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

[Tobias Hartmann]

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