RFR(XS): 8229855: C2 fails with assert(false) failed: bad AD file

[Vladimir Ivanov]

> http://cr.openjdk.java.net/~thartmann/8229855/webrev.00/

-        IfNode *iff_le  = jump_if_fork_int(key_val, test_val, 
BoolTest::le, if_prob(cnt, total_cnt), if_cnt(cnt));
+        IfNode *iff_le  = jump_if_fork_int(key_val, test_val, 
BoolTest::lt, if_prob(cnt, total_cnt), if_cnt(cnt));

Would be nice to rename iff_le to iff_lt as well.

Otherwise, looks good.

Best regards,
Vladimir Ivanov

> For the switch statement in TestJumpTable.java, C2 creates the following binary search tree with a
> jump table at (*):
> 
> if (key_val > 45) {
>    if (key_val != 60) {
>      if (key_val <= 60) {
>        (*)
>      } else {
>        [...]
>      }
>    } else {
>      [...]
>    }
> } else {
>    [...]
> }
> 
> The jump table at (*) handles the following key_val ranges: {46..50}, {51}, {52..59}.
> Parse::jump_switch_ranges -> Parse::create_jump_tables adds a CastII to narrow the type of key_val
> to type int:46..59 (type int:0..13 after normalizing to zero):
> http://hg.openjdk.java.net/jdk/jdk/file/44cb1f517839/src/hotspot/share/opto/parse2.cpp#l884
> 
> Now after some rounds of loop unrolling, key_val is known to be int:60..260 (normalized to
> int:14..214) in one of the loop bodies. As a result, the narrow CastII node is replaced by TOP and
> we crash in the matcher because the JumpNode user didn't go away.
> 
> The problem is that the key_val != 60 && key_val <= 60 combination is *not* found to be always false
> if key_val becomes known as int:60..260 because C2's type system does not narrow the type after the
> first check. The fix is to simply strengthen the second check to ensure that both control and data
> paths die consistently.
> 
> I would like to push this simple fix into JDK 14 and follow-up in JDK 15 with an enhancement that
> adds a general optimization for such integer checks:
> https://bugs.openjdk.java.net/browse/JDK-8236721
> 
> I've quickly hacked a prototype that shows promising results (and would also fix this issue):
> http://cr.openjdk.java.net/~thartmann/8236721/webrev.00/
> 
> Thanks,
> Tobias
>