RFR: 8263718: unused-result warning happens at os_linux.cpp

[Yasumasa Suenaga]

On Thu, 18 Mar 2021 10:03:04 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> I can't really comment on which is better -- it's up to you hotspot developers.
>> 
>> Nevertheless, my personal preference is that `(void)` would have been best since it's a well-established idiom. If gcc is buggy about this, then I think I'd preferred to use this workaround with a comment `// the ! is needed to workaround gcc bug`, to keep with the idiom as much as possible. Pragmas also work, but I personally consider them ugly and a last resort. But once again, don't listen to what I'm saying :-)
>
> I agree with Magnus. BTW, we are sure that alloca() call is not simply optimized away, right? Otherwise I would assign the return value to some file static volatile holder.

First, I think it is the best to remove `alloca()`. According to the comment, it seems to aim to reduce cache pollution. But I wonder why this `alloca()` call resolves it because stack memory will be allocated in each threads - they should be different physical memory. This code has existed since initial load, so I cannot find JBS ticket for this. So I'm not sure we can remove it yet. (comments are welcome!)

If we decide to remain this code, we need to avoid unused-result warning from GCC. I think we can use pragma because other pragmas (format-nonliteral, format-security, stringop-truncation") are used in HotSpot. In addition, this behavior does not seem to determine to be a bug in GCC (status is UNCONFIRMED). However I will agree to use `(void)!` if it is still preferred based on them.

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PR: https://git.openjdk.java.net/jdk/pull/3042