old code gen

Old Code Generator (prior to GHC 7.8)

Material below describes old code generator that was used up to GHC 7.6 and was retired in 2012. This page is not maintained and is here only for historical purposes. See Code generator page for an up to date description of the current code generator.

Storage manager representations

See The Storage Manager for the Layout of the stack.

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/rts/storage/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 18
```
which 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::Int
```
This 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

See compiler/GHC/Cmm/CLabel.hs

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	Info table
srt	Static reference table
srtd	Static reference table descriptor
entry	Entry code (function, closure)
slow	Slow entry code (if any)
ret	Direct return address
vtbl	Vector table
n_alt	Case alternative (tag n)
dflt	Default case alternative
btm	Large bitmap vector
closure	Static closure
con_entry	Dynamic Constructor entry code
con_info	Dynamic Constructor info table
static_entry	Static Constructor entry code
static_info	Static Constructor info table
sel_info	Selector info table
sel_entry	Selector entry code
cc	Cost centre
ccs	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.

Modules

`CodeGen`

Top level, only exports codeGen.

Called from HscMain for each module that needs to be converted from Stg to Cmm.

For each such module codeGen does three things:

cgTopBinding for the StgBinding
cgTyCon for the TyCon (These are constructors not constructor calls).
mkModuleInit for the module

mkModuleInit generates several boilerplate initialization functions that:

regiser the module,
creates an Hpc table,
setup its profiling info (InitConstCentres, code coverage info initHpc), and
calls the initialization functions of the modules it imports.

If neither SCC profiling or HPC are used, then the initialization code short circuits to return.

If the module has already been initialized, the initialization function just returns.

The Ghc.TopHandler and Ghc.Prim modules get special treatment.

cgTopBinding is a small wrapper around cgTopRhs which in turn disptaches to:

cgTopRhsCons for StgRhsCons (these are bindings of constructor applications not constructors themselves) and
cgTopRhsClosure for StgRhsClosure.

cgTopRhsCons and cgTopRhsClosure are located in CgCon and CgClosure which are the primary modules called by CodeGen.

`CgCon`

TODO

`CgClosure`

TODO

`CgMonad`

The monad that most of codeGen operates inside

Reader
State
(could be Writer?)
fork
flatten

`CgExpr`

Called by CgClosure and CgCon.

Since everything in STG is an expression, almost everything branches off from here.

This module exports only one function cgExpr, which for the most part just dispatches to other functions to handle each specific constructor in StgExpr.

Here are the core functions that each constructor is disptached to (though some may have little helper functions called in addition to the core function):

`StgApp`	Calls to `cgTailCall` in `CgTailCall`
`StgConApp`	Calls to `cgReturnDataCon` in `CgCon`
`StgLit`	Calls to `cgLit` in `CgUtil` and `performPrimReturn` in `CgTailCall`
`StgOpApp`	Is a bit more complicated see below.
`StgCase`	Calls to `cgCase` in `CgCase`
`StgLet`	Calls to `cgRhs` in `CgExpr`
`StgLetNoEscape`	Calls to `cgLetNoEscapeBindings` in `CgExpr`, but with a little bit of wrapping by `nukeDeadBindings` and `saveVolatileVarsAndRegs`.
`StgSCC`	Calls to `emitSetCCC` in `CgProf`
`StgTick`	Calls to `cgTickBox` in `CgHpc`
`StgLam`	Does not have a case because it is only for `CoreToStg`'s work.

Some of these cases call to functions defined in cgExpr. This is because they need a little bit of wrapping and processing before calling out to their main worker function.

`cgRhs`	For `StgRhsCon` calls out to `buildDynCon` in `CgCon`. For `StgRhsClosure` calls out to `mkRhsClosure`. In turn, `mkRhsClosure` calls out to `cgStdRhsClosure` for selectors and thunks, and calls out to `cgRhsClosure` in the default case. Both these are defined in `CgClosure`.

