1. Analysis of Zinc
Eugene Yokota (@eed3si9n)
Lightbend

2. everything changes

3. separate compilation

4. separate compilation
A.scala
A.class B.scala
B.class

5. separate compilation
A.scala
A.jar B.scala
B.class

6. A.scala
A.class B.scala
B.class
A.sig
"Types are a property of expressions
and thus exist [only] at compile time"
— Creative Scala, Gurnell and Welsh

7. A.scala
A.class B.scala
B.class
A.sig
"I turned myself into a pickle, Morty!"
— Rick and Morty

8. incremental
(re)compilation =
minimality
• compiles sources at most once per build
only if they transitively depend on
sources that changed since the previous
build

9. naïve incremental
(re)compilation
A.scala B.scala C1.scala
C3.scala
C2.scala
C4.scala
• step 1

10. naïve incremental
(re)compilation
• step 2
A.scala B.scala C1.scala
C3.scala
C2.scala
C4.scala
initial invalidation

11. naïve incremental
(re)compilation
• step 3
A.scala B.scala C1.scala
C3.scala
C2.scala
C4.scala
transitive invalidation

12. naïve incremental
(re)compilation
• step 3
A.scala B.scala C1.scala
C3.scala
C2.scala
C4.scala
problem: results to compiling
all/most sources in the graph

13. name hashing

14. name hashing
• created by Grzegorz Kossakowski in
2013 (sbt/sbt#1026)

15. name hashing
• we need to prevent invalidation from
spreading like a wildfire

16. name hashing
• step 1 (initial compilation)
A.scala B.scala C1.scala
C3.scala
C2.scala
C4.scala
• inheritance (strong)
• member ref (weak)

17. name hashing
• step 2
A.scala B.scala C1.scala
C3.scala
C2.scala
C4.scala
• collect method signatures and
other information

18. name hashing
• step 3 (second compilation)
• compile initial invalidation
• update signature info for A
A.scala B.scala C1.scala
C3.scala
C2.scala
C4.scala

19. name hashing
• step 4
API changes: compare the
signature information before and
after compilation
A.scala B.scala C1.scala
C3.scala
C2.scala
C4.scala

20. name hashing
• step 5
member ref: transitively invalidated
only if a "used name" changed its
signature
A.scala B.scala C1.scala
C3.scala
C2.scala
C4.scala

21. name hashing
• step 4
inheritance: always transitively
invalidated
A.scala B.scala C1.scala
C3.scala
C2.scala
C4.scala

22. name hashing
package example
class B {
def foo: Unit = {
val a = new A
a.something
}
}
package example
class A {
def something: Int = 0
}

23. name hashing
used names:
example.B -> "A
example.B -> "AnyRef
example.B -> "B
example.B -> "Int
example.B -> "Object
example.B -> "Unit
example.B -> "a
example.B -> "example
example.B -> "example;A;init;
example.B -> "java;lang;Object;init;
example.B -> "scala
example.B -> "something

24. name hashing
package example
class B {
def foo: Unit = {
val a = new A
a.something
}
}
package example
class A {
// was 0
def something: Int = 2
}
• body change doesn't matter

25. name hashing
package example
class B {
def foo: Unit = {
val a = new A
a.something
}
}
package example
class A {
def something: Int = 0
// new
def somethingElse: Int = 1
}
• unused names do not matter

26. name hashing
package example
class C extends A {
def something: Int = 1
}
package example
class A {
def something: Int = 0
// new
def somethingElse: Int = 1
}
• inheritance propagates
invalidation

27. name hashing
• implementation body changes
• changes to unused methods / vals
prevents invalidation spread based on

28. dependency origins
subprojects local library JARs
• we only need
signature (API)
changes from
upstream

29. next steps

30. liberation from a machine

31. liberation from a machine
2 items
/Users/eed3si9n/work/zinc-example/A.scala -> /Users/eed3si9n/work/zinc-
example/target/scala-2.13/classes/example/A.class
/Users/eed3si9n/work/zinc-example/B.scala -> /Users/eed3si9n/work/zinc-
example/target/scala-2.13/classes/example/B.class
library dependencies:
4 items
/Users/eed3si9n/work/zinc-example/A.scala -> /Users/eed3si9n/.coursier/
cache/v1/https/repo1.maven.org/maven2/org/scala-lang/scala-library/2.13.1/
scala-library-2.13.1.jar
/Users/eed3si9n/work/zinc-example/A.scala -> /Users/eed3si9n/.sdkman/
candidates/java/8.0.222.hs-adpt/jre/lib/rt.jar
/Users/eed3si9n/work/zinc-example/B.scala -> /Users/eed3si9n/.coursier/
cache/v1/https/repo1.maven.org/maven2/org/scala-lang/scala-library/2.13.1/
scala-library-2.13.1.jar
/Users/eed3si9n/work/zinc-example/B.scala -> /Users/eed3si9n/.sdkman/
candidates/java/8.0.222.hs-adpt/jre/lib/rt.jar
library class names:
2 items
/Users/eed3si9n/.coursier/cache/v1/https/repo1.maven.org/maven2/org/scala-
lang/scala-library/2.13.1/scala-library-2.13.1.jar -> scala.Int
/Users/eed3si9n/.sdkman/candidates/java/8.0.222.hs-adpt/jre/lib/rt.jar ->
java.lang.Object

32. repeatable build = build as function
sbt/zinc#712

33. (modular) incremental build pipelining
sbt/zinc#744
subprojects local library JARs
parser
etc
typer, pickler etc jvm
refcheck,
erasure
parser
etc
typer, pickler etc jvm
refcheck,
erasure
parser
etc
typer, pickler etc jvm
refcheck,
erasure
early/foo.jar
early/bar.jar

34. Thank You and stay healthy!

35. 2020.03 mixtape
COVID-19 real-time update in US (https://
coronavirus.1point3acres.com/en)
Coronavirus update and guidance (https://
www.flattenthecurve.com/)