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

[Vladimir Ivanov]

Looks good.

Just a cosmetic suggestion (feel free to ignore it):
it looks like if (k == n->req() - 1) is better suited for 
PhaseChaitin::merge_multidefs() than 
PhaseChaitin::possibly_merge_multidef().

Best regards,
Vladimir Ivanov

On 1/19/15 9:35 PM, Igor Veresov wrote:
> 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 <http://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
>> <mailto: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
>