Symbol entries

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Symbol entries

Architecture

Warning: Last updated for FPC version 1.0.x

There are different possible types of symbols, each one having different fields then the others. Each symbol type has a specific signature to indicate what kind of entry it is. Each entry in the symbol table is actually one of the symbol entries described in the following sections. The relationship between a symbol entry, a type definition, and the type name symbol entry is shown in the following figure:

http://www.pjh2.de/fpc/CompilerInternalsFigure06.png

Symbol entry types

Warning: Last updated for FPC version 1.0.x

Base symbol type (TSym)

All entries in the symbol table are derived from this base object which contains information on the symbol type as well as information on the owner of this symbol entry.

type
  PSym = ^TSym;
  TSym = object(TSymTableEntry)
    SymOptions: TSymOptions;  // Indicate the access scope of the symbol
    FileInfo: TFilePosInfo;
    Refs: Longint;            // Indicates how many times this label is refered in the
                              // parsed code (is only used with variable and assembler
                              // label symbols).
    LastRef: PRef;
    DefRef: PRef;
    LastWritten: PRef;
    RefCount: Longint;        // Browser information indicating the reference count
    Typ: TSymTyp;             // Indicates the symbol type
    IsStabWritten: Boolean;   // Set to TRUE if the stabs debugging information has
                              // been written for this symbol.
  end;

TSymTyp	Description
AbstractSym	This is a special abstract symbol (this should never occur)
VarSym	This symbol is a variable declaration in the var section, or a var parameter.
TypeSym	This symbol is a type name
ProcSym	This symbol is a routine or method name
UnitSym	This symbol is a unit name
ProgramSym	This symbol is the main program name
ConstSym	This symbol is a constant
EnumSym	This symbol is an enumeration symbol (an element in an enumeration)
TypedConstSym	This symbol is pre-initialized variable (pascal typed constant)
ErrorSym	This symbol is created for error generation
SysSym	This symbol represents an inlined system unit routine
LabelSym	This symbol represents a label in a label pascal declaration
AbsoluteSym	This symbol represents an the symbol following an absolute variable declaration
PropertySym	This symbol is a property name
FuncRetSym	This symbol is the name of the return value for functions
MacroSym	This symbol is a macro symbol (just like #define in C)

Label symbol (TLabelSym)

The label symbol table entry is only created when a pascal label is declared via the label keyword. The object has the following fields which are available for use publicly:

type
  PLabelSym = ^TLabelSym;
  TLabelSym = object(TSym)
    Used: Boolean;                 // Set to TRUE if this pascal label is used using a goto
                                   // or in an assembler statement
    Defined: Boolean;              // Set to TRUE if this label has been declared
    Lab: PAsmLabel;                // Points to the actual assembler label structure which
                                   // will be emitted by the code generator
    Code: Pointer;
  end;

Unit symbol (TUnitSym)

The unit symbol is created and added to the symbol table each time that the uses clause is parsed and a unit name is found, it is also used when compiling a unit, with the first entry in that symbol table being the unit name being compiled. The unit symbol entry is actual part of a linked list which is used in the unit symbol table.

type
  PUnitSym = ^TUnitSym;
  TUnitSym = object(TSym)
    UnitSymTable: PUnitSymTable;  // Pointer to the global symbol table for that
                                  // unit, containing entries for each public?
                                  // symbol in that unit
    PrevSym: PUnitSym;            // Pointer to previous entry in the linked list
  end;

Macro symbol (TMacroSym)

The macro synbols are used in the preprocessor for conditional compilation statements. There is one such entry created for each $define directive, it contains the value of the define (stored as a string).

type
  PMacroSym = ^TMacroSym;
  TMacroSym = object(TSym)
    Defined: Boolean;              // TRUE if the symbol has been defined with a 
                                   // $define directive, or false if it has been 
                                   // undefined with a $undef directive
    Defined_At_Startup: Boolean;   // TRUE if the symbol is a system wide define
    Is_Used: Boolean;              // TRUE if the define has been used such as in 
                                   // a $ifdef directive.
    BufText: PChar;                // The actual string text of the define
    BufLength: Longint;            // The actual string length of the define
  end;

Error symbol (TErrorSym)

This symbol is actually an empty symbol table entry. When the parser encounters an error when parsing a symbol, instead of putting nothing in the symbol table, it puts this symbol entry. This avoids illegal memory accesses later in parsing.

