RFR: 8240370: Provide Intel JCC Erratum opt-out

[Vladimir Ivanov]

Hi Erik,

> New webrev: http://cr.openjdk.java.net/~eosterlund/8240370/webrev.01/

IMO it makes sense to support both forcibly turning the flag on and off 
from the command-line.

So, I prefer:

   bool VM_Version::compute_has_intel_jcc_erratum() {
+  if (!FLAG_IS_DEFAULT(IntelJccErratumMitigation)) {
+    return IntelJccErratumMitigation;
+  }

Or even:

   if (FLAG_IS_DEFAULT(IntelJccErratumMitigation)) {
     _has_intel_jcc_erratum = compute_has_intel_jcc_erratum();
   } else {
     _has_intel_jcc_erratum = IntelJccErratumMitigation;
   }

Best regards,
Vladimir Ivanov

>> /Claes
>>
>> On 2020-03-03 13:23, Erik Österlund wrote:
>>> Hi,
>>>
>>> Today, the Intel JCC Erratum mitigation is automatically enabled when 
>>> running on an affected machine.
>>> While it is strongly recommended that customers run code in 
>>> environments with the micro code updates
>>> provided by Intel to mitigate the erratum, and hence are probably 
>>> almost always better off with the
>>> mitigation, there could be value in being able to opt out to analyze 
>>> the performance characteristics
>>> of an application, as a diagnostic option.
>>>
>>> This patch adds a diagnostic option that allows you to opt out with: 
>>> -XX:-IntelJccErratum
>>>
>>> Webrev:
>>> http://cr.openjdk.java.net/~eosterlund/8240370/webrev.00/
>>>
>>> Bug:
>>> https://bugs.openjdk.java.net/browse/JDK-8240370
>>>
>>> Thanks,
>>> /Erik
>