On Thu, 12 Sep 2024 15:38:18 GMT, Thomas Stuefe <stu...@openjdk.org> wrote:
>> src/hotspot/share/opto/lcm.cpp line 272:
>>
>>> 270: const TypePtr* tptr;
>>> 271: if ((UseCompressedOops || UseCompressedClassPointers) &&
>>> 272: (CompressedOops::shift() == 0 ||
>>> CompressedKlassPointers::shift() == 0)) {
>>
>> Could you explain this change? It seems like it may affect C2's implicit
>> null check analysis even for `-XX:-UseCompactObjectHeaders`. In particular,
>> for the following configurations, the changed condition evaluates to true
>> before the change and false after it, regardless of whether
>> `UseCompactObjectHeaders` is enabled:
>>
>> (!UseCompressedOops, UseCompressedClassPointers,
>> CompressedKlassPointers::shift() != 0)
>> ( UseCompressedOops, !UseCompressedClassPointers, CompressedOops::shift() !=
>> 0)
>
> Hi @robcasloz
>
> The `CompressedKlassPointers` utility class is not usable anymore with
> `-UseCompressedClassPointers`. One change is that if
> `UseCompressedClassPointers` is off, `CompressedKlassPointers` stays
> uninitialized. And that makes more sense then to rely on the static
> initialization values of `CompressedOops::_shift`.
Thanks for the explanation. I wonder if the test is necessary at all, or one
could simply use `base->get_ptr_type()` unconditionally, which defaults to
`base->bottom_type()->isa_ptr()` anyway for non-compressed pointers. But this
simplification would be in any case out of the scope of this changeset.
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1758356268
