Compiler Back End as Plugin

On Wed, Nov 23, 2011 at 7:55 PM, Chris Dow wrote:
> Is it possible to create a new backend for the Scala compiler as a
> plugin?

Sure. (But genjvm might do its thing too.)

> Based on the Scaladoc and the examples I have seen it looks
> like you can add additional phases but you can't replace phases (i.e
> replacing the GenJVM phase).

Are you determined to do it with a plugin? It's easy to do if you
subclass Global. I'm not aware of anything in the plugin architecture
that lets you evict phases.

> If you can't replace phases then how does the GenMSIL phase hook into
> the compiler?

There are "platform phases":

protected def computePlatformPhases() = platform.platformPhases
foreach { sub =>
addToPhasesSet(sub, otherPhaseDescriptions(sub.phaseName))
}

In MSILPlatform that's these:

def platformPhases = List(
genMSIL // generate .msil files
)

In JavaPlatform it's these:

def platformPhases = List(
flatten, // get rid of inner classes
genJVM // generate .class files
) ++ depAnalysisPhase

But GenMSIL isn't a plugin, it's part of the compiler.

Hi,

Subclassing Global is the easiest way to go if you want another back-end. That's what I did for Ozma, aka Scala-Mozart [1].

Sébastien

[1] https://github.com/sjrd/ozma/blob/master/src/compiler/scala/tools/nsc/ozma/OzmaGlobal.scala

On Thu, Nov 24, 2011 at 05:30, Paul Phillips <paulp@improving.org> wrote:

On Wed, Nov 23, 2011 at 7:55 PM, Chris Dow <chris.dow8@gmail.com> wrote:
> Is it possible to create a new backend for the Scala compiler as a
> plugin?

Sure.  (But genjvm might do its thing too.)

> Based on the Scaladoc and the examples I have seen it looks
> like you can add additional phases but you can't replace phases (i.e
> replacing the GenJVM phase).

Are you determined to do it with a plugin? It's easy to do if you
subclass Global.  I'm not aware of anything in the plugin architecture
that lets you evict phases.

> If you can't replace phases then how does the GenMSIL phase hook into
> the compiler?

There are "platform phases":

 protected def computePlatformPhases() = platform.platformPhases
foreach { sub =>
   addToPhasesSet(sub, otherPhaseDescriptions(sub.phaseName))
 }

In MSILPlatform that's these:

 def platformPhases = List(
   genMSIL   // generate .msil files
 )

In JavaPlatform it's these:

 def platformPhases = List(
   flatten,    // get rid of inner classes
   genJVM      // generate .class files
 ) ++ depAnalysisPhase

But GenMSIL isn't a plugin, it's part of the compiler.

For compiler phases, there's the -Yskip command-line option:

// Each subcomponent supplies a phase, which are chained together.
//   If -Ystop:phase is given, neither that phase nor any beyond it is added.
//   If -Yskip:phase is given, that phase will be skipped.

After a prototype compiler-plugin works, it's always possible to move that code to a built-in phase. Plugin development is faster (just the plugin needs recompiling, it's loaded dynamically on each compiler run).

My take on writing compiler plugins: http://lamp.epfl.ch/~magarcia/ScalaCompilerCornerReloaded/

Miguel