[llvm] [clang] [clang-tools-extra] [PowerPC] Implement llvm.set.rounding intrinsic (PR #67302)

[Chen Zheng via cfe-commits]

https://github.com/chenzheng1030 commented:

Maybe we can do some perf test between this expansion for set rounding mode and the system library's version for `fesetround()`. On AIX, I saw some improvements were introduced in the system library's implementation.

https://github.com/llvm/llvm-project/pull/67302