RFR: 8280476: [macOS] : hotspot arm64 bug exposed by latest clang [v2]

[Daniel D.Daugherty]

On Fri, 4 Feb 2022 10:46:22 GMT, Andrew Dinn <adinn at openjdk.org> wrote:

>>> First thing to note is that `replicate` is never actually called with `count == 0`.
>> 
>> This is crucial: it means we have no UB.
>> 
>> Could I ask you to take the opportunity add a little of this commentary to the rather opaque code? Pretty please? :-)
>
>> Could I ask you to take the opportunity add a little of this commentary to the rather opaque code? Pretty please? :-)
> 
> Sure,this stuff really needs to be better documented in the code itself. I'll do that as a follow-up to this PR.

>>    First thing to note is that replicate is never actually called with count == 0.

> This is crucial: it means we have no UB.

The UB occurs when `count == 1` and `nbits == 64`.

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PR: https://git.openjdk.java.net/jdk/pull/7270