RFR: 8227633: avoid comparing this pointers to NULL - was : RE: this-pointer NULL-checks in hotspot codebase [-Wtautological-undefined-compare]

[Baesken, Matthias]

Hello Erik,   thanks for the input .

We still have a few places in the HS codebase where "this" is compared to NULL. 
When compiling with xlc16 / xlclang we get these warnings : 

 warning: 'this' pointer cannot be null in well-defined C++ code; comparison may be assumed to always evaluate to false [-Wtautological-undefined-compare] 

so those places should be removed where possible.


I adjusted 3  checks , please review !



Bug/webrev :

http://cr.openjdk.java.net/~mbaesken/webrevs/8227633.0/

https://bugs.openjdk.java.net/browse/JDK-8227633

Thanks , Matthias


> -----Original Message-----
> From: Erik Österlund <erik.osterlund at oracle.com>
> Sent: Freitag, 12. Juli 2019 10:22
> To: Baesken, Matthias <matthias.baesken at sap.com>; 'hotspot-
> dev at openjdk.java.net' <hotspot-dev at openjdk.java.net>
> Subject: Re: this-pointer NULL-checks in hotspot codebase [-Wtautological-
> undefined-compare]
> 
> Hi Matthias,
> 
> Removing such NULL checks seems like a good idea in general due to the
> undefined behaviour.
> Worth mentioning though that there are some tricky ones, like in
> markOopDesc* where this == NULL
> means that the mark word has the "inflating" value. So we explicitly
> check if this == NULL and
> hope the compiler will not elide the check. Just gonna drop that one
> here and run for it.
> 
> Thanks,
> /Erik
> 
> On 2019-07-12 09:48, Baesken, Matthias wrote:
> > Hello , when looking  into  the  recent  xlc16 / xlclang   warnings I came
> across  those  3 :
> >
> > /nightly/jdk/src/hotspot/share/adlc/formssel.cpp:1729:7: warning: 'this'
> pointer cannot be null in well-defined C++ code;
> > comparison may be assumed to always evaluate to true [-Wtautological-
> undefined-compare]
> >    if( this != NULL ) {
> >        ^~~~    ~~~~
> >
> > /nightly/jdk/src/hotspot/share/adlc/formssel.cpp:3416:7: warning: 'this'
> pointer cannot be null in well-defined C++ code;
> > comparison may be assumed to always evaluate to false [-Wtautological-
> undefined-compare]
> >    if( this == NULL ) return;
> >
> > /nightly/jdk/src/hotspot/share/libadt/set.cpp:46:7: warning: 'this' pointer
> cannot be null in well-defined C++ code;
> > comparison may be assumed to always evaluate to false [-Wtautological-
> undefined-compare]
> >    if( this == NULL ) return os::strdup("{no set}");
> >
> >
> > Do you think the  NULL-checks can be removed or is there still some value
> in doing them ?
> >
> > Best regards, Matthias