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   This file documents the use and the internals of the GNU compiler.

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File: gcc.info,  Node: Initialization,  Next: Macros for Initialization,  Prev: Label Output,  Up: Assembler Format

How Initialization Functions Are Handled
----------------------------------------

   The compiled code for certain languages includes "constructors"
(also called "initialization routines")--functions to initialize data
in the program when the program is started.  These functions need to be
called before the program is "started"--that is to say, before `main'
is called.

   Compiling some languages generates "destructors" (also called
"termination routines") that should be called when the program
terminates.

   To make the initialization and termination functions work, the
compiler must output something in the assembler code to cause those
functions to be called at the appropriate time.  When you port the
compiler to a new system, you need to specify how to do this.

   There are two major ways that GCC currently supports the execution of
initialization and termination functions.  Each way has two variants.
Much of the structure is common to all four variations.

   The linker must build two lists of these functions--a list of
initialization functions, called `__CTOR_LIST__', and a list of
termination functions, called `__DTOR_LIST__'.

   Each list always begins with an ignored function pointer (which may
hold 0, -1, or a count of the function pointers after it, depending on
the environment).  This is followed by a series of zero or more function
pointers to constructors (or destructors), followed by a function
pointer containing zero.

   Depending on the operating system and its executable file