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Reduce the size of WrappedArray#8842

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diesalbla merged 3 commits intoscala:2.12.xfrom
mkeskells:2.12.x_wrapped_array
Apr 11, 2020
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Reduce the size of WrappedArray#8842
diesalbla merged 3 commits intoscala:2.12.xfrom
mkeskells:2.12.x_wrapped_array

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@mkeskells mkeskells commented Mar 30, 2020

WrappedArray is used in varargs, and IMO its rare to need the elemTag:ClassTag[T]

As its backed by a ClassValue cache anyway is seems unnecessary to cache it in array as well, and this reduces the size of the WrappedArray by 8 bytes, and removes and object ref from the GC traversal

@scala-jenkins scala-jenkins added this to the 2.12.12 milestone Mar 30, 2020
@mkeskells mkeskells requested a review from retronym March 30, 2020 08:08
@mkeskells mkeskells changed the title reduce the size of WrappedArray [after #8841] reduce the size of WrappedArray Mar 30, 2020
@mkeskells mkeskells force-pushed the 2.12.x_wrapped_array branch from 1cd2901 to b8226e1 Compare April 6, 2020 08:04
@SethTisue SethTisue added library:collections PRs involving changes to the standard collection library performance the need for speed. usually compiler performance, sometimes runtime performance. labels Apr 6, 2020
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retronym commented Apr 6, 2020 •
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Aside from reducing the class footprint to that of Some, it will save allocation of the ClassTag or its lookup in the ClassTag.cache, so LGTM.

scala> val data = new Array[Object](0)
data: Array[Object] = Array()

scala> println(org.openjdk.jol.info.ClassLayout.parseInstance(Some("")).toPrintable)
scala.Some object internals:
 OFFSET  SIZE               TYPE DESCRIPTION                               VALUE
      0     4                    (object header)                           0d 00 00 00 (00001101 00000000 00000000 00000000) (13)
      4     4                    (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
      8     4                    (object header)                           42 04 01 f8 (01000010 00000100 00000001 11111000) (-134151102)
     12     4   java.lang.Object Some.value                                (object)
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total


scala> println(org.openjdk.jol.info.ClassLayout.parseInstance(data.toSeq).toPrintable)
scala.collection.mutable.WrappedArray$ofRef object internals:
 OFFSET  SIZE                     TYPE DESCRIPTION                               VALUE
      0     4                          (object header)                           05 00 00 00 (00000101 00000000 00000000 00000000) (5)
      4     4                          (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
      8     4                          (object header)                           43 58 01 f8 (01000011 01011000 00000001 11111000) (-134129597)
     12     1                  boolean ofRef.bitmap$0                            false
     13     3                          (alignment/padding gap)
     16     4   scala.reflect.ClassTag ofRef.elemTag                             null
     20     4       java.lang.Object[] ofRef.array                               []
Instance size: 24 bytes
Space losses: 3 bytes internal + 0 bytes external = 3 bytes total

@retronym retronym changed the title [after #8841] reduce the size of WrappedArray Reduce the size of WrappedArray Apr 7, 2020
@retronym retronym mentioned this pull request Apr 7, 2020
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@mkeskells mkeskells added the performance:do_not_allocate Changes to avoid object allocations label Apr 10, 2020
@diesalbla diesalbla merged commit 4f9b70d into scala:2.12.x Apr 11, 2020
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library:collections PRs involving changes to the standard collection library performance:do_not_allocate Changes to avoid object allocations performance the need for speed. usually compiler performance, sometimes runtime performance.

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