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copied!<p>GHC Core is the <a href="http://research.microsoft.com/~simonpj/papers/ext-f/" rel="noreferrer">System FC</a> language into which all Haskell is translated. The (approximate) grammar for Core is given by:</p> <p><img src="https://i.stack.imgur.com/RP6fQ.png" alt="enter image description here"></p> <p>Core is closely related to the simpler and better known <a href="http://en.wikipedia.org/wiki/System_F" rel="noreferrer">System F</a>. All <a href="http://research.microsoft.com/en-us/um/people/simonpj/papers/comp-by-trans-scp.ps.gz" rel="noreferrer">transformations GHC does on the Core level</a> are type-preserving refactorings of this Core representation, to improve performance. And, not so well known, you can write directly in Core to program GHC.</p> <p>GHC Core fits in the compiler pipeline (as it was in 2002, sans-LLVM and CMM):</p> <p><img src="https://i.stack.imgur.com/PtsCf.png" alt="enter image description here"></p> <p>The primary documents to learn about GHC Core are:</p> <ul> <li><strong><a href="http://www.haskell.org/ghc/docs/papers/core.ps.gz" rel="noreferrer">An External Representation for the GHC Core Language</a>, Tolmach, 2001</strong></li> <li><a href="https://github.com/ghc/ghc/blob/master/compiler/coreSyn/CoreSyn.hs#L141" rel="noreferrer">ghc/compiler/CoreSyn</a>, the GHC definition itself</li> <li><strong><a href="http://research.microsoft.com/en-us/um/people/simonpj/papers/inlining/inline-jfp.ps.gz" rel="noreferrer">Secrets of the Glasgow Haskell Compiler inliner</a></strong>, Peyton Jones and Marlow, 1999. Core is described in Section 2.3, including details on the occurrence analysis annotations.</li> <li><strong><a href="http://research.microsoft.com/pubs/67064/comp-by-trans-scp.ps.gz" rel="noreferrer">A transformation-based optimiser for Haskell</a></strong>, Peyton Jones and Santos, 1998. Core is described in S3, including a discussion of polymorphism and operational readings of Core.</li> </ul> <p>Related material that can aid understanding:</p> <ul> <li>The <a href="http://www.haskell.org/ghc/docs/latest/html/users_guide/ext-core.html" rel="noreferrer">GHC -fext-core output</a></li> <li>I spent a lot of time learning Core by reading GHC source. Some is described in my <a href="https://web.archive.org/web/20170118091715/https://www.cse.unsw.edu.au/~pls/thesis/dons-thesis.ps.gz" rel="noreferrer">undergraduate thesis</a> from 2002, from page 16.</li> <li>From using the <a href="http://hackage.haskell.org/package/ghc-core" rel="noreferrer">ghc-core</a> tool, to generate Core in a format I find pleasing.</li> </ul> <p>Core in turn is translated into STG code, which looks something like:</p> <p><img src="https://i.stack.imgur.com/Gaj9X.png" alt="enter image description here"></p> <p>The funny names in Core are encoded in the "Z-encoding":</p> <p><img src="https://i.stack.imgur.com/J9pqR.png" alt="enter image description here"></p> <p>GHC Core's types and kinds (from Tolmach's paper):</p> <p><img src="https://i.stack.imgur.com/eNvd2.png" alt="enter image description here"></p> <p>Finally, GHC's <a href="https://ghc.haskell.org/trac/ghc/wiki/Commentary/PrimOps" rel="noreferrer">primops</a> appear regularly in GHC Core output, when you have optimized your Haskell down to the basic instructions GHC knows about. The primop set is given as a set of Core functions in <a href="https://ghc.haskell.org/trac/ghc/browser/ghc/compiler/prelude/primops.txt.pp#L12" rel="noreferrer">a pre-processed file.</a> </p>

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