RFR(S): 8087341: C2 doesn't optimize redundant memory operations with G1

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8087341/webrev.00/

C2 currently doesn’t optimize the field load in the following code:

    static Object field;

    static Object m(Object o) {
        field = o;
        return field;
    }

It should return o but instead loads the value back from memory. The reason it misses such simple optimization is that the G1 post barrier has a memory barrier with a wide effect on the memory state. C2 doesn’t optimize this either:

        object.field = other_object;
        object.field = other_object; 

Same applies to -XX:+UseConcMarkSweepGC -XX:+UseCondCardMark

That memory barrier was added to have a memory barrier instruction and doesn’t have to have a wide memory effect.

Roland.