[9] RFR(S): 8174164: SafePointNode::_replaced_nodes breaks with irreducible loops

[Roland Westrelin]

> Then in what cases your new check (_idx >= idx) fails?

Here is an example:

    static void test_helper(A a) {
        a.m(); // virtual call with polluted profile
    }

    static void inlined() {}

    static A field = new A();
    static Object test() {
        A a1 = field;
        if (a1.getClass() != A.class) { // always fail
            return null;
        }
        A a2 = field;
        inlined();
        test_helper(a2);
        return a1;
    }


Without my change, C2 inlines the a.m() call. The exact class test in
the test() method for a1 results in a call to replace_in_map that
replaces the load of field in the map with a CheckCastPP and records
that pair in the replaced nodes. On return from inlined(), the load of
field (from a2 = field) is replaced by the CheckCastPP when the replaced
nodes are processed.

If we drop the inlined() call:

    static Object test() {
        A a1 = field;
        if (a1.getClass() != A.class) { // always fail
            return null;
        }
        A a2 = field;
        test_helper(a2);
        return a1;
    }

then C2 doesn't inline the a.m() call anymore. If we put the inlined()
call back and with the change we're discussing, C2 doesn't inline the
a.m() call either (because the CheckCastPP is an "old" node on return
from the inlined call).

Again, the replaced nodes were added to propagate casts done in callees
to callers. This example just happens to optimize well by accident.

The other fix I see would be to disable replaced nodes if one of the
callers was found to have a irreducible loop by ciTypeFlow. It doesn't
look much better to me because OSR compilations would be randomly
affected.

Roland.