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From: "amacleod at redhat dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/94779] Bad optimization of simple switch
Date: Thu, 17 Dec 2020 16:38:39 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94779

--- Comment #18 from Andrew Macleod <amacleod at redhat dot com> ---
(In reply to Jakub Jelinek from comment #17)
> (In reply to Martin Li=C5=A1ka from comment #16)

> >=20
> > EVRP knows the proper range:
> > 2->4  (F) x_6(D) : 	unsigned int [0, 1][4, 4]
>=20
> EVRP ATM invokes both the old code and ranger and only ranger knows the
> above.
> There is a param to adjust the behavior.
> Anyway, if something isn't able to work with the detailed ranges (up to 2=
55
> subranges), type conversions will ensure that one gets a single summary
> range ([0, 4] in this case likely) and possibly the switch opts still do
> that.
>

Indeed.  Ranger knows, but at this point most of the client consumers such =
as
folding and simplification still only deal with value_range, which means th=
ey
will revert to using the best approximation using only a pair, which would =
be
[0,4] in this case as you observe.

One of the work items for the next release is to multi-range enable all the=
se
consumers that can make use of the information.=