RFR: 8340620: Fix -Wzero-as-null-pointer-constant warnings for CompressedOops

[Quan Anh Mai]

On Wed, 25 Sep 2024 08:36:37 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> src/hotspot/share/oops/compressedOops.inline.hpp line 53:
>> 
>>> 51:   // Assume a null base casts to zero.  Otherwise we need more complexity that
>>> 52:   // we can't test, since this is true for all currently supported platforms.
>>> 53:   assert(0 == reinterpret_cast<uintptr_t>(nullptr), "null pointer value not zero?");
>> 
>> Could this be a static_assert?
>
> +1. Although I would expect any sane compiler to fold it, maybe it is still not optimized with something like `-O0`. Or maybe just move these asserts to `CompressedOops::initialize`, so whatever happens, happens once.

This cannot be used in `static_assert` because `reinterpret_cast` is not allowed here.

I believe [`reinterpret_cast<uintptr_t>(nullptr)` will always return 0](https://en.cppreference.com/w/cpp/language/reinterpret_cast). You may need to do it the other way around.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21172#discussion_r1775048234