Syntax ambiguities with "." and "(" #18
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anatoliykmetyuk
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Mar 4, 2015
In Scala 2.8.2, an optimization was added to create a static
cache for Symbol literals (ie, the results of `Symbol.apply("foo"))`.
This saves the map lookup on the second pass through code.
This actually was broken somewhere during the Scala 2.10 series,
after the addition of an overloaded `apply` method to `Symbol`.
The cache synthesis code was made aware of the overload and brought
back to working condition recently, in scala#3149.
However, this has uncovered a latent bug when the Symbol literals are
defined with traits.
One of the enclosed tests failed with:
jvm > t8933b-run.log
java.lang.IllegalAccessError: tried to access field MotherClass.symbol$1 from class MixinWithSymbol$class
at MixinWithSymbol$class.symbolFromTrait(A.scala:3)
at MotherClass.symbolFromTrait(Test.scala:1)
This commit simply disables the optimization if we are in a trait.
Alternative fixes might be: a) make the static Symbol cache field
public / b) "mixin" the static symbol cache. Neither of these
seem worth the effort and risk for an already fairly situational
optimization.
Here's how the optimization looks in a class:
% cat sandbox/test.scala; qscalac sandbox/test.scala && echo ":javap C" | qscala;
class C {
'a; 'b
}
Welcome to Scala version 2.11.5-20141106-145558-aa558dce6d (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_20).
Type in expressions to have them evaluated.
Type :help for more information.
scala> :javap C
Size 722 bytes
MD5 checksum 6bb00189166917254e8d40499ee7c887
Compiled from "test.scala"
public class C
{
public static {};
descriptor: ()V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=0, args_size=0
0: getstatic AndreVanDelft#16 // Field scala/Symbol$.MODULE$:Lscala/Symbol$;
3: ldc AndreVanDelft#18 // String a
5: invokevirtual AndreVanDelft#22 // Method scala/Symbol$.apply:(Ljava/lang/String;)Lscala/Symbol;
8: putstatic AndreVanDelft#26 // Field symbol$1:Lscala/Symbol;
11: getstatic AndreVanDelft#16 // Field scala/Symbol$.MODULE$:Lscala/Symbol$;
14: ldc AndreVanDelft#28 // String b
16: invokevirtual AndreVanDelft#22 // Method scala/Symbol$.apply:(Ljava/lang/String;)Lscala/Symbol;
19: putstatic AndreVanDelft#31 // Field symbol$2:Lscala/Symbol;
22: return
public C();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial AndreVanDelft#34 // Method java/lang/Object."<init>":()V
4: getstatic AndreVanDelft#26 // Field symbol$1:Lscala/Symbol;
7: pop
8: getstatic AndreVanDelft#31 // Field symbol$2:Lscala/Symbol;
11: pop
12: return
}
fixup
anatoliykmetyuk
pushed a commit
to anatoliykmetyuk/subscript
that referenced
this issue
Mar 4, 2015
These methods are "signature polymorphic", which means that compiler
should not:
1. adapt the arguments to `Object`
2. wrap the repeated parameters in an array
3. adapt the result type to `Object`, but instead treat it as it
it already conforms to the expected type.
Dispiritingly, my initial attempt to implement this touched the type
checker, uncurry, erasure, and the backend.
However, I realized we could centralize handling of this in the typer
if at each application we substituted the signature polymorphic
symbol with a clone that carried its implied signature, which is
derived from the types of the arguments (typechecked without an
expected type) and position within and enclosing cast or block.
The test case requires Java 7+ to compile so is currently embedded
in a conditionally compiled block of code in a run test.
We ought to create a partest category for modern JVMs so we can
write such tests in a more natural style.
Here's how this looks in bytecode. Note the `bipush` / `istore`
before/after the invocation of `invokeExact`, and the descriptor
`(LO$;I)I`.
```
% cat sandbox/poly-sig.scala && qscala Test && echo ':javap Test$#main' | qscala
import java.lang.invoke._
object O {
def bar(x: Int): Int = -x
}
object Test {
def main(args: Array[String]): Unit = {
def lookup(name: String, params: Array[Class[_]], ret: Class[_]) = {
val lookup = java.lang.invoke.MethodHandles.lookup
val mt = MethodType.methodType(ret, params)
lookup.findVirtual(O.getClass, name, mt)
}
def lookupBar = lookup("bar", Array(classOf[Int]), classOf[Int])
val barResult: Int = lookupBar.invokeExact(O, 42)
()
}
}
scala> :javap Test$#main
public void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC
Code:
stack=3, locals=3, args_size=2
0: aload_0
1: invokespecial AndreVanDelft#18 // Method lookupBar$1:()Ljava/lang/invoke/MethodHandle;
4: getstatic AndreVanDelft#23 // Field O$.MODULE$:LO$;
7: bipush 42
9: invokevirtual AndreVanDelft#29 // Method java/lang/invoke/MethodHandle.invokeExact:(LO$;I)I
12: istore_2
13: return
LocalVariableTable:
Start Length Slot Name Signature
0 14 0 this LTest$;
0 14 1 args [Ljava/lang/String;
13 0 2 barResult I
LineNumberTable:
line 16: 0
}
```
I've run this test across our active JVMs:
```
% for v in 1.6 1.7 1.8; do java_use $v; pt --terse test/files/run/t7965.scala || break; done
java version "1.6.0_65"
Java(TM) SE Runtime Environment (build 1.6.0_65-b14-466.1-11M4716)
Java HotSpot(TM) 64-Bit Server VM (build 20.65-b04-466.1, mixed mode)
Selected 1 tests drawn from specified tests
.
1/1 passed (elapsed time: 00:00:02)
Test Run PASSED
java version "1.7.0_71"
Java(TM) SE Runtime Environment (build 1.7.0_71-b14)
Java HotSpot(TM) 64-Bit Server VM (build 24.71-b01, mixed mode)
Selected 1 tests drawn from specified tests
.
1/1 passed (elapsed time: 00:00:07)
Test Run PASSED
java version "1.8.0_25"
Java(TM) SE Runtime Environment (build 1.8.0_25-b17)
Java HotSpot(TM) 64-Bit Server VM (build 25.25-b02, mixed mode)
Selected 1 tests drawn from specified tests
.
1/1 passed (elapsed time: 00:00:05)
Test Run PASSED
```
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As noted on the Syntax documentation page:
This needs to be implemented. For now, [a .] is not accepted by the parser.
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