[libc] [llvm] [libc][math] Implement single precision lgamma function (PR #205490)

[via llvm-commits]

zimmermann6 wrote:

the OpenGroup standard says that signgam is unspecified for NaN, -Inf or a negative integer. Thus for x=-0x1.eep+6 it should be set if you follow that standard.

https://github.com/llvm/llvm-project/pull/205490