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

[Yasumasa Suenaga]

On Fri, 19 Mar 2021 00:02:21 GMT, Yasumasa Suenaga <ysuenaga at openjdk.org> wrote:

>>> The alloca was added as a performance boost for hyperthreaded systems
>>> back in 2003 for JDK 5:
>>> 
>>> "A per-thread offset was added to each thread's stack to randomize the
>>> cachelines of hot stack frames (aka, stack coloring)."
>> 
>> IIUC it relates to share DTLB between two logical processors when Hyperthreading is enabled.
>> 
>>> I'm running some of our benchmarks to see if removing it makes a
>>> difference ... but chances are I'm not going to be running it on the
>>> kind of machines for which it was introduced.
>> 
>> Thanks!
>> 
>>> > 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.
>>> 
>>> I'd reluctantly prefer to use the pragma as an official mechanism for
>>> avoiding the warning.
>> 
>> Ok, OpenJDK folks who discuss in this PR are not prefer to use pragma, so I will use `(void)!` if it is needed.
>> Let's see the result of benchmark and discuss what should we do.
>
>> > The alloca was added as a performance boost for hyperthreaded systems
>> > back in 2003 for JDK 5:
>> > "A per-thread offset was added to each thread's stack to randomize the
>> > cachelines of hot stack frames (aka, stack coloring)."
>> 
>> IIUC it relates to share DTLB between two logical processors when Hyperthreading is enabled.
> 
> `alloca()` call might be less effective if ASLR is enabled. It can be configured by [randomize_va_space](https://www.kernel.org/doc/html/latest/admin-guide/sysctl/kernel.html#randomize-va-space), and I guess it is enabled in a lot of x86 systems.

> BTW this is not the first time we have had this issue with gcc making a
> function deprecated and casting to void not fixing it. Unfortunately I
> can't remember enough of the previous case's details to actually look it
> up and see what we resolved to do. :(

I found [JDK-6879689](https://bugs.openjdk.java.net/browse/JDK-6879689), and it fixed unused-return as following:

https://github.com/openjdk/jdk/blob/9b5a9b61899cf649104c0ff70e14549f64a89561/src/hotspot/share/adlc/archDesc.cpp#L1082-L1083

I prefer it rather than `(void)!`.

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