RFR: 8256831: MIPS Zero builds fail with undefined __atomic_compare_exchange_8

[Jie Fu]

On Sun, 22 Nov 2020 14:40:12 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> If you try to build `linux-mipsel-zero-fastdebug`, this happens:
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> I think it relates to [JDK-8253970](https://bugs.openjdk.java.net/browse/JDK-8253970) that introduced `atomic_compare_exchange` on those paths, but maybe the issue exists for longer. 
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> Various other reports for build failures like this suggest the binary should link with `libatomic`. GCC's `libatomic` is the library that provides runtime support for atomics not supported in hardware, which seems to include 8-byte CAS for MIPS, but not any other OpenJDK platform.
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> The alternative would be to massage the otherwise generic Zero code to unimplement the 8-byte CAS.
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> Attention @DamonFool, who must be running into this problem for their MIPS builds?
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> Testing:
>  - [x] Linux mipsel Zero fastdebug build (together with JDK-8256829 fix)

Thanks @shipilev for fixing it.

I don't have a mips machine at hand.
Maybe, people from Loongson can verify this fix.  @theaoqi , @xiangzhai
Thanks.

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PR: https://git.openjdk.java.net/jdk/pull/1375