|Version 4 (modified by guest, 9 years ago) (diff)|
GHC Commentary: The Code Generator
Storage manager representations
The code generator needs to know the layout of heap objects, because it generates code that accesses and constructs those heap objects. The runtime also needs to know about the layout of heap objects, because it contains the garbage collector. How can we share the definition of storage layout such that the code generator and the runtime both have access to it, and so that we don't have to keep two independent definitions in sync?
Currently we solve the problem this way:
- C types representing heap objects are defined in the C header files, see for example includes/Closures.h.
- A C program, includes/mkDerivedConstants.c, #includes the runtime headers. This program is built and run when you type make or make boot in includes/. It is run twice: once to generate includes\DerivedConstants.h, and again to generate includes/GHCConstants.h.
- The file DerivedConstants.h contains lots of #defines like this:
#define OFFSET_StgTSO_why_blocked 18which says that the offset to the why_blocked field of an StgTSO is 18 bytes. This file is #included into includes/Cmm.h, so these offests are available to the hand-written .cmm files.
- The file GHCConstants.h contains similar definitions:
oFFSET_StgTSO_why_blocked = 18::IntThis time the definitions are in Haskell syntax, and this file is #included directly into compiler/main/Constants.lhs. This is the way that these offsets are made available to GHC's code generator.
Generated Cmm Naming Convention
Labels generated by the code generator are of the form <name>_<type> where <name> is <Module>_<name> for external names and <unique> for internal names. <type> is one of the following:
- Info table
- Static reference table
- Static reference table descriptor
- Entry code (function, closure)
- Slow entry code (if any)
- Direct return address
- Vector table
- Case alternative (tag n)
- Default case alternative
- Large bitmap vector
- Static closure
- Dynamic Constructor entry code
- Dynamic Constructor info table
- Static Constructor entry code
- Static Constructor info table
- Selector info table
- Selector entry code
- Cost centre
- Cost centre stack
Many of these distinctions are only for documentation reasons. For example, _ret is only distinguished from _entry to make it easy to tell whether a code fragment is a return point or a closure/function entry.