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From: "marxin at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/94779] Bad optimization of simple switch
Date: Fri, 18 Dec 2020 12:30:18 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94779

--- Comment #21 from Martin Li=C5=A1ka <marxin at gcc dot gnu.org> ---
> See my comment above.  It isn't any integration of VRP, just asking the
> ranger about the range, and it isn't useless because to be able to optimi=
ze
> properly,
> you need to figure out for each value one of the 3 possibilities (handled
> explicitly by switch and well defined, handled by default and never
> reachable or UB).

I've got the point that we have these 3 possibilities for each case.
However, we also have it for "holes" which are removed and we can't disting=
uish
them in between __builtin_unreachable and falling to the default case.

I believe a better place for Ranger is to improve:
simplify_using_ranges::simplify_switch_using_ranges (gswitch *stmt)

Note that even if CSWITCH does not do a transformation other passes can sti=
ll
benefit from a more canonical gswitch form.

That said, I don't like the idea of using Ranger in cswitch because similar
should do each pass that somehow works with switches.=