RFR: 8340620: Fix -Wzero-as-null-pointer-constant warnings for CompressedOops
Stefan Karlsson
stefank at openjdk.org
Wed Sep 25 07:51:35 UTC 2024
On Tue, 24 Sep 2024 23:26:08 GMT, Kim Barrett <kbarrett 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.
Looks good. One inquiry below.
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?
-------------
Marked as reviewed by stefank (Reviewer).
PR Review: https://git.openjdk.org/jdk/pull/21172#pullrequestreview-2327422676
PR Review Comment: https://git.openjdk.org/jdk/pull/21172#discussion_r1774716887
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