codegen.txt

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CODEGEN.TXT - notes on code generation
===========

(A) Philosophy
    ----------

    - front-end to create virtual file(s) of code-generation commands

    - after processing the input file(s), these files are interpreted to
      invoke specific code-generation routines

    - intermediate file used for each function being generated and for other
      interesting stuff such as module data, unitialized data (_BSS), and
      for constants

    - a file is created by CgFrontFileCreate which returns a CGFILE* ptr.

    - a file is closed by calling CgFrontCloseFile

    - code is emitted to the current file; you can change the current file
      by calling CgFrontSwitchFile



(B) Header Files
    ------------

    CGDATA.H    - code-generation data
                - included by CGFRONT.H

    CGFRONT.H   - communication with front-end
                - this is file to be included in C++ modules

    CGIO.H      - i/o on code-generation file
                - included by CGFRONT.H

    IC.H        - file listing intermediate opcodes
                - never included directly

    ICODES.H    - enumerate opcodes
                - included by CGFRONT.H


(C) Format of Intermediate Code
    ---------------------------

    - defined by CGINTER as:
        byte    opcode;
        CGVALUE value;

    - CGVALUE is union of "signed long", "unsigned long", "void *"

    - instructions are placed in the current file by optionally calling
      a routine to set the value (0 is used as the value when no
      value-setting  routine is called) and then calling a routine to set
      the opcode.

    - opcode-setting routines:
        - CgFrontCode       -- set opcode in a code segment
        - CgFrontDataInit   -- set opcode in constant segment
        - CgFrontDataUninit -- set opcode in uninitialized-data segment

    - value-setting routines:
        - CgFrontValueUint  -- set "unsigned long" value
        - CgFrontValueInt   -- set "signed long" value
        - CgFrontValuePtr   -- set "void *" value


(D) Opcode Conventions
    ------------------


    Internal Opcodes:
    -----------------

    IC_EOF              -- mark end-of-virtual-file

    IC_NEXT             -- move to next block in virtual file

    IC_DEF_SEG segid    -- set output segment


    Code Labels and goto's:
    -----------------------

    IC_LABGET_CS count  -- allocate "count" labels for control sequences

    IC_LABFREE_CS count -- free "count" labels for control sequences

    IC_LABGET_GOTO      -- allocate label for GOTO

    IC_LABGET_BIG       -- allocate "big" label

    IC_LABDEF_CS lab#   -- define control-sequence label

    IC_LABDEF_SW lab#   -- define "switch" label
                        -- used internally

    IC_LABDEF_GOTO lab# -- define GOTO label

    IC_LABDEF_BIG lab#  -- define "big" label

    IC_LABEL_CS lab#    -- set control-sequence label for next IC_GOTO_NEAR

    IC_LABEL_SW lab#    -- set switch label for next IC_GOTO_NEAR

    IC_LABEL_GOTO lab#  -- set goto label for next IC_GOTO_NEAR

    IC_GOTO_NEAR cond   -- goto near label (control sequence, goto, switch)
                        -- target label set by IC_LABEL_CS, IC_LABEL_SW, or
                           IC_LABEL_GOTO

    IC_GOTO_BIG lab#    -- unconditional goto "big" label

    IC_SWITCH_BEG       -- start a switch statement
                        -- selection expression will be generated
                        -- should call CgSwitchId to get switch id
                           immediately before opcode is generated

    IC_SWITCH_END id    -- complete a switch statement
                        -- "id" is value returned by CgSwitchId

    IC_SWITCH_CASE case -- next CASE in switch statement
                        -- "case" is constant value to select the case
                        -- creates a switch label

    IC_SWITCH_DEFAULT 0 -- DEFAULT in switch statement
                        -- should call CgFrontLabelSw to get default label
                           immediately before opcode is generated
                        -- creates a switch label


    FeedBackRoutines: called to get label numbers:
    ----------------------------------------------

    These routines are called to obtain the next available label number, and
    so should be called before labels are allocated.

    CgFrontLabelCs      -- label # for control sequence

    CgFrontLabelSw      -- label # for switch

    CgFrontLabelGoTo    -- label # for goto

    CgFrontLabelBig     -- label # for "big"


    Expressions:
    ------------

    The code generator requires the type of expression elements to be
    specified for each operand or operator node.  This is set by the
    following opcode:

    IC_SET_TYPE type    -- set the type


    Operands are specified as follows:

    IC_LEAF_CONST_INT value -- set an integer value

    IC_LEAF_CONST_FLT value -- set a floating-point value

    IC_LEAF_NAME_FRONT sym  -- set operand specified by symbol-table entry

    IC_LEAF_NAME_BACK hand  -- set operand specified by back-handle "hand"
                            -- not known if req'd

    IC_LEAF_NAME_TEMP       -- set a temporary
                            -- not known if req'd


    Operators are defined in CGDEFS.H and the C++ extensions are defined
    in CODEGEN.H. Operators are specified as follows:

    IC_OPR_TERNARY op       -- ternary operator
                            -- IC_SET_TYPE has been used to set type
                            -- "op" is the O_xxx code

