RFR (S) : 8014362 : Need to expose some processor features via Unsafe interface

[Vladimir Kozlov]

On 5/21/13 4:08 AM, David Chase wrote:
> I believe I was doing monkey-see-monkey-do.
> Other not-always-present machine features options seemed to be coded that way, so I copied it.
> Till we know it's a modern processor, it's not safe (I can verify that it does indeed crash if you try new instructions on an old chip).

David C. is correct. We set flags for new instructions to false by default because it is most common case (old cpus). 
Also it allows us to track incorrect settings on command line when an instruction is not available.

Vladimir

>
> David
>
> On 2013-05-21, at 12:49 AM, David Holmes <david.holmes at oracle.com> wrote:
>
>> David,
>>
>> I don't quite understand the handling of UseCLMUL. Why do you set it to false initially:
>>
>>   99   product(bool, UseCLMUL, false,           \
>> 100           "Control whether CLMUL instructions can be used on x86/x64")      \
>>
>> then force it to true if supported:
>>
>> 493   // Use CLMUL instructions if available.
>> 494   if (supports_clmul()) {
>> 495     if (FLAG_IS_DEFAULT(UseCLMUL)) {
>> 496       UseCLMUL = true;
>> 497     }
>> 498   } else if (UseCLMUL) {
>>
>> I would expect it to be true by default and set to false if not supported. That way the value in globals_x86.hpp really is the default value.
>>
>> That said we seem to handle a number of these flags in what I would consider an odd way.
>>
>> David
>> -----
>>
>>
>>
>> On 18/05/2013 7:28 AM, David Chase wrote:
>>> There was a spacing change that did not get reverted.
>>> This time, for sure.
>>> Note that this sets a property that won't get reset without a companion change in the JDK.
>>> (And the companion change is getting a bit of a pushback, though I hope to find, once I digest
>>> all the benchmarks I ran this afternoon, that it's not as scary as feared.)
>>>
>>> http://cr.openjdk.java.net/~drchase/8014362/webrev.04
>>>
>>> On 2013-05-17, at 4:30 PM, Vladimir Kozlov <vladimir.kozlov at oracle.com> wrote:
>>>
>>>> Somehow globals.hpp showed up in the webrev. Otherwise changes are good.
>>>>
>>>> Vladimir
>>>>
>>>> On 5/17/13 1:28 PM, David Chase wrote:
>>>>> New webrev:
>>>>>
>>>>> http://cr.openjdk.java.net/~drchase/8014362/webrev.03/
>>>>>
>>>>> It compiles (and runs) on both x86 and Sparc.
>>>>>
>>>>> David
>>>>>
>>>>> On 2013-05-16, at 11:32 PM, Vladimir Kozlov <vladimir.kozlov at oracle.com> wrote:
>>>>>
>>>>>> Okay, I agree with X86_ONLY().
>>>>>>
>>>>>> Vladimir
>>>>>
>>>
>