RFR(M) 8068881: SIGBUS in C2 compiled method weblogic.wsee.jaxws.framework.jaxrpc.EnvironmentFactory$SimulatedWsdlDefinitions.<init>

[Igor Veresov]

I just amended the change with an extended comment on why it’s enough to track base registers only and ignore multi-register effects (like multi-registers lrg definitions and fat projections), it is subtle, but saves unnecessary work:

    // We just updated the last edge, now null out the value produced by the instruction itself,
    // since we're only interested in defs implicitly defined by the uses. We are actually interested
    // in tracking only redefinitions of the multidef lrgs in the same register. For that matter it's enough
    // to track changes in the base register only and ignore other effects of multi-register lrgs
    // and fat projections. It is also ok to ignore defs coming from singledefs. After an implicit
    // overwrite by one of those our register is guaranteed to be used by another lrg and we won't
    // attempt to merge it.


I also tightened the code following the comment to do updates only for multidefs lrgs (postaloc.cpp):

    lrg = _lrg_map.live_range_id(n);
-    if (lrg > 0) {
+    if (lrg > 0 && lrgs(lrg).is_multidef()) {
      OptoReg::Name reg = lrgs(lrg).reg();
      reg2defuse.at(reg).clear();
    }

Webrev updated in place.

Sorry about the last minute change.
igor

> On Jan 18, 2015, at 12:25 PM, Igor Veresov <igor.veresov at oracle.com> wrote:
> 
> After register allocation we may end up with nodes in the same block using different inputs that are in fact a part of a multidef lrg. Since that would confuse the scheduler, the post-allocation copy removal attempts to select a one of the inputs and replace all the uses that refer to the same value within the block. That works most of the time except when we try to replace an input coming from a phi that has the only user. In that case the phi goes dead along with the spill copy that merges the values, which produces incorrect code. Unfortunately there is no way to make a proper selection of a reaching def - the easiest counter example is having to select from two phis.
> 
> The solution to the problem is to introduce a node that acts like a phi but is without control (or rather something like MergeMem) that would merge the defs (that are really the same value and are a part of a multidef lrg). The following change adds a new node (MachMerge) and a pass after the post-allocation copy removal to insert them when needed. Even though it’s a separate pass it’s a very fast linear traversal.
> 
> Webrev: http://cr.openjdk.java.net/~iveresov/8068881/webrev.01/  
> 
> Tested with the failing method in weblogic, jprt, CTW 
> 
> Thanks,
> igor

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