Undefined reference __powf_finite with clang 9.0.1, Linux 5.5.4-arch1-1 and glibc 2.31-1

[roxlu]

Clang fails to compile Filament with a recently updated linux system. When using the -ffast-math flag you'll run into errors like: undefined reference to __powf_finite. You can still build Filament by adding -fno-builtin to you CXXFLAGS.

[stasgora]

Running into the same problem here, have you been able to figure out the cause and a proper solution other than adding -fno-builtin manually?

[stasgora]

After a little digging that's what I figured out:
The issue is introduced with the 2.31 version of the GNU C Library.
The following symbols from LLVM libc++ math.h become undefined:

U __asinf_finite
U __atan2f_finite
U __exp2f_finite
U __exp2_finite
U __expf_finite
U __log2f_finite
U __powf_finite

Looking at the changelog the cause seems to be:

* The <math.h> functions that round their results to a narrower type
  now have corresponding type-generic macros in <tgmath.h>, as defined
  in TS 18661-1:2014 and TS 18661-3:2015 as amended by the resolution
  of Clarification Request 13 to TS 18661-3.