RFR: 8281429: PhiNode::Value() is too conservative for tripcount of CountedLoop

[Christian Hagedorn]

On Tue, 15 Mar 2022 16:02:54 GMT, Roland Westrelin <roland at openjdk.org> wrote:

> The type for the iv phi of a counted loop is computed from the types
> of the phi on loop entry and the type of the limit from the exit
> test. Because the exit test is applied to the iv after increment, the
> type of the iv phi is at least one less than the limit (for a positive
> stride, one more for a negative stride).
> 
> Also, for a stride whose absolute value is not 1 and constant init and
> limit values, it's possible to compute accurately the iv phi type.
> 
> This change caused a few failures and I had to make a few adjustments
> to loop opts code as well.

src/hotspot/share/opto/cfgnode.cpp line 1129:

> 1127:                   julong diff = (uhi - ulo - 1) / (-stride_t->lo_as_long()) * (-stride_t->lo_as_long());
> 1128:                   julong ufirst = hi->lo_as_long() - diff;
> 1129:                   first = reinterpret_cast<jlong &>(ufirst);

Could we also use a `static_cast` here and further down instead?

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PR: https://git.openjdk.java.net/jdk/pull/7823