RFR: 8159370: Add FlagGuard for easier modification of flags for unit tests

[Jesper Wilhelmsson]

Looks good!
/Jesper

Den 14/6/16 kl. 12:58, skrev Erik Helin:
> Hi all,
>
> this patch adds a small utility class that makes it easier for TEST_VM
> unit tests to safely modify flags. The class FlagGuard will store the
> value of the flag in its constructor and then restore the flag in its
> destructor. This is similar IntFlagSetting, UIntFlagSetting etc, but the
> FlagGuard differs in two aspects:
> - it does not set a new value
> - it is "untyped", you can use FlagGuard for flags with any type
>
> I have also added a macro, GUARD_FLAG, to make it more convenient to
> guard a flag. A test might now look like:
>
>   TEST_VM(G1Policy, test_calc_max_old_cset_length) {
>     GUARD_FLAG(MaxGCPauseMillis);
>     GUARD_FLAG(ParallelGCThreads);
>
>     MaxGCPauseMillis = 500;
>     ParallelGCThreads = 10;
>     ASSERT_EQ(10u, G1Policy::calc_max_old_cset_length());
>
>     MaxGCPauseMillis = 50;
>     ParallelGCThreads = 1;
>     ASSERT_EQ(2u, G1Policy::calc_max_old_cset_length());
>
>     // All guarded flags are restored here
>   }
>
> Bug:
> https://bugs.openjdk.java.net/browse/JDK-8159370
>
> Webrev:
> http://cr.openjdk.java.net/~ehelin/8159370/00/
>
> Testing:
> - JPRT
>
> Thanks,
> Erik
>