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Re: RFC: lra-constraints.c and TARGET_HARD_REGNO_CALL_PART_CLOBBERED question/patch

On Thu, 2018-07-12 at 07:17 +0100, Richard Sandiford wrote:
> So it only calls targetm.hard_regno_call_part_clobbered if such a
> call is known to exist somewhere between the two references to
> regno (although we don't have the calls themselves to hand).
> Thanks,
> Richard

Having the specific calls is the problem here because the normal ABI
and the SIMD ABI are going to have different values for caller_save
and part_clobbered.  But I think I have found a better way to address

Looking at 'need_for_call_save_p', when 'flag_ipa_ra' is set, we look
at 'actual_call_used_reg_set' to see if we need to save a
register.  But even if a particular register doesn't show up as used,
if 'hard_regno_call_part_clobbered' is set for that register we are
going to return true and say we need a save/restore of the
register.  On Aarch64, if I 'really' didn't use the register I
shouldn't need to save/restore it.  If I used it but it is callee saved
then on 'normal' functions I may need to save/restore it (to protect
the upper bits) but on SIMD functions I do not need to save/restore it
at all because the callee will save/restore the entire register and not
just the lower 64 bits.

I think that the patch I want to create is: when 'flag_ipa_ra' is true,
remove the check of 'hard_regno_call_part_clobbered' from
'need_for_call_save_p'.  Instead, check
'hard_regno_call_part_clobbered' in 'process_bb_lives' (where we know
exactly what function is being called) and use that to set
'actual_call_used_reg_set'.  In process_bb_live we know exactly what
function we are calling and can check to see if it is a 'normal'
function or a SIMD function.

Steve Ellcey

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