    IC_OPR_BINARY op        -- binary operator
                            -- IC_SET_TYPE has been used to set type
                            -- "op" is the O_xxx code

    IC_OPR_UNARY op         -- unary operator
                            -- IC_SET_TYPE has been used to set type
                            -- "op" is the O_xxx code

    When the binary operator is O_INDEX (indexing) the opcode should be
    immediately preceded by:

    IC_OPR_INDEX type       -- set the type to be indexed
                            -- IC_SET_TYPE has been used to set type of index
                            -- the "current type" at this point will be the
                               type of the subscript

    Once an expression has been generated, one of the following should be
    specified:

    IC_EXPR_DONE            -- evaluate expression and retain final value

    IC_EXPR_TRASH           -- evaluate expression and ignore final value

    A partially-computed expression can be duplicated pushed and later
    popped by the following opcodes. The pushing and popping uses another
    stack from the normal stack used for expression evaluation.

    IC_EXPR_PUSH            -- duplicate and push the expression

    IC_EXPR_POP             -- pop a pushed expression


    Scope Processing:
    -----------------

    IC_FILE_OPEN scope      -- start of file scope
                            -- "scope" is scope for file

    IC_FILE_CLOSE scope     -- completion of file scope
                            -- "scope" is scope for file

    IC_FUNCTION_OPEN sym    -- start of function scope
                            -- "sym" is the function SYMBOL

    IC_FUNCTION_CLOSE sym   -- completion of function scope
                            -- "sym" is the function SYMBOL

    IC_BLOCK_OPEN scope     -- start of block scope
                            -- "scope" is scope for the block

    IC_BLOCK_CLOSE scope    -- completion of block scope
                            -- "scope" is scope for the block

    IC_BLOCK_EXIT scope     -- generated before a jump
                            -- DTORs blocks up to "scope"

    IC_BLOCK_PART symbol    -- generated before a jump
                            -- DTORs symbols in current block, from "symbol"
                               onwards

    IC_BLOCK_ENTER scope    -- generated before a jump
                            -- initializes for DTORing down to "scope"

    IC_BLOCK_SRC scope      -- sets current scope

    IC_DTOR_SYMBOL sym      -- indicate symbol needs DTORing


    Procedure calls:
    ----------------

    IC_PROC_TEMP            -- create temporary for procedure
                            -- not known if req'd

    IC_PROC_RETURN bool     -- return statement in procedure
                            -- IC_SET_TYPE has been used to set type
                            -- if bool==1, return expression is generated

    IC_CALL_SETUP sym       -- start a direct call using function 'sym'
                            -- IC_SET_TYPE has been used to set type
                            -- IC_LEAF_NAME_FRONT used to specify function

    IC_CALL_SETUP_IN type   -- start an indirect call
                            -- IC_SET_TYPE has been used to set type
                            -- "type" is TYPE for temporary symbol
                            -- expression for call address has been gen'ed

    IC_CALL_PARM            -- add parameter for call
                            -- IC_SET_TYPE has been used to set type
                            -- last expression computed is parameter

    IC_CALL_EXEC_DIRECT     -- execute the direct call
                            -- IC_SET_TYPE has been used to set type

    IC_CALL_EXEC_INDIRECT   -- execute the indirect call
                            -- IC_SET_TYPE has been used to set type

    IC_CALL_SETUP_RT #      -- start a run-time call
                            -- # is code for run-time function
                            -- call effected by IC_CALL_EXEC_INDIRECT

    Symbol Handling
    ---------------

    IC_BACK_ALLOC sym       -- allocate back handle for "sym"

    IC_BACK_FINI sym        -- complete usage for "sym"
                            -- not known if req'd

    IC_BACK_FREE sym        -- free back handle for "sym"


    Data Generation
    ---------------

    IC_DATA_LABEL sym       -- generate label for "sym"

    IC_DATA_PTR_OFFSET off  -- specify offset for use with IC_DATA_PTR_SYM

    IC_DATA_PTR_SYM sym     -- generate pointer relative to "sym"
                            -- offset specified by IC_DATA_PTR_OFFSET added
                               to address of "sym"
                            -- type of pointer set by IC_SET_TYPE

    IC_DATA_INT value       -- generate integer with value "value"
                            -- type of integer set by IC_SET_TYPE

    IC_DATA_FLT ptr         -- generate float with value indicated by "ptr"
                            -- type of float set by IC_SET_TYPE

    IC_DATA_SIZE value      -- specify size for use with upcoming instruction:
                                -- IC_DATA_BYTES
                                -- IC_DATA_REPLICATE
                                -- IC_DATA_UNDEF

    IC_DATA_TEXT string     -- generate text (translated to target codeset)
                            -- size and pointer in string STRING_CONSTANT

    IC_DATA_BYTES bytes     -- generate string of bytes (not translated)
                            -- size set by IC_DATA_SIZE

    IC_DATA_REPLICATE byte  -- replicate "byte" value
                            -- size set by IC_DATA_SIZE

    IC_DATA_UNDEF size      -- set "size" undefined value(s)

    IC_DATA_ALIGN value     -- align to indicated value