RFR: 8308504: C2: "malformed control flow" after JDK-8303466 [v3]

[Christian Hagedorn]

On Mon, 24 Jul 2023 11:18:10 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> I took that bug over from Emanuel because he's away:
>> https://github.com/openjdk/jdk/pull/14331
>> 
>> I tried adding a `CastII` to narrow the limit of the loop as I
>> suggested in a comment on the PR but I found that doesn't work in all
>> cases: if the type of the initial value for the loop variable is not
>> narrow enough, then the narrower type for the limit doesn't help
>> narrow the loop phi type.
>> 
>> What I propose instead is to add an assert predicate that catches when
>> the main loop is unreachable but the zero trip count doesn't constant
>> fold. For that to work, the order of predicates must be preserved when
>> they are copied or updated. I had to make some small changes to
>> guarantee that.
>
> Roland Westrelin has updated the pull request incrementally with one additional commit since the last revision:
> 
>   review

> I tried adding a CastII to narrow the limit of the loop as I
suggested in a comment on the PR but I found that doesn't work in all
cases: if the type of the initial value for the loop variable is not
narrow enough, then the narrower type for the limit doesn't help
narrow the loop phi type.

I hoped that this would be enough. Can you show an example where this failed?

src/hotspot/share/opto/loopPredicate.cpp line 1505:

> 1503:                                (stride > 0) != (scale > 0), overflow);
> 1504:   add_template_assertion_predicate_helper(predicate_proj, reason, upper_bound_proj, bol,
> 1505:                                           overflow ? Op_If : iff->Opcode());

Is this supposed to replace L1506-1510?

src/hotspot/share/opto/loopPredicate.cpp line 1524:

> 1522:   bol = rc_predicate(loop, new_proj, scale, offset, max_value, limit, stride, rng, (stride > 0) != (scale > 0),
> 1523:                      overflow);
> 1524:   add_template_assertion_predicate_helper(predicate_proj, reason, new_proj, bol, overflow ? Op_If : iff->Opcode());

You probably need to assign the result to `new_proj`?

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PR Review: https://git.openjdk.org/jdk/pull/14973#pullrequestreview-1543608845
PR Review Comment: https://git.openjdk.org/jdk/pull/14973#discussion_r1272338663
PR Review Comment: https://git.openjdk.org/jdk/pull/14973#discussion_r1272369875