[clang] [Clang] Implement __builtin_fpclassify (PR #190848)

[Yeongu Choe via cfe-commits]

YeonguChoe wrote:

> Why is this needed? It updates clang to use the `llvm.is.fpclass` intrinsic rather than direct operations that deduce the same information, but is that necessary?

Hello Andy

I may be wrong. But I think lowering C/C++ to intrinsic operation is better, because intrinsic operation is lowered to different machine IR depending on target during instruction selection phase.

https://github.com/llvm/llvm-project/pull/190848