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

[Kim Barrett]

> On Jun 14, 2016, at 6:58 AM, Erik Helin <erik.helin at oracle.com> wrote:
> 
> 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

Is there a reason why this is being added to the unit test framework,
rather than being a generalization of the existing [U]IntFlagSetting
&etc and placed nearby?  That’s where I would expect to find it.

> 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