https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95252
Jim Wilson <wilson at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Ever confirmed|0 |1
Last reconfirmed| |2020-05-25
Status|UNCONFIRMED |NEW
--- Comment #1 from Jim Wilson <wilson at gcc dot gnu.org> ---
It appears to be failing in the rename register (rnreg) pass. This is because
the unspec patterns for the save/restore calls don't mention the registers that
they use/modify. This confuses rename reg into thinking that live regs are
dead, and it accidentally clobbers them before the save call. This worked OK
when save/restore calls could only be at the beginning or end of a function.
But now that this works with tail calls and shrink wrapping, we can get them in
inner blocks. Since the different save/restore calls use/modify different sets
of registers, fixing this gets a little complicated. Maybe we can just use the
max list of registers because listing extra ones shouldn't matter?
Another solution is to disable the rename register pass when -msave-restore is
used. This isn't doing any checking for whether regs can be used in compressed
instructions or not. This is currently encoded in REG_ALLOC_ORDER which this
pass doesn't use. The result is that this is probably increasing code size
which is undesirable when -msave-restore it used. Disabling this would reduce
code size and fix the -msave-restore problem.
The rename register pass does use the PREFERRED_RENAME_CLASS hook that we
haven't defined. We should try defining this to convert registers classes to
subsets that only include the regs that can be used in compressed instructions.
This might result in a code size decrease. If this works, then the rename reg
pass should not be disabled, and we should find a way to fix the save/restore
pattern register lists instead.
I need to do some builds and experiments to verify this info.