`cgLetNoEscapeBindings`	Wraps a call to `cgLetNoEscapeRhs` with `addBindsC` depending on whether it is called on a recursive or a non-recursive binding. In turn `cgLetNoEscapeRhs` wraps `cgLetNoEscapeClosure` defined in `CgLetNoEscapeClosure`.

StgOpApp has a number of sub-cases.

StgFCallOp
StgPrimOp of a TagToEnumOp
StgPrimOp that is primOpOutOfLine
StgPrimOp that returns Void
StgPrimOp that returns a single primitive
StgPrimOp that returns an unboxed tuple
StgPrimOp that returns an enumeration type

(It appears that non-foreign-call, inline PrimOps are not allowed to return complex data types (e.g. a |Maybe|), but this fact needs to be verified.)

Each of these cases centers around one of these three core calls:

emitForeignCall in CgForeignCall
tailCallPrimOp in CgTailCall
cgPrimOp in CgPrimOp

There is also a little bit of argument and return marshelling with the following functions

Argument marshelling	`shimForeignCallArg`, `getArgAmods`
Return marshelling	`dataReturnConvPrim`, `primRepToCgRep`, `newUnboxedTupleRegs`
Performing the return	`emitReturnInstr`, `performReturn`, `returnUnboxedTuple`, `ccallReturnUnboxedTuple`

In summary the modules that get called in order to handle a specific expression case are:

Also called for top level bindings by `CodeGen`

`CgCon`	for `StgConApp` and the `StgRhsCon` part of `StgLet`
`CgClosure`	for the `StgRhsClosure` part of `StgLet`

Core code generation

`CgTailCall`	for `StgApp`, `StgLit`, and `StgOpApp`
`CgPrimOp`	for `StgOpApp`
`CgLetNoEscapeClosure`	for `StgLetNoEscape`
`CgCase`	for `StgCase`

Profiling and Code coverage related

`CgProf`	for `StgSCC`
`CgHpc`	for `StgTick`

Utility modules that happen to have the functions for code generation

`CgForeignCall`	for `StgOpApp`
`CgUtil`	for `cgLit`

Note that the first two are the same modules that are called for top level bindings by CodeGen, and the last two are really utility modules, but they happen to have the functions needed for those code generation cases.

Memory and Register Management

`CgBindery`	Module for `CgBindings` which maps variable names to all the volitile or stable locations where they are stored (e.g. register, stack slot, computed from other expressions, etc.) Provides the `addBindC`, `modifyBindC` and `getCgIdInfo` functions for adding, modifying and looking up bindings.

`CgStackery`	Mostly utility functions for allocating and freeing stack slots. But also has things on setting up update frames.

`CgHeapery`	Functions for allocating objects that appear on the heap such as closures and constructors. Also includes code for stack and heap checks and `emitSetDynHdr`.

Function Calls and Parameter Passing

(Note: these will largely go away once CPS conversion is fully implemented.)

`CgPrimOp`, `CgTailCall`, `CgForeignCall`	Handle different types of calls.
`CgCallConv`	Use by the others in this category to determine liveness and to select in what registers and stack locations arguments and return values get stored.

Misc utilities

`Bitmap`	Utility functions for making bitmaps (e.g. `mkBitmap` with type `[Bool] -> Bitmap`)
`ClosureInfo`	Stores info about closures and bindings. Includes information about memory layout, how to call a binding (`LambdaFormInfo`) and information used to build the info table (`ClosureInfo`).
`SMRep`	Storage manager representation of closures. Part of ClosureInfo but kept separate to "keep nhc happy."
`CgUtils`	TODO
`CgInfoTbls`	TODO

Special runtime support

`CgTicky`	Ticky-ticky profiling
`CgProf`	Cost-centre profiling
`CgHpc`	Support for the Haskell Program Coverage (hpc) toolkit, inside GHC.
`CgParallel`	Code generation for GranSim (GRAN) and parallel (PAR). All the functions are dead stubs except `granYield` and `granFetchAndReschedule`.