RFR(XS): 8143307: Crash in C2 local code motion

[Lindenmaier, Goetz]

Hi Vladimir, 

thanks for looking at this!

> Is this Itanium specific problem? Who produces flags for branch?
> I don't see Branch_loop node in .ad files.
We saw it in our zArch node.  "Branch_loop" is a pseudo-name for the
mach node I choose because the Z-name is too cryptic.  
That node is similar to sparc:
instruct cmpI_reg_branchLoopEnd(cmpOp cmp, iRegI op1, iRegI op2, label labl, flagsReg icc) %{
  match(CountedLoopEnd cmp (CmpI op1 op2));
so the issue should also apply to openJDK.

> Changes are fine but I am concern that could be other places where we
> assume that there is an input node which produces flags.
Oh, yes, for other nodes it's the compare that's checked.
I didn't grok that! Then, the cause of the error is a different one, 
and the fix should check for RegFlags before dereferencing once
more.  See my adapted changeset:
http://cr.openjdk.java.net/~goetz/webrevs/8143307-lcmCrash/webrev.01/

Best regards,
  Goetz.




> 
> Thanks,
> Vladimir
> 
> On 11/19/15 5:21 AM, Lindenmaier, Goetz wrote:
> > Hi,
> >
> > We see assert(i < _max) failed: oob: i=1, _max=1  in C2s local code motion.
> >
> > lcm.cpp:502 accesses e->in(1)->in(1) where e is an Op_CountedLoopEnd.
> >
> > e->in(1) should return a loop iteration node. In the failing case the
> > loop iteration node has been optimized out and replaced by a Proj, which
> > has no in(1).
> >
> > In detail, in the ideal world, I could observe the following
> > optimization steps:
> >
> > 1.) If( Proj_lt (Cmp(AddI(Proj_res(Call(...)), -1) LoadI(...) ) )
> >
> > 2.) CountedLoopEnd( Proj_lt (Cmp(AddI(Proj_res(Call(...)), -1)
> > LoadI(...) ) )
> >
> > 3.) CountedLoopEnd( Proj_le (Cmp(Proj_res(Call(...)), LoadI(...) ) )
> >
> > 4.) CountedLoopEnd( Proj_le(Cmp(x,y))) was matched into one mach
> node:
> >
> >       Branch_loopEnd(Proj_res(Call(...)), LoadI(...) )
> >
> > On pattern 4.) e->in(1)->in(1) fails.
> >
> > Fix: check whether e->in(1)->req() > 1.
> >
> > Please review this change. I please need a sponsor.
> >
> > http://cr.openjdk.java.net/~goetz/webrevs/8143307-
> lcmCrash/webrev.00/
> >
> > Best regards,
> >
> >    Goetz.
> >