[Bug tree-optimization/78332] [ARM] Negative costs of ivopts groups

["amker at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78332

amker at gcc dot gnu.org changed:

           What    |Removed                     |Added
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                 CC|                            |amker at gcc dot gnu.org

--- Comment #1 from amker at gcc dot gnu.org ---
I think there are two issues here.
First is the negative cost.  It's because we book saved cost of auto-increment
addressing mode to use-cand cost, rather than the candidate itself.  Given it
only makes sense to use auto-increment candidate for the use why it is created,
I think we can change the strategy by booking saved cost to candidate it self. 
This issue could be improved, but not a bug I think.

Second is auto-increment candidate is preferred too much.  This is again a cost
model issue.  A. to how much auto-increment should be preferred;  B. should
register cost be tightened because currently the effect is only spill cost in
loop is considered, cost of epilogue/prologue pop/push of more registers is not
handled.  Also addressing mode costs returned by backend need to be considered
too.  I will further investigate this later.