Scala 2.13 inline optimizer unable to correctly look up `.head` method in `scala.collection.LinearSeq`

[mdedetrich]

Reproduction steps

The current Scala 2.12/2.13 optimizer appears to be unable to correctly look up the .head method from the Scala scala.collection.LinearSeq when you happen to use @inline on a method that ends up transitively calling .head. The context of this is the current PR at apache/pekko#305. If you checkout the branch and run sbt ++2.13.10 actor/compile you will get the following error messages

[error] /Users/mdedetrich/github/incubator-pekko/actor/src/main/scala-2.13/org/apache/pekko/util/ByteIterator.scala:370:50: org/apache/pekko/util/ByteIterator$MultiByteArrayIterator::org$apache$pekko$util$ByteIterator$MultiByteArrayIterator$$current()Lorg/apache/pekko/util/ByteIterator$ByteArrayIterator; is annotated @inline but could not be inlined:
[error] Failed to check if org/apache/pekko/util/ByteIterator$MultiByteArrayIterator::org$apache$pekko$util$ByteIterator$MultiByteArrayIterator$$current()Lorg/apache/pekko/util/ByteIterator$ByteArrayIterator; can be safely inlined to org/apache/pekko/util/ByteIterator$MultiByteArrayIterator without causing an IllegalAccessError. Checking instruction INVOKEINTERFACE scala/collection/LinearSeq.head ()Ljava/lang/Object; (itf) failed:
[error] The method head()Ljava/lang/Object; could not be found in the class scala/collection/LinearSeq or any of its parents.
[error]       getToArray(xs, offset, n, 1) { getByte } { current.getBytes(_, _, _) }
[error]                                                  ^
[error] /Users/mdedetrich/github/incubator-pekko/actor/src/main/scala-2.13/org/apache/pekko/util/ByteIterator.scala:385:63: org/apache/pekko/util/ByteIterator$MultiByteArrayIterator::org$apache$pekko$util$ByteIterator$MultiByteArrayIterator$$current()Lorg/apache/pekko/util/ByteIterator$ByteArrayIterator; is annotated @inline but could not be inlined:
[error] Failed to check if org/apache/pekko/util/ByteIterator$MultiByteArrayIterator::org$apache$pekko$util$ByteIterator$MultiByteArrayIterator$$current()Lorg/apache/pekko/util/ByteIterator$ByteArrayIterator; can be safely inlined to org/apache/pekko/util/ByteIterator$MultiByteArrayIterator without causing an IllegalAccessError. Checking instruction INVOKEINTERFACE scala/collection/LinearSeq.head ()Ljava/lang/Object; (itf) failed:
[error] The method head()Ljava/lang/Object; could not be found in the class scala/collection/LinearSeq or any of its parents.
[error]       getToArray(xs, offset, n, 8) { getDouble(byteOrder) } { current.getDoubles(_, _, _)(byteOrder) }
...

The current method is defined as

@inline private def current: ByteArrayIterator = iterators.head

More context can be found at https://contributors.scala-lang.org/t/scala-2-12-2-13-inliner-is-too-aggressive/6191. Will also try to create a minimised example

Problem

It appears that specifically for Scala 2.13, the optimizer has issues in inlining methods that happen to call .head from scala.collection.LinearSeq in the call chain. I suspect that this may be a result of the new collections included in Scala 2.13.

[mdedetrich]

@lrytz I have made a minimal reproduction at https://github.com/mdedetrich/scala-213-collections-inliner-bug .

[scalway]

Here is mdedetrich's example moved to scala-cli script:

// file:scala-213-collections-inliner-bug.scala

//> using scala "2.13.nightly"
//> using option "-opt-inline-from:<sources>" 
//> using option "-opt:l:inline"

import scala.collection.LinearSeq

object Main {
  val iterators: LinearSeq[Int] = Nil
  @inline def current: Int = iterators.head
  val asString = current.toString
}

to compile it for nightly 2.13 with optimiser warnings:
scala-cli compile -Wopt:any-inline-failed 'scala-213-collections-inliner-bug.scala'

it produces warning (same as in sbt):

[warn] ./scala-213-collections-inliner-bug.scala:11:18
[warn] Main$::current()I is annotated @inline but could not be inlined:
[warn] Failed to check if Main$::current()I can be safely inlined to Main$ 
       without causing an IllegalAccessError. 
       Checking instruction INVOKEINTERFACE 
       scala/collection/LinearSeq.head ()Ljava/lang/Object; 
       (itf) failed:
[warn] The method head()Ljava/lang/Object; 
       could not be found in the class scala/collection/LinearSeq 
       or any of its parents.
[warn]   val asString = current.toString

to compile it for nightly 2.12 (-Wopt is not supported there):
scala-cli compile --scala 2.12.nightly 'scala-213-collections-inliner-bug.scala'
there is no warning in 2.12 whatsoever but it could be that such warnings are not implemented (as -Wopt is not supported)?

[mdedetrich]

@SethTisue Would it be out of the question to include this fix in the next upcoming Scala 2.13.11 RC/release?

EDIT: I just noticed that the bot already assigned it for Scala 2.13.12 so I guess its too late for that.

[SethTisue]

The bot behavior is just a default behavior that we can overrule, but I suggest that we let this one go until 2.13.12. Last-minute changes are tempting but risky.

[som-snytt]

Thanks, Seth. I might up late on Friday nite anyway, but I'd just as soon not be on call because I broke something.

[mdedetrich]

but I'd just as soon not be on call because I broke something.

I thought thats how we all enjoyed our Friday nights 😆

[som-snytt]

lrytz has pushed the PR into correcter territory.

[mdedetrich]

Since people love it when more work is pushed on them, what are the chances of this being backported into 2.12? This is on the presumption that the bug is a general one and was only shown in Scala 2.13 due to the collections rework (I could be wrong here).

If I scrap together some time I might be able to look into it a bit later if its not picked up, it would largely be shamelessly copying @som-snytt prior art anyways.