[llvm] RuntimeLibcalls: Stop opting out of exp10 (PR #148604)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> For my understanding: this is expected to ICE because there is no possible libcall here (since `exp10l` is `f64` on Apple) and not because ICEs are expected when the default target lib isn't known to provide a symbol, right?

These are equivalent. There needs to be a known symbol that provides the behavior needed to implement exp10.fp128. The "default target lib" concept doesn't really exist, it's a historical implementation artifact. The compiler needs accurate knowledge about a concrete target

>  Otherwise I would be a bit concerned about our ability to make `fp128` work cross-platform by vendoring needed symbols (unrelated to this change of course).

In theory the backend needs to do something to provide the intrinsic behavior. This is currently built on top of emitting libm calls to shoddy host math libraries. In a better world we would have complete coverage of all functions x types in compiler-rt and ignore the host libraries, or rip out most math intrinsics and disallow types we don't have a direct compiler implementation of 



https://github.com/llvm/llvm-project/pull/148604