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

[Christian Hagedorn]

Thank you Erik for your review!

Best regards,
Christian

On 14.05.20 11:47, Erik Österlund wrote:
> Hi Christian,
> 
> Thanks for taking care of this. Looks good to me.
> 
> Thanks,
> /Erik
> 
> On 2020-05-14 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
>