[15] RFR(S): 8244433: Remove saving of RSP in Assembler::pusha_uncached()

[Vladimir Kozlov]

Hi, Christian

Changes looks good.

I looked and this code existed from day one. And I don't remember we ever had an issue with possible RSP corrupted 
values. I don't think you need to worry about this after you fixed print code.

Thanks,
Vladimir

On 5/14/20 5:35 AM, Christian Hagedorn wrote:
> Thank you Tobias for your review!
> 
> Best regards,
> Christian
> 
> On 14.05.20 14:31, Tobias Hartmann wrote:
>> Hi Christian,
>>
>> this looks good to me.
>>
>> Best regards,
>> Tobias
>>
>> On 14.05.20 11:38, Christian Hagedorn wrote:
>>> Hi
>>>
>>> Please review the following enhancement for x86:
>>> https://bugs.openjdk.java.net/browse/JDK-8244433
>>> http://cr.openjdk.java.net/~chagedorn/8244433/webrev.00/
>>>
>>> This removes the move instruction for saving the actual value of RSP in
>>> Assembler::pusha_uncached()/pusha(). The original value of RSP is normally not used on the stack as
>>> the value of RSP will automatically be restored after popa to the same value before doing pusha.
>>> There are two locations, however, where we need to know the original value of RSP in order to print
>>> it. But these places can also compute the correct value of RSP by using the new value of RSP after
>>> pusha and adding 16 * wordSize to it. I fixed those.
>>>
>>> We still keep the same alignment by subtracting 16 * wordSize from RSP in pusha. Does anybody see
>>> any potential problems by not saving the value of RSP on the stack with pusha?
>>>
>>> Either way, as Erik Ö. has pointed out, the Windows x64 ABI does not specify a red zone of 128
>>> bytes. If we are unlucky and get an interrupt between saving RSP and decrementing RSP in the current
>>> code, we could end up with a corrupt value for RSP on Windows. Therefore, we do need to fix
>>> pusha_uncached() if we still want to save the old value of RSP. For example, we could first subtract
>>> 16 * wordSize and then calculate the correct value:
>>>
>>> subq(rsp, 16 * wordSize);
>>> movq(Address(rsp, 11 * wordSize), rsp);
>>> addq(Address(rsp, 11 * wordSize), 16 * wordSize);
>>>
>>> Thank you!
>>>
>>> Best regards,
>>> Christian