... | ... | @@ -6,5 +6,22 @@ A `.cmm` file is rather like C--. The syntax is almost C-- (a few constructs ar |
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We 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 [includes/Cmm.h](/trac/ghc/browser/ghc/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.
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## Reading references
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For more information on Cmm (GHC's implementation of C--), see [the Cmm language](commentary/compiler/cmm-type) page. |
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Reading 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! :)*)
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- An overview of Cmm is given in [ David Terei's bachelor thesis](https://davidterei.com/downloads/papers/terei:2009:honours_thesis.pdf) (chapter 2.4.3).
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- The comments in the beginning of [compiler/cmm/CmmParse.y](/trac/ghc/browser/ghc/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](http://blog.ezyang.com/2013/07/no-grammar-no-problem/) on how to understand the constructs of Cmm by using the grammar.
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- Cmm has a preprocessor like the one in C and many of the macros are defined in [includes/Cmm.h](/trac/ghc/browser/ghc/includes/Cmm.h).
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- 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](https://github.com/ghc/ghc/commit/a7c0387d20c1c9994d1100b14fbb8fb4e28a259e) explains the details.
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- Cmm is also described [on this wiki](commentary/compiler/cmm-type), but it is written before the new syntax was introduced.
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- 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](http://arashrouhani.com/papers/master-thesis.pdf) (sections 4.2 and 4.2.1).
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## Other information
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It 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.
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We 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](http://www.cminusminus.org/) and the [ specification](http://www.cs.tufts.edu/~nr/c--/extern/man2.pdf). |