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diff --git a/docs/comm/the-beast/simplifier.html b/docs/comm/the-beast/simplifier.html deleted file mode 100644 index 4dbce7765b..0000000000 --- a/docs/comm/the-beast/simplifier.html +++ /dev/null @@ -1,86 +0,0 @@ -<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN"> -<html> - <head> - <META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=ISO-8859-1"> - <title>The GHC Commentary - The Mighty Simplifier</title> - </head> - - <body BGCOLOR="FFFFFF"> - <h1>The GHC Commentary - The Mighty Simplifier</h1> - <p> - Most of the optimising program transformations applied by GHC are - performed on an intermediate language called <em>Core,</em> which - essentially is a compiler-friendly formulation of rank-2 polymorphic - lambda terms defined in the module <a - href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/coreSyn/CoreSyn.lhs/"><code>CoreSyn.lhs</code>.</a> - The transformation engine optimising Core programs is called the - <em>Simplifier</em> and composed from a couple of modules located in the - directory <a - href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/simplCore/"><code>fptools/ghc/compiler/simplCore/</code>.</a> - The main engine of the simplifier is contained in <a - href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/simplCore/Simplify.lhs"><code>Simplify.lhs</code>.</a> - and its driver is the routine <code>core2core</code> in <a - href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/simplCore/SimplCore.lhs"><code>SimplCore.lhs</code>.</a> - <p> - The program that the simplifier has produced after applying its various - optimisations can be obtained by passing the option - <code>-ddump-simpl</code> to GHC. Moreover, the various intermediate - stages of the optimisation process is printed when passing - <code>-dverbose-core2core</code>. - - <h4><a name="loopBreaker">Recursive Definitions</a></h4> - <p> - The simplification process has to take special care when handling - recursive binding groups; otherwise, the compiler might loop. - Therefore, the routine <code>reOrderRec</code> in <a - href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/simplCore/OccurAnal.lhs"><code>OccurAnal.lhs</code></a> - computes a set of <em>loop breakers</em> - a set of definitions that - together cut any possible loop in the binding group. It marks the - identifiers bound by these definitions as loop breakers by enriching - their <a href="basicTypes.html#occInfo">occurrence information.</a> Loop - breakers will <em>never</em> be inlined by the simplifier; thus, - guaranteeing termination of the simplification procedure. (This is not - entirely accurate -- see <a href="#rules">rewrite rules</a> below.) - - The processes finding loop breakers works as follows: First, the - strongly connected components (SCC) of the graph representing all - function dependencies is computed. Then, each SCC is inspected in turn. - If it contains only a single binding (self-recursive function), this is - the loop breaker. In case of multiple recursive bindings, the function - attempts to select bindings where the decision not to inline them does - cause the least harm - in the sense of inhibiting optimisations in the - code. This is achieved by considering each binding in turn and awarding - a <em>score</em> between 0 and 4, where a lower score means that the - function is less useful for inlining - and thus, a better loop breaker. - The evaluation of bingings is performed by the function - <code>score</code> locally defined in <code>OccurAnal</code>. - - Note that, because core programs represent function definitions as - <em>one</em> binding choosing between the possibly many equations in the - source program with a <code>case</code> construct, a loop breaker cannot - inline any of its possibly many alternatives (not even the non-recursive - alternatives). - - <h4><a name="rules">Rewrite Rules</a></h4> - <p> - The application of rewrite rules is controlled in the module <a - href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/simplCore/Simplify.lhs"><code>Simplify.lhs</code></a> - by the function <code>completeCall</code>. This function first checks - whether it should inline the function applied at the currently inspected - call site, then simplifies the arguments, and finally, checks whether - any rewrite rule can be applied (and also whether there is a matching - specialised version of the applied function). The actual check for rule - application is performed by the function <code><a - href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/specialise/Rules.lhs">Rules</a>.lookupRule</code>. - <p> - It should be note that the application of rewrite rules is not subject - to the loop breaker check - i.e., rules of loop breakers will be applied - regardless of whether this may cause the simplifier to diverge. - - <p><small> -<!-- hhmts start --> -Last modified: Wed Aug 8 19:25:33 EST 2001 -<!-- hhmts end --> - </small> - </body> -</html> |