https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84443
--- Comment #2 from Segher Boessenkool <segher at gcc dot gnu.org> --- If you want some specific machine code for something complex like this, it is much easier to write the machine code than to fight the compiler. That said: 1) "flags" is stored in the same register everywhere. This is a problem in expand: it puts the return value in the same pseudo in the whole function. This is a bad idea because (if it did not) after optimisation the return value is just a hard register (r3) and putting all _other_ uses in the same register is a pessimisation; like here (and in many other cases, there are other PRs for this!) it causes that pseudo to be put in a callee-save register (r30). 2) That should be fixed in expand (to enable other optimisations with the return pseudo), but IRA should also be smarter about live-range splitting for shrink-wrapping. 3) Separate shrink-wrapping should deal with CRFs. I have a prototype, it has a few small problems (we need to handle things a little differently for pretty much every ABI, sigh), and it does not help very much until the other problems are solved; GCC 9 work); 4) Shrink-wrapping itself could also do more live-range splitting than it currently does.
