[Bug target/98884] Implement empty struct optimisations on ARM

["david at westcontrol dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98884

--- Comment #2 from David Brown <david at westcontrol dot com> ---
Yes, ABI issues were my initial thought too.  If so, then optimising away the
assignments while leaving the stack manipulation (and possibly register
allocations) in place would still be a significant improvement.

However, I note that clang has no problem with generating ideal code here for
the ARM - it is not bothered by the ABI.  There could be several reasons for
that.  Perhaps the clang folk got the ABI wrong and the optimisation is not
valid for the ARM EABI.  Maybe the EABI used on the "arm (none)" target doesn't
specify these details, meaning the optimisation is valid there even if it is
not valid on "arm (linux)" targets.  I don't know the details of the ABIs well
enough to answer.

If it is an ABI issue, then I'd be quite happy with an ARM-specific flag to
enable an variation on the ABI that lets the compiler skip empty types
entirely.  When compiling for the Cortex-M devices, you rarely link much to
pre-compiled code (other than the C library) and it's usually fine to break
standards a bit to get more optimal code (like using "-ffast-math").

It would of course be best to have a general solution that works for all ARM
users (and ideally other targets too) without needing a flag.