RFR: 8258897: wrong translation of capturing local classes inside nested lambdas
Jia Yanwei
github.com+3896345+hltj at openjdk.java.net
Tue Dec 29 02:46:57 UTC 2020
On Fri, 27 Nov 2020 12:37:24 GMT, Jia Yanwei <github.com+3896345+hltj at openjdk.org> wrote:
>> **Issue:**
>> `javac` crashes with a NPE when compiling such code:
>> - a local class captured a reference type local variable of a enclosed lambda (which contains multiple local variables with different types)
>> - instance the local class in a nested lambda
>>
>> Affects all versions from Java 8 to 16-ea, here is a example output with OpenJDK 15.0.1:
>>
>> An exception has occurred in the compiler (15.0.1). Please file a bug against the Java compiler via the Java bug reporting page (http://bugreport.java.com) after checking the Bug Database (http://bugs.java.com) for duplicates. Include your program, the following diagnostic, and the parameters passed to the Java compiler in your report. Thank you.
>> java.lang.NullPointerException: Cannot read field "sym" because "this.lvar[1]" is null
>> at jdk.compiler/com.sun.tools.javac.jvm.Code.emitop0(Code.java:571)
>> at jdk.compiler/com.sun.tools.javac.jvm.Items$LocalItem.load(Items.java:399)
>> at jdk.compiler/com.sun.tools.javac.jvm.Gen.genArgs(Gen.java:889)
>> at jdk.compiler/com.sun.tools.javac.jvm.Gen.visitNewClass(Gen.java:1942)
>> at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCNewClass.accept(JCTree.java:1800)
>> at jdk.compiler/com.sun.tools.javac.jvm.Gen.genExpr(Gen.java:864)
>> at jdk.compiler/com.sun.tools.javac.jvm.Gen.visitExec(Gen.java:1723)
>> at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCExpressionStatement.accept(JCTree.java:1532)
>> at jdk.compiler/com.sun.tools.javac.jvm.Gen.genDef(Gen.java:597)
>> at jdk.compiler/com.sun.tools.javac.jvm.Gen.genStat(Gen.java:632)
>> at jdk.compiler/com.sun.tools.javac.jvm.Gen.genStat(Gen.java:618)
>> at jdk.compiler/com.sun.tools.javac.jvm.Gen.genStats(Gen.java:669)
>> at jdk.compiler/com.sun.tools.javac.jvm.Gen.visitBlock(Gen.java:1084)
>> at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCBlock.accept(JCTree.java:1047)
>> at jdk.compiler/com.sun.tools.javac.jvm.Gen.genDef(Gen.java:597)
>> at jdk.compiler/com.sun.tools.javac.jvm.Gen.genStat(Gen.java:632)
>> at jdk.compiler/com.sun.tools.javac.jvm.Gen.genMethod(Gen.java:954)
>> at jdk.compiler/com.sun.tools.javac.jvm.Gen.visitMethodDef(Gen.java:917)
>> at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCMethodDecl.accept(JCTree.java:893)
>> at jdk.compiler/com.sun.tools.javac.jvm.Gen.genDef(Gen.java:597)
>> at jdk.compiler/com.sun.tools.javac.jvm.Gen.genClass(Gen.java:2395)
>> at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.genCode(JavaCompiler.java:756)
>> at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.generate(JavaCompiler.java:1644)
>> at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.generate(JavaCompiler.java:1612)
>> at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:973)
>> at jdk.compiler/com.sun.tools.javac.main.Main.compile(Main.java:317)
>> at jdk.compiler/com.sun.tools.javac.main.Main.compile(Main.java:176)
>> at jdk.compiler/com.sun.tools.javac.Main.compile(Main.java:59)
>> at jdk.compiler/com.sun.tools.javac.Main.main(Main.java:45)
>> printing javac parameters to: /tmp/javac.20201126_141249.args
>>
>> The minimal reproducing code is:
>>
>> java
>> class Main {
>> static Runnable runnable = () -> {
>> boolean b0 = false;
>> String s0 = "hello";
>>
>> class Local {
>> int i = s0.length();
>> }
>>
>> Runnable dummy = () -> new Local();
>> };
>> }
>>
>> If the captured variable is a primitive type, the code will compile but cause a runtime crash at startup. e.g.:
>>
>> java
>> public class CaptureInt {
>> static Runnable runnable = () -> {
>> boolean b0 = false;
>> int i0 = 5;
>>
>> class Local {
>> int i = i0 + 2;
>> }
>>
>> Runnable dummy = () -> new Local();
>> };
>>
>> public static void main(String args[]) {
>> }
>> }
>>
>> **Reason & Solution:**
>>
>> During compilation, the captured variable was correctly synthetized as a synthetic argument in the synthetic methods for the nested lambda after `desugar()`.
>> But the synthetic argument was not used when loading free variables, and it always use the original symbol for the original variable as if not in a nested lambda.
>> My experimental solution is substituting the symbols in free variables that were captured as synthetic arguments in nested lambda.
>> Welcome any advice and better solution.
>
> I had signed OCA, but named as `jyw`, the email and full name are identical.
Bernard's reply:
The problem seems to be that 'lambdaTranslationMap' links the
untranslated local variable to its capture. So, what do you think of
the following solution to keep the translated local variable's mapping
with the original symbol (both examples and langtools:tier1 are OK on
jdk14u)?
diff
diff --git a/src/jdk.compiler/share/classes/com/sun/tools/javac/comp/LambdaToMethod.java
b/src/jdk.compiler/share/classes/com/sun/tools/javac/comp/LambdaToMethod.java
--- a/src/jdk.compiler/share/classes/com/sun/tools/javac/comp/LambdaToMethod.java
+++ b/src/jdk.compiler/share/classes/com/sun/tools/javac/comp/LambdaToMethod.java
@@ -2057,7 +2057,13 @@
};
break;
case LOCAL_VAR:
- ret = new VarSymbol(sym.flags() & FINAL,
sym.name, sym.type, translatedSym);
+ ret = new VarSymbol(sym.flags() & FINAL,
sym.name, sym.type, translatedSym) {
+ @Override
+ public Symbol baseSymbol() {
+ //keep mapping with original symbol
+ return sym;
+ }
+ };
((VarSymbol) ret).pos = ((VarSymbol) sym).pos;
break;
case PARAM:
diff --git a/src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Lower.java
b/src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Lower.java
--- a/src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Lower.java
+++ b/src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Lower.java
@@ -1221,7 +1221,7 @@
//sym is a local variable - check the lambda translation map to
//see if sym has been translated to something else in
the current
//scope (by LambdaToMethod)
- Symbol translatedSym = lambdaTranslationMap.get(sym);
+ Symbol translatedSym =
lambdaTranslationMap.get(sym.baseSymbol());
if (translatedSym != null) {
tree = make.at(tree.pos).Ident(translatedSym);
}
-------------
PR: https://git.openjdk.java.net/jdk/pull/1479
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