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

[Aleksey Shipilev]

On Wed, 25 Sep 2024 07:47:21 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

>> Please review this change that fixes -Wzero-as-null-pointer-constant warnings
>> in CompressedOops code.  These all relate to CompressedOops::base().
>> 
>> I also added a couple of asserts to verify our assumptions about null pointer
>> constants being representationally zero. That isn't a Standard-conforming
>> assumption, but holds for all platforms we currently support. I considered,
>> and even explored, a couple of different options.
>> 
>> (1) Continue to have CompressedOops::base() be a pointer, but avoid that
>> assumption, being more careful about how zero-valued pointers are treated. But
>> that adds significant complexity that we can't test, since we don't support
>> any platforms needing that extra work.
>> 
>> (2) Change CompressedOops::base() to an integral adjustment.  This is probably
>> the correct approach, but is much more intrusive and wide ranging in the
>> changes required.  Maybe something for the future.
>> 
>> Testing: mach5 tier1-5
>> GHA testing, verifying builds on some platforms not supported by Oracle.
>> 
>> There are some simple changes to s390 and ppc code that I haven't tested,
>> beyond verifying compilation.
>
> 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.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21172#discussion_r1774809759