diff options
Diffstat (limited to 'ghc/docs/comm/the-beast/mangler.html')
| -rw-r--r-- | ghc/docs/comm/the-beast/mangler.html | 79 |
1 files changed, 0 insertions, 79 deletions
diff --git a/ghc/docs/comm/the-beast/mangler.html b/ghc/docs/comm/the-beast/mangler.html deleted file mode 100644 index 1ad80f0d5c..0000000000 --- a/ghc/docs/comm/the-beast/mangler.html +++ /dev/null @@ -1,79 +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 Evil Mangler</title> - </head> - - <body BGCOLOR="FFFFFF"> - <h1>The GHC Commentary - The Evil Mangler</h1> - <p> - The Evil Mangler (EM) is a Perl script invoked by the <a - href="driver.html">Glorious Driver</a> after the C compiler (gcc) has - translated the GHC-produced C code into assembly. Consequently, it is - only of interest if <code>-fvia-C</code> is in effect (either explicitly - or implicitly). - - <h4>Its purpose</h4> - <p> - The EM reads the assembly produced by gcc and re-arranges code blocks as - well as nukes instructions that it considers <em>non-essential.</em> It - derives it evilness from its utterly ad hoc, machine, compiler, and - whatnot dependent design and implementation. More precisely, the EM - performs the following tasks: - <ul> - <li>The code executed when a closure is entered is moved adjacent to - that closure's infotable. Moreover, the order of the info table - entries is reversed. Also, SRT pointers are removed from closures that - don't need them (non-FUN, RET and THUNK ones). - <li>Function prologue and epilogue code is removed. (GHC generated code - manages its own stack and uses the system stack only for return - addresses and during calls to C code.) - <li>Certain code patterns are replaced by simpler code (eg, loads of - fast entry points followed by indirect jumps are replaced by direct - jumps to the fast entry point). - </ul> - - <h4>Implementation</h4> - <p> - The EM is located in the Perl script <a - href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/driver/mangler/ghc-asm.lprl"><code>ghc-asm.lprl</code></a>. - The script reads the <code>.s</code> file and chops it up into - <em>chunks</em> (that's how they are actually called in the script) that - roughly correspond to basic blocks. Each chunk is annotated with an - educated guess about what kind of code it contains (e.g., infotable, - fast entry point, slow entry point, etc.). The annotations also contain - the symbol introducing the chunk of assembly and whether that chunk has - already been processed or not. - <p> - The parsing of the input into chunks as well as recognising assembly - instructions that are to be removed or altered is based on a large - number of Perl regular expressions sprinkled over the whole code. These - expressions are rather fragile as they heavily rely on the structure of - the generated code - in fact, they even rely on the right amount of - white space and thus on the formatting of the assembly. - <p> - Afterwards, the chunks are reordered, some of them purged, and some - stripped of some useless instructions. Moreover, some instructions are - manipulated (eg, loads of fast entry points followed by indirect jumps - are replaced by direct jumps to the fast entry point). - <p> - The EM knows which part of the code belongs to function prologues and - epilogues as <a href="../rts-libs/stgc.html">STG C</a> adds tags of the - form <code>--- BEGIN ---</code> and <code>--- END ---</code> the - assembler just before and after the code proper of a function starts. - It adds these tags using gcc's <code>__asm__</code> feature. - <p> - <strong>Update:</strong> Gcc 2.96 upwards performs more aggressive basic - block re-ordering and dead code elimination. This seems to make the - whole <code>--- END ---</code> tag business redundant -- in fact, if - proper code is generated, no <code>--- END ---</code> tags survive gcc - optimiser. - - <p><small> -<!-- hhmts start --> -Last modified: Sun Feb 17 17:55:47 EST 2002 -<!-- hhmts end --> - </small> - </body> -</html> |