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

[Erik Österlund]

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