hg: valhalla/valhalla: 8198748: [lworld] Javac should insert null checks at casts to value types.

[forax at univ-mlv.fr]

Thanks Srikanth, 
it's very helpful, currently i only deal with java 6 code (i control the compiler) for the transformations i was thinking about, but with Java 11 being a LTS, i suppose the target 6 will disappear and i will have to deal with this new kind of nullcheck. 

Rémi 

> De: "Srikanth" <srikanth.adayapalam at oracle.com>
> À: "Remi Forax" <forax at univ-mlv.fr>
> Cc: "valhalla-dev" <valhalla-dev at openjdk.java.net>
> Envoyé: Lundi 12 Mars 2018 08:46:12
> Objet: Re: hg: valhalla/valhalla: 8198748: [lworld] Javac should insert null
> checks at casts to value types.

> On Monday 12 March 2018 12:29 PM, Remi Forax wrote:

>> Hi Srikanth,
>> thanks for doing that.

>> There is already a code path in the compiler to generate nullchecks using
>> getClass() + pop,
>> i think you should reuse this code instead of adding another way to do a
>> nullcheck.

> Hello Remi,

> Javac *already* employs two distinct mechanisms to implement null checks:

> Please see: Gen#genNullCheck

> /** Generate a null check from the object value at stack top. */ private void
> genNullCheck (JCTree tree) { code .statBegin(tree. pos ) ; if (
> allowBetterNullChecks ) {
>        callMethod(tree.pos() , syms . objectsType , names . requireNonNull , List. of (
>         syms . objectType ) , true ) ; } else {
>        callMethod(tree.pos() , syms . objectType , names . getClass , List. nil () ,
>         false ) ; } code .emitop0( pop ) ; }

> where the controlling boolean is initialized as:

> allowBetterNullChecks = target .hasObjects() ;

> and target.hasObjects() is really:

> /** Does the target JDK contains the java.util.Objects class? */ public boolean
> hasObjects () { return compareTo( JDK1_7 ) >= 0 ; }

> In short, I am not expressly selecting the method of implementing the null
> check, only inserting a null-check AST node.

> So this pattern is already emitted by javac and any/all analyzers should anyway
> be upgraded (should have been upgraded already)

> Thanks!
> Srikanth

>> And with my hat of backporter/bytecode analyzer writer, adding a new runtime
>> dependency, here, to Objects.requireNonNull, makes things more complex because
>> some analyzers may have to be enhanced to recognize this new pattern.

>> cheers,
>> Rémi

>> ----- Mail original -----

>>> De: "Srikanth" [ mailto:srikanth.adayapalam at oracle.com |
>>> <srikanth.adayapalam at oracle.com> ] À: "valhalla-dev" [
>>> mailto:valhalla-dev at openjdk.java.net | <valhalla-dev at openjdk.java.net> ]
>>> Envoyé: Lundi 12 Mars 2018 07:32:13
>>> Objet: Re: hg: valhalla/valhalla: 8198748: [lworld] Javac should insert null
>>> checks at casts to value types.

>>> On Monday 12 March 2018 11:51 AM, [ mailto:srikanth.adayapalam at oracle.com |
>>> srikanth.adayapalam at oracle.com ] wrote:

>>>> Changeset: 7c1c51c340e6
>>>> Author:    sadayapalam
>>>> Date:      2018-03-12 11:46 +0530
>>>> URL: [ http://hg.openjdk.java.net/valhalla/valhalla/rev/7c1c51c340e6 |
>>>> http://hg.openjdk.java.net/valhalla/valhalla/rev/7c1c51c340e6 ] 8198748:
>>>> [lworld] Javac should insert null checks at casts to value types.

>>> Notes:

>>> Where Point is a value type, code generated for casting an object to
>>> Point as in:

>>> Object o = ...

>>> Point p = (Point) o;

>>> is:

>>>          2: aload_1
>>>          3: dup
>>>          4: invokestatic  #2                  // Method
>>> java/util/Objects.requireNonNull:(Ljava/lang/Object;)Ljava/lang/Object;
>>>          7: pop
>>>          8: checkcast     #3                  // class Point
>>>         11: astore_2

>>> If the cast occurs in source code compiled at a level where value types
>>> are not recognized as such, then there is no insertion of null check.

>>> (FWIW, Javac is not in the business of nullness tracking, so these
>>> checks will be inserted even where a cursory glance of source code
>>> context could inform a reader that no null check would be needed)

>>> Thanks
>>> Srikanth

>>>> ! src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Lower.java
>>>> ! src/jdk.compiler/share/classes/com/sun/tools/javac/jvm/ClassReader.java
>>>> + test/langtools/tools/javac/valhalla/lworld-values/CastNoNullCheckTest.java
>>>> + test/langtools/tools/javac/valhalla/lworld-values/CastNullCheckTest.java