [APFloat] Add exp function for APFloat::IEEESsingle using expf implementation from LLVM libc. (PR #143959)

[Joshua Cranmer via llvm-commits]

https://github.com/jcranmer-intel commented:

I'm also concerned about having a function declaration but not definition if LLVM_INTEGRATE_LIBC is off.

https://github.com/llvm/llvm-project/pull/143959