Changes between Version 11 and Version 12 of Commentary/Compiler/NewCodeGen


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Timestamp:
Dec 11, 2007 12:30:29 PM (6 years ago)
Author:
simonpj
Comment:

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  • Commentary/Compiler/NewCodeGen

    v11 v12  
    44 
    55Status: 
     6 * The Rep swamp is drained: see [wiki:Commentary/Compiler/BackEndTypes] 
    67 * Code generator: first draft done. 
    78 * Control-flow opt: simple ones done 
     
    114115   * In a few cases, use native calls (notably eval) 
    115116 
    116 -------------------------------------- 
    117 == The `Rep` swamp == 
    118  
    119 I have completed a major representation change, affecting both old and new code generators, of the  various `Rep` types.  It's pervasive in that it touches a lot of files; and in the native code-gen very many lines are changed.  The new situation is much cleaner. 
    120  
    121 Here are the highlights of the new design. 
    122  
    123 === `CmmType` === 
    124  
    125 There is a new type `CmmType`, defined in module `CmmExpr`, which is just what it sounds like: it's the type of a `CmmExpr` or a `CmmReg`.   
    126    * A `CmmType` is ''abstract'': its representation is private to `CmmExpr`.  That makes it easy to change representation. 
    127    * A `CmmType` is actually just a pair of a `Width` and a category (`CmmCat`). 
    128    * The `Width` type is exported and widely used in pattern-matching, but it does what it says on the tin: width only.   
    129    * In contrast, the `CmmCat` type is entirely private to `CmmExpr`.  It is just an enumeration that allows us to distinguish: floats, gc pointers, and other.  
    130  
    131 Other important points are these: 
    132  
    133  * Each `LocalReg` has a `CmmType` attached; this replaces the previous unsavoury combination of `MachRep` and `CmmKind`.  Indeed, both of the latter are gone entirely. 
    134  
    135  * Notice that a `CmmType` accurately knows about gc-pointer-hood. Ultimately we will abandon static-reference-table generation in STG syntax, and instead generate SRTs from the Cmm code.  We'll need to update the RTS `.cmm` files to declare pointer-hood. 
    136  
    137  * The type `TyCon.PrimRep` remains; it enumerates the representations that a Haskell value can take.  Differences from `CmmType`: 
    138    * `PrimRep` contains `VoidRep`, but `CmmType` has no zero-width form. 
    139    * `CmmType` includes sub-word width values (e.g. 8-bit) which `PrimRep` does not. 
    140    The function `primRepCmmType` converts a non-void `PrimRep` to a `CmmType`. 
    141  
    142 === `MachOp` === 
    143  
    144 The `MachOp` type enumerates (in machine-independent form) the available machine instructions.  The principle they embody is that ''everything except the width is embodied in the opcode''.  In particular, we have 
    145  * `MO_S_Lt`, `MO_U_Lt`, and `MO_F_Lt` for comparison (signed, unsigned, and float). 
    146  * `MO_SS_Conv`, `MO_SF_Conv` etc, for conversion (`SS` is signed-to-signed, `SF` is signed-to-float, etc). 
    147 These constructor all take `Width` arguments. 
    148  
    149 The `MachOp` data type is defined in `CmmExpr`, not in a separate `MachOp` module. 
    150  
    151 === Foreign calls and hints === 
    152  
    153 In the new Cmm representation (`ZipCfgCmmRep`), but not the old one, arguments and results to all calls, including foreign ones, are ordinary `CmmExpr` or `CmmReg` respectively.  The extra information we need for foreign calls (is this signed?  is this an address?) are kept in the calling convention.  Specifically: 
    154  
    155  * `MidUnsafeCall` calls a `MidCallTarget` 
    156  * `MidCallTarget` is either a `CallishMachOp` or a `ForeignTarget` 
    157  * In the latter case we supply a `CmmExpr` (the function to call) and a `ForeignConvention` 
    158  * A `ForeignConvention` contains the C calling convention (stdcall, ccall etc), and a list of `ForiegnHints` for arguments and for results. (We might want to rename this type.) 
    159  
    160 This simple change was horribly pervasive.  The old Cmm rep (and Michael Adams's stuff) still has arguments and results being (argument,hint) pairs, as before. 
    161  
    162 === Native code generation and the `Size` type === 
    163  
    164 The native code generator has an instruction data type for each architecture.  Many of the instructions in these data types used to have a `MachRep` argument, but now have a `Size` argument instead.  In fact, so far as the native code generators are concerned, these `Size` types (which can be machine-specific) are simply a plug-in replacement for `MachRep`, with one big difference: '''`Size` is completely local to the native code generator''' and hence can be changed at will without affecting the rest of the compiler. 
    165  
    166 `Size` is badly named, but I inherited the name from the previous code. 
    167  
    168 I rather think that many instructions should have a `Width` parameter, not a `Size` parameter.  But I didn't feel confident to change this.  Generally speaking the NCG is a huge swamp and needs re-factoring.