[Bug target/124165] [16 Regression] Many std::map<string, string> -> long compilation time at -O0 (late_pro_and_epilogue pass involved) since r16-7563

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124165

--- Comment #9 from Richard Biener <rguenth at gcc dot gnu.org> ---
Oh, and DF on hardregs is of course notoriously bad if you want any of this
in a precise way.

Instead of that you'd want to use DF UD/DU chains or fire up RTL SSA.  Or
do it on your own with a simple hardreg lattice and a single walk over all insn
of the function.