Array[Double](1) not optimized by compiler

[OndrejSpanel]

Array is currently optimized by compiler.

Originally posted by @texasbruce in #6247 (comment)

Recently I have hit an issue in a real application - I was surprised to see Double boxing in it. Reduced to the bare minimum the following code produces an array construction which is not optimized:

var el = Array[Double](1)

var el = Array[Double](1.0)

Both produce the following byte code:

       0: aload_0
       1: invokespecial #27                 // Method java/lang/Object."<init>":()V
       4: aload_0
       5: getstatic     #33                 // Field scala/Array$.MODULE$:Lscala/Array$;
       8: getstatic     #38                 // Field scala/runtime/ScalaRunTime$.MODULE$:Lscala/runtime/ScalaRunTime$;
      11: iconst_1
      12: newarray       double
      14: dup
      15: iconst_0
      16: dconst_1
      17: dastore
      18: invokevirtual #42                 // Method scala/runtime/ScalaRunTime$.wrapDoubleArray:([D)Lscala/collection/immutable/ArraySeq;
      21: getstatic     #47                 // Field scala/reflect/ClassTag$.MODULE$:Lscala/reflect/ClassTag$;
      24: invokevirtual #51                 // Method scala/reflect/ClassTag$.Double:()Lscala/reflect/ManifestFactory$DoubleManifest;
      27: invokevirtual #55                 // Method scala/Array$.apply:(Lscala/collection/immutable/Seq;Lscala/reflect/ClassTag;)Ljava/lang/Object;
      30: checkcast     #56                 // class "[D"
      33: putfield      #18                 // Field el:[D
      36: return

A slightly different variants are optimized as expected:

var el = Array(1.0)
var el: Array[Double] = Array(1)

See also https://stackoverflow.com/q/64482385/16673

I see the issue in both Scala 2.12.12 and 2.13.3. I do not see the issue in Dotty 0.27

[SethTisue]

("good first issue" is speculative. a newcomer could investigate, and it's plausible that an easy solution exists, given that the other forms get optimized as expected)

[jxnu-liguobin]

Hello @SethTisue , what knowledge need to solve this problem? Where is the related code ?

[SethTisue]

As I commented on the other ticket you asked about, my “You Are a Scala Contributor” video and/or slides are good for getting oriented, as are the README and CONTRIBUTING files in scala/scala.

You should already be very comfortable coding in Scala. And you should only attempt this bug if you want to learn something about Scala compiler internals, starting with https://docs.scala-lang.org/overviews/reflection/symbols-trees-types.html . This is a "good first issue" (we suspect, we hope) for those who have that desire. (Having studied other compilers isn't a prerequisite.)

The optimization in question is implemented at https://github.com/scala/scala/blob/2.13.x/src/compiler/scala/tools/nsc/transform/CleanUp.scala#L595-L609

A few recent PRs closely related to this: scala/scala#9356, scala/scala#9461