What is the order of the scala compiler phases?

Question

I want to sharpen my picture of the phases of scala compilation. I know that certain things have to happen in the compiler, but don't really know the order in which they happen and how the order should affect my programming.

Am I right in saying that the following things are the full list of what the compiler does?

• parse program
• types checked
• do erasure
• implicit conversion
• byte-code generated
• optimize

If so, what is the order that it does these phases? How does this order affect the programmer, especially the type-level programmer?

Answer 1

You see the phases, their order and explanation by using scalac -Xshow-phases.

In 2.11, use -Xshow-phases -Ydebug to show both enabled and disabled phases.

Here is how it's for 2.10.0:

 » scalac -Xshow-phases

             phase name  id  description
             ----------  --  -----------
                 parser   1  parse source into ASTs, perform simple desugaring
                  namer   2  resolve names, attach symbols to named trees
         packageobjects   3  load package objects
                  typer   4  the meat and potatoes: type the trees
                 patmat   5  translate match expressions
         superaccessors   6  add super accessors in traits and nested classes
             extmethods   7  add extension methods for inline classes
                pickler   8  serialize symbol tables
              refchecks   9  reference/override checking, translate nested objects
           selectiveanf  10  
           selectivecps  11  
                uncurry  12  uncurry, translate function values to anonymous classes
              tailcalls  13  replace tail calls by jumps
             specialize  14  @specialized-driven class and method specialization
          explicitouter  15  this refs to outer pointers, translate patterns
                erasure  16  erase types, add interfaces for traits
            posterasure  17  clean up erased inline classes
               lazyvals  18  allocate bitmaps, translate lazy vals into lazified defs
             lambdalift  19  move nested functions to top level
           constructors  20  move field definitions into constructors
                flatten  21  eliminate inner classes
                  mixin  22  mixin composition
                cleanup  23  platform-specific cleanups, generate reflective calls
                  icode  24  generate portable intermediate code
                inliner  25  optimization: do inlining
inlineExceptionHandlers  26  optimization: inline exception handlers
               closelim  27  optimization: eliminate uncalled closures
                    dce  28  optimization: eliminate dead code
                    jvm  29  generate JVM bytecode
               terminal  30  The last phase in the compiler chain

Answer 2

Have you looked here? That page points to http://www.scala-lang.org/sites/default/files/sids/nielsen/Thu,%202009-05-28,%2008:13/compiler-phases-sid.pdf (PDF), which describes the compiler phases for Scala 2.8.

Answer 3

You really should not worry about the order of compiler phases unless you're writing a compiler plugin. Even for type-level programming, you should focus on the language semantics instead.

As far as erasure is concerned, for example, it's important to understand static vs. dynamic types and the model of separate compilation. Similarly, it's important to understand the conditions under which the compiler will apply an implicit conversion. Where these occur in the compilation cycle is just an implementation detail.

Answer 4

Scala.js 0.6.23

     phase name  id  description
     ----------  --  -----------
         parser   1  parse source into ASTs, perform simple desugaring
     jspretyper   2  capture pre-typer only tree info (for Scala.js)
          namer   3  resolve names, attach symbols to named trees
 packageobjects   4  load package objects
          typer   5  the meat and potatoes: type the trees
      jsinterop   6  prepare ASTs for JavaScript interop
         patmat   7  translate match expressions
 superaccessors   8  add super accessors in traits and nested classes
     extmethods   9  add extension methods for inline classes
        pickler  10  serialize symbol tables
       xsbt-api  11  
xsbt-dependency  12  
      refchecks  13  reference/override checking, translate nested objects
        uncurry  14  uncurry, translate function values to anonymous classes
      tailcalls  15  replace tail calls by jumps
     specialize  16  @specialized-driven class and method specialization
  explicitouter  17  this refs to outer pointers
        erasure  18  erase types, add interfaces for traits
    posterasure  19  clean up erased inline classes
       lazyvals  20  allocate bitmaps, translate lazy vals into lazified defs
     lambdalift  21  move nested functions to top level
   constructors  22  move field definitions into constructors
        flatten  23  eliminate inner classes
          mixin  24  mixin composition
         jscode  25  generate JavaScript code from ASTs
        cleanup  26  platform-specific cleanups, generate reflective calls
     delambdafy  27  remove lambdas
          icode  28  generate portable intermediate code
        inliner  xx  optimization: do inlining
 inlinehandlers  xx  optimization: inline exception handlers
       closelim  xx  optimization: eliminate uncalled closures
       constopt  xx  optimization: optimize null and other constants
            dce  xx  optimization: eliminate dead code
            jvm  29  generate JVM bytecode
  xsbt-analyzer  30  
       terminal  31  the last phase during a compilation run

Scala-Native 0.3.7

     phase name  id  description
     ----------  --  -----------
         parser   1  parse source into ASTs, perform simple desugaring
          namer   2  resolve names, attach symbols to named trees
 packageobjects   3  load package objects
          typer   4  the meat and potatoes: type the trees
  nativeinterop   5  prepare ASTs for Native interop
         patmat   6  translate match expressions
 superaccessors   7  add super accessors in traits and nested classes
     extmethods   8  add extension methods for inline classes
        pickler   9  serialize symbol tables
       xsbt-api  10  
xsbt-dependency  11  
      refchecks  12  reference/override checking, translate nested objects
        uncurry  13  uncurry, translate function values to anonymous classes
      tailcalls  14  replace tail calls by jumps
     specialize  15  @specialized-driven class and method specialization
  explicitouter  16  this refs to outer pointers
        erasure  17  erase types, add interfaces for traits
    posterasure  18  clean up erased inline classes
       lazyvals  19  allocate bitmaps, translate lazy vals into lazified defs
     lambdalift  20  move nested functions to top level
   constructors  21  move field definitions into constructors
        flatten  22  eliminate inner classes
          mixin  23  mixin composition
            nir  24  
        cleanup  25  platform-specific cleanups, generate reflective calls
     delambdafy  26  remove lambdas
          icode  27  generate portable intermediate code
        inliner  xx  optimization: do inlining
 inlinehandlers  xx  optimization: inline exception handlers
       closelim  xx  optimization: eliminate uncalled closures
       constopt  xx  optimization: optimize null and other constants
            dce  xx  optimization: eliminate dead code
            jvm  28  generate JVM bytecode
  xsbt-analyzer  29  
       terminal  30  the last phase during a compilation run