RFR (M) 8152753: C1 intrinsics for CompareAndExchange (x86)

[Aleksey Shipilev]

On 04/07/2016 06:41 PM, Roland Westrelin wrote:
> 
>>>   http://cr.openjdk.java.net/~shade/8152753/webrev.01/
> 
> That c1_LIR.cpp change:
> 
> 931       if (!opCompareAndSwap->_exchange)
> do_input(opCompareAndSwap->_cmp_value);
> 
> doesn't seem right. Why do you need it?

CompareAndSwap produces boolean result, and kills cmp_value and
new_value. CompareAndExchange produces the "old"/null value result,
which is stored at the same position as cmp_value.

So, if you omit that line, LinearScan asserts when you are trying to use
the result of CompareAndExchange. AFAIU, removing the "input" property
from cmp_value, but leaving "temp" makes things back in order for
CompareAndExchange.

Thanks,
-Aleksey

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