fusion-plugin
Motivation
The goal of stream fusion is to eliminate constructors of
internal state used in a stream. For example, in case of
streamly streams, the
constructors of Step
type, Yield
, Skip
and Stop
would get
eliminated by fusion. Similarly, constructors of any other intermediate
state types get eliminated when stream fusion works correctly. See the papers
in the reference section for more details on stream fusion.
Stream fusion depends on the GHC case-of-case transformations
eliminating intermediate constructors. Case-of-case transformation in
turn depends on inlining. During core-to-core transformations GHC may
create several internal bindings (e.g. join points) which may not get
inlined because their size is too big. Even though we know that after
fusion the resulting code would be smaller and more efficient. The
programmer cannot force inlining of these bindings as there is no way
for the programmer to address these bindings at the source level because
they are internal, generated during core-to-core transformations. As a result
stream fusion fails unpredictably depending on whether GHC was able to inline
the internal bindings or not.
See GHC ticket #17075 for
more details.
Solution
This plugin provides the programmer with a way to annotate certain
types using a Fuse
pragma from the
fusion-plugin-types
package. The programmer would annotate the types that are to be
eliminated by fusion. During the simplifier phase the plugin goes
through the relevant bindings and if one of these types are found
inside a binding then that binding is marked to be inlined
irrespective of the size.
Using the plugin
This plugin was primarily motivated by
streamly but it can be used in
general.
To use this plugin, add this package to your build-depends
and pass the following to your ghc-options:
ghc-options: -O2 -fplugin=Fusion.Plugin
Plugin options
Note: dump-core does not work for GHC-9.0.1 and above
-fplugin-opt=Fusion.Plugin:dump-core
: dump core after each
core-to-core transformation. Output from each transformation is printed
in a different file.
-fplugin-opt=Fusion.Plugin:verbose=1
: report unfused functions. Verbosity
levels 2
, 3
, 4
can be used for more verbose output.
See also
If you are a library author looking to annotate the types, you need to
use the
fusion-plugin-types
package.
Contributing
All contributions are welcome! The code is available under Apache-2.0
license on github. In
case you have any questions or suggestions please contact the
maintainers.
We would be happy to see this work getting integrated with GHC as a fix for
GHC ticket #17075, any help
with that would be appreciated.
References