(XS) RFR: 8056183: os::is_MP() always reports true when NMT is enabled

[David Holmes]

Thanks Staffan!

David

On 9/09/2014 3:55 PM, Staffan Larsen wrote:
> Looks good to me.
>
> Thanks,
> /Staffan
>
> On 9 sep 2014, at 07:50, David Holmes <david.holmes at oracle.com> wrote:
>
>> webrev:
>>
>> http://cr.openjdk.java.net/~dholmes/8056183/webrev/
>>
>> This is a confidential bug so here is the background:
>>
>> The NMT code uses atomics very early in the initialization process, before _processor_count has been initialized, and so it would hit the assertion in is_MP that _processor_count > 0. To workaround that it was assumed that if NMT were present then we could just assume a MP system and have is_MP return true. That change was made under 8046598.
>>
>> However that is an invalid assumption on some platforms and will lead to incorrect code generation. The real fix for the original assertion failure is to just remove the assertion as weakening it to check >= 0 would serve no point. Further we already assert that _processor_count is > 0 when we actually initialize it. Having is_MP return false during the early stages of initialization is okay because the VM is single-threaded at that point. However, we must be vigilant to ensure that we don't make any long term decisions (such as code generation choices) based on is_MP during this pre-initialization phase.
>>
>> So this change reverts the change from 8046598 and removes the assertion.
>>
>> Testing: JPRT
>>
>> Pushing to hs-rt/hotspot
>>
>> Thanks,
>> David
>