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wrong local variable table for methods containing while loops #9179

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scabug opened this issue Feb 24, 2015 · 6 comments
Closed

wrong local variable table for methods containing while loops #9179

scabug opened this issue Feb 24, 2015 · 6 comments
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@lrytz
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bytecode
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2.12.0-M5

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@scabug
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scabug commented Feb 24, 2015

Local variables defined outside of a while loop sometimes are not visible from the body of this loop in the debugger.

{code}object Example {
def main(args: Array[String]) {
var x = ""
while (x != null) {
foo()
x = null
}
bar()
}

def foo() {}
def bar() {}
}
{code}

Bytecode instructions of the method main:

{code} 0 ldc #16
2 astore_2
3 aload_2
4 ifnonnull 12 (+8)
7 aload_0
8 invokevirtual #19 <Example$.bar>
11 return
12 aload_0
13 invokevirtual #22 <Example$.foo>
16 aconst_null
17 astore_2
18 goto 3 (-15)
{code}

Local variable x has attributes start_pc = 3, length = 8 in the local variable table. So it is not visible for a debugger inside the while statement body, when instructions 12 - 18 are executed.

It seems that this problem occurs when while statement condition has the form A != B, where A and B are reference type expressions.
@scabug
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scabug commented Feb 24, 2015

Imported From: https://issues.scala-lang.org/browse/SI-9179?orig=1
Reporter: Nikolay Tropin (niktrop)
Affected Versions: 2.11.5

@scabug
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scabug commented Mar 12, 2015

@retronym said:
This seems to be fixed under the experimental backend, GenBCode.

That backend will become the default in 2.12. We aren't making non-critical bug fixes to GenASM anymore, so I'm going to mark this as fixed for 2.12.

I'd appreciate if you could verify this under scala 2.11 compiling with -Ybackend:GenCode, and take a broader look at the debugger experience to check for any regressions in bytecode positions.

 % scalac -Ybackend:GenBCode -Ydebug -Xprint:typer sandbox/test.scala && echo ':javap -v -c Example$' | scala
...
  public void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=3, args_size=2
         0: ldc           #16                 // String
         2: astore_2
         3: aload_2
         4: ifnull        21
         7: goto          10
        10: aload_0
        11: invokevirtual #19                 // Method foo:()V
        14: aconst_null
        15: pop
        16: aconst_null
        17: astore_2
        18: goto          3
        21: aload_0
        22: invokevirtual #22                 // Method bar:()V
        25: return
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            2      23     2     x   Ljava/lang/String;
            0      26     0  this   LExample$;
            0      26     1  args   [Ljava/lang/String;

@scabug
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scabug commented Mar 12, 2015

@lrytz said:
we should still add a test case

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scabug commented Mar 12, 2015

@retronym said:
Right you are.

@scabug
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scabug commented Mar 13, 2015

Nikolay Tropin (niktrop) said:
It works on 2.11.4 with -Ybackend:GenBCode. I'll switch to this backend to test it further.

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scabug commented Apr 25, 2016

@lrytz said:
scala/scala#5122

@scabug scabug closed this as completed May 25, 2016
@scabug scabug added the bytecode label Apr 7, 2017
@scabug scabug added this to the 2.12.0-M5 milestone Apr 7, 2017
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