Arun Kumar S Jadhav
I'm looking at the implementation of Haskell done by nhc. The precise G-code semantics are still unclear to me. I wanted help in two matters
1) It would be great if someone explained the symbol table structure. Because many G-code seemed to refer to symbol table.
Internal to the compiler, G-code instructions refer to functions and constructors by their index value in the symbol table, a simple Int. For example, HEAP_CON 32 means to write a constructor into a heap cell, where 32 is its index in the global symbol table. Use the compiler option +CTS -abound -CTS to see the symbol table immediately before G-code-generation. The symbol entries are like this: (48, InfoConstr 48 Prelude.Right IEsel (infixl{-def-} ,9) \/ b c . c -> (5 b c) [Nothing] 5) The number 48 is the entry number (duplicated inside the "Info" structure). The Info structure is defined in src/compiler98/Info.hs. * This particular value is a constructor (InfoConstr). * Its name is "Prelude.Right". * The value "IEsel" refers to whether it is imported/re-exported from this module. * It has default fixity and precedence (infixl 9). * Its type is "\/ b c . c -> (5 b c)", written in big-lambda notation where a number refers to another entry in the symbol table. Thus, in the context of two types, b and c, Prelude.Right takes an argument of type c and returns a value of type (Either b c) (where entry 5 in the symbol table is an InfoData for "Either"). * [Nothing] is a list of fieldnames for this constructor (there are none). * The final number 5 is the type to which this constructor belongs (Either). However, when the G-code is stored as bytecode in the target .hc file, there is no longer a global symbol table available. Instead, each function symbol (FN_) is associated with a local constant table (CT_) which stores references to all and only those functions and constructors used by this function. So for instance, the G-code instruction HEAP_CVAL_I3 now means to take the 3rd value from the local constant table and write it to a heap cell. More details of the bytecode representation are in the documentation at http://haskell.org/nhc98/implementation-notes/bytecode
2) The file which generates target code from G-code and the related files.
Generation of a .hc file containing the G-code bytecode is performed by src/compiler98/GcodeLowC.hs using the mechanisms defined in src/compiler98/EmitState.hs to resolve local and global labels. Regards, Malcolm