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

[Yangfei (Felix)]

Hi,

    Thanks for reviewing this.
    Committed to jdk/submit repo and test result received looks good.
    Will do the push.

Thanks,
Felix

> -----Original Message-----
> From: Vladimir Kozlov [mailto:vladimir.kozlov at oracle.com]
> Sent: Wednesday, July 29, 2020 1:28 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
> 
> Yes, this looks good.
> 
> Thanks,
> Vladimir K
> 
> On 7/28/20 5:10 AM, Yangfei (Felix) wrote:
> > 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/shar
> >> e/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
> >>>