tested with Clang 15 and Cray Compiling Environment 11 (clang) passing -O3 -fno-honor-nans to PETSc CC/CXX flags on

https://gitlab.com/jkruzik/htfeti

mpirun -n 8 ./ddHTFETI -mh 10 -mhc 1 -qps_view_convergence -qpt_dualize_B_nest_extension -linear -qps_ksp_monitor_true_residual -qps_rtol 1e-6

last QPSSolve DIVERGED due to DIVERGED_INDEFINITE_MAT, KSPReason=-10, required 26 iterations

Note: -fno-honor-nans is turned on by -ffinite-math-only, which is enabled by -ffast-math
Note 2: I did not observe any benefit in using -ffast-math flags and omitting -fno-honor-nans for the above program compiled with -O3 on Archer2 (but no extensive testing was done)