RFR: 8253375: OSX build fails with Xcode 12.0 (12A7209)

[Kim Barrett]

On Thu, 24 Sep 2020 21:28:01 GMT, Paul Hohensee <phh at openjdk.org> wrote:

> Please review this small patch to enable the OSX build using Xcode 12.0.
> 
> Thanks,
> Paul

No, don't do this.  In the original, double is used to obtain the desired
alignmnent.  Changing the element type to short reduces the alignment
requirement for the variable.  initialize_shared_locs will then adjust the
alignment, potentially shrinking the effective array size.  So this is a
real change that I think shouldn't be made.

I think changing the declaration for locs_buf to any of the following gives
equivalent behavior to the current code. I don't know whether any of them
will trigger the new clang warning though. I don't have access to that
version right now, and the documentation for the warning is useless (like so
much of clang's warning options documentation).

(1) alignas(double) char locs_buf[20 * sizeof(double)];
(but alignas is not yet in the "permitted features" list in the Style Guide:
https://bugs.openjdk.java.net/browse/JDK-8252584)

(2) union { char locs_buf[20 * sizeof(double)]; double align; };

(3) std::aligned_storage_t<20 * sizeof(double)> locs_buf;
and change (relocInfo*)locs_buf => (relocInfo*)&locs_buf
and #include <type_traits>
This has the benefit of being explicit that we're just stack allocating a
block of storage.

I'll make a wild guess that (1) and (2) will still warn, though char arrays
are somewhat special in the language so maybe they won't.  I'm pretty sure
(3) should do the trick.

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Changes requested by kbarrett (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/348