Procedure symbol (TProcSym)

The procedure symbol is created each time a routine is defined in the code. This can be either a forward definition or the actual implementation of the routine. After creation, the symbol is added into the appropriate symbol table stack.

type
  PProcSym = ^TProcSym;
  TProcSym = object(TSym)
    Is_Global: Boolean;           // Set if the routine is exported by the unit
    Definition: PProcDef;         // Procedure definition, including parameter 
                                  // information and return values
  end;

Type symbol (TTypeSym)

The type symbol is created each time a new type declaration is done, the current symbol table stack is then inserted with this symbol. Furthermore, each time the compiler compiles a module, the default base types are initialized and added into the symbol table (psystem.pas) The type symbol contains the name of a type, as well as a pointer to its type definition.

type
  PTypeSym = ^TTypeSym;
  TTypeSym = object(TSym)
    ResType: TType;               // Contains base type information as well as the type
                                  // definition
  end;

Variable symbol (TVarSym)

Variable declarations, as well as parameters which are passed onto routines are declared as variable symbol types. Access information, as well as type information and optimization information are stored in this symbol type.

type
  PVarSym = ^TVarSym;
  TVarSym = object(TSym)
    Reg: TRegister;              // If the value is a register variable, the reg field will
                                 // be different then R_NO
    VarSpez: TVarSpez;           // Indicates the variable type (parameters only) (Cf. 32).
    Address: Longint;            // In the case where the variable is a routine parameter,
                                 // this indicates the positive offset from theframe_pointer 
                                 // to access this variable. In the caseof a local variable,
                                 // this field indicates the negative offset from the 
                                 // frame_pointer. to access this variable.
    LocalVarSym: PVarSym;
    VarType: TType;              // Contains base type information as well as the type
                                 // definition
    VarOptions: TVarOptions;     // Flags for this variable (Cf. 31)
    VarState: TVarState;         // Indicates the state of the variable, if it’s used or
                                 // declared
  end;

TVarOptions	Description
vo_regable	The variable can be put into a hardware general purpose register
vo_is_c_var	The variable is imported from a C module
vo_is_external	The variable is declared external
vo_is_Dll_var	The variable is a shared library variable
vo_is_thread_var	The variable is declared as being thread safe
vo_fpuregable	The variable can be put into a hardware floating point register
vo_is_local_copy
vo_is_const	unused and useless
vo_is_exported	The variable is declared as exported in a dynamic link library

TVarSpez	Description
vs_value	This is a value parameter
vs_const	This is a constant parameter, property or array
vs_var	This is a variable parameter

Property symbol (TPropertySym)

type
  PPropertySym = ^TPropertySym;
  TPropertySym = object(TSym)
    PropOptions: TPropertyOptions;    // ???
    PropType: TType;                  // Indicates the type of the property
    PropOverriden: PPropertySym;      // ???
    IndexType: TType;
    Index: Longint;                   // ????
    Default: Longint;                 // ???
    ReadAccess: PSymList;             // ???
    WriteAccess: PSymList;            // ???
    StoredAccess: PSymList;           // ???
  end;

Return value of function symbol

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Absolute declared symbol (TAbsoluteSym)

This symbol represents a variable declared with the absolute keyword. The address of the TVarSym object holds the address of the variable in the case of an absolute address variable.

The possible types of absolute symbols, are from an external object reference, an absolute address (for certain targets only), or on top of another declared variable.

type
  PAbsoluteSym = ^TAbsoluteSym;
  TAbsoluteSym = object(TVarSym)
    AbsTyp: TAbsoluteTyp;         // Indicates the type of absolute symbol it is 
    AbsSeg: Boolean;              // ???
    Ref: PSym;                    // In case abstyp is tovar, this field indicates the
                                  // symbol which is overlaid with this symbol. Otherwise
                                  // this field is unused.
    AsmName: PString;             // In case abstyp is toasm, this field indicates label
                                  // name for the variable.
  end;

TAbsoluteTyp	Description
toVar	The symbol will be declared on top of another symbol (variable or typed constant)
toAsm	The variable is imported from an external module
toAddr	The variable is declared as being at an absolute address

Typed constant symbol

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Constant symbol (TConstSym)

This symbol type will contain all constants defined and encountered during the parsing. The values of the constants are also set in this symbol type entry.

type
  PConstSym = ^TConstSym;
  TConstSym = object(TSym)
    ConstType: TType;       // Type information for this constant (?).
    ConstTyp: TConstTyp;    // Indicates the type of the constant
    ResStrIndex: Longint;   // If this is a resource string constant, it indicates the
                            // index in the resource table
    Value: Longint;         // In certain cases, contains the value of the constant
    Len: Longint;
  end;

Enumeration symbol

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Program symbol

The program symbol type (TProgramSym) is used to store the name of the program, which is declared using program in the pascal source. This symbol type is currently unused in FreePascal.

Sys symbol

The TSysSym symbol type is used to load indexes into the symbol table of the internal routines which are inlined directly by the compiler. It has a single field, which is the index of the inline routine.

Symbol interface

Next chapter: Type information