RFR(s): 8235410: Enable handshakes on Linux x86 (32-bit)

[David Holmes]

On 6/12/2019 7:22 pm, Robbin Ehn wrote:
> Thanks David!
> 
> On 2019-12-05 22:36, David Holmes wrote:
>>> diff -r 636d71e53732 src/hotspot/cpu/x86/globals_x86.hpp
>>> --- a/src/hotspot/cpu/x86/globals_x86.hpp    Wed Dec 04 10:26:32 2019 
>>> +0100
>>> +++ b/src/hotspot/cpu/x86/globals_x86.hpp    Thu Dec 05 14:13:57 2019 
>>> +0100
>>> @@ -89,12 +89,7 @@
>>>
>>>   define_pd_global(intx, InitArrayShortSize, 8*BytesPerLong);
>>>
>>> -#if defined(_LP64) || defined(_WINDOWS)
>>>   define_pd_global(bool, ThreadLocalHandshakes, true);
>>> -#else
>>> -// get_thread() is slow on linux 32 bit, therefore off by default
>>
>> Why would get_thread be slow on linux 32-bit? Have we discussed this 
>> when handshakes went in? I'm feeling surprised now to see only 32-bit 
>> Linux was disabled.
> 
> You actually explained it RFR for 32-bit TLH:
> 
> "Thread::current() already uses compiler-based TLS. get_thread (on 
> non-Windows x86) make a VM call to Thread::current(). If we want to get 
> faster here we have to introduce compiler specific hooks directly into 
> the TLS implementation (not nice to do initially or maintain). "
> 
> :)

Sad how much one forgets :(

Right get_thread in 32-bit non-Windows has to make the VM call, whereas 
on 64-bit we have the r15_thread register dedicated.

David

> /Robbin
> 
> 
>>
>> Thanks,
>> David
>>
>>> -define_pd_global(bool, ThreadLocalHandshakes, false);
>>> -#endif
>>>
>>>   #define ARCH_FLAGS(develop, \
>>>                      product, \