Changes between Version 6 and Version 7 of Commentary/Rts/Cmm

May 4, 2014 6:51:41 PM (15 months ago)

Add lots of reading resources


  • Commentary/Rts/Cmm

    v6 v7  
    77We use the C preprocessor on `.cmm` files, making extensive use of macros to make writing this low-level code a bit less tedious and error-prone.  Most of our C-- macros are in [[GhcFile(includes/Cmm.h)]]. One useful fact about the macros is `P_` is an alias for `gcptr`, and you should not use it for non-garbage-collected pointers.
    9 For more information on Cmm (GHC's implementation of C--), see [wiki:Commentary/Compiler/CmmType the Cmm language] page.
     10== Reading references ==
     12Reading material for learning Cmm is somewhat scattered, so I (Arash) have created a list of useful links. Since the Cmm language is changing as GHC changes, I have prioritized. (''Feel free to add/remove/modify this this list! :)'')
     15 * An overview of Cmm is given in [ David Terei's bachelor thesis] (chapter 2.4.3).
     16 * The comments in the beginning of [[GhcFile(compiler/cmm/CmmParse.y)]] is super-useful and kept up to date. The rest of the file contains the ''grammar'' of the language. Afraid of grammars? Edward Yang wrote this fantastic [ blog post] on how to understand the constructs of Cmm by using the grammar. 
     17 * Cmm has a preprocessor like the one in C and many of the macros are defined in [[GhcFile(includes/Cmm.h)]].
     18 * In 2012, Simon Marlow extended the Cmm language by adding a new high-level syntax which can be used when you don't need low-level access (like registers). The [ commit] explains the details.
     19 * Cmm is also described [wiki:Commentary/Compiler/CmmType on this wiki], but it is written before the new syntax was introduced.
     20 * Stack frame types are created using ``INFO_TABLE_RET``, the syntax can be confusing since there are both ''arguments'' and ''fields'', I (Arash) have not seen anything like it in other programming languages. I tried to explain it in my [ master thesis] (sections 4.2 and 4.2.1).
     22== Other information ==
     24It can take time to learn Cmm. One unintuitive thing to watch out for is that there are no function calls in low-level cmm code. The new syntax from 2012 allows function calls but you should know that they are kind of magical.
     26We say that '''Cmm''' is GHC's implementation of '''C--'''. This naming scheme is not done consistently everywhere, unfortunately. If you are interested in C-- (which have diverged from Cmm), you can check out the [ website] and the [ specification].