RFR(S): 8250609: C2 crash in IfNode::fold_compares

[Yangfei (Felix)]

Hi,

> -----Original Message-----
> From: Vladimir Kozlov [mailto:vladimir.kozlov at oracle.com]
> Sent: Tuesday, July 28, 2020 3:15 AM
> To: Yangfei (Felix) <felix.yang at huawei.com>; hotspot-compiler-
> dev at openjdk.java.net
> Subject: Re: RFR(S): 8250609: C2 crash in IfNode::fold_compares
> 
> It happens because 'lo' is new node created just now and have no uses yet.
> For such new nodes we usually add dummy use to avoid removal from graph:
> 
> http://hg.openjdk.java.net/jdk/jdk/file/c379dc750a02/src/hotspot/share/op
> to/convertnode.cpp#l403

Thanks for the suggestions.  Yes, that will also fix the issue. 
New webrev: http://cr.openjdk.java.net/~fyang/8250609/webrev.01/ 
Performed the same tests as before.  Does it look better? 

Felix
 
> On 7/27/20 5:27 AM, Yangfei (Felix) wrote:
> > Hi,
> >
> > Bug: https://bugs.openjdk.java.net/browse/JDK-8250609
> > Webrev: http://cr.openjdk.java.net/~fyang/8250609/webrev.00/
> >
> > In IfNode::fold_compares_helper, C2 tries to fold 2 CmpI into a single
> CmpU.
> > At the crash site in IfNode::fold_compares_helper:
> >   995   if (lo && hi) {
> >   996     // Merge the two compares into a single unsigned compare by
> building (CmpU (n - lo) (hi - lo))
> >   997     Node* adjusted_val = igvn->transform(new SubINode(n,  lo));
> >   998     if (adjusted_lim == NULL) {
> >   999       adjusted_lim = igvn->transform(new SubINode(hi, lo));
> > 1000     }
> >
> > At line 997, we have:
> > (gdb) p lo->dump()
> >   641    AddI    === _  513  92  [[]]
> > $1 = void
> >
> > After the transformation at line 997, we have
> > (gdb) p lo->dump()
> >   641    AddI    === _ _ _  [[]]   [34200641]
> > $3 = void
> >
> > Then node 641 was used at line 999, which triggers the crash.
> > Patch fixes the issue by delaying transformation in IfNode::fold_compares
> temporarily.
> > Tier1-3 tested on aarch64-linux-gnu & x86_64-linux-gnu.
> > Newly added test fail without the patch and pass otherwise.
> > Suggestions?
> >
> > Thanks,
> > Felix
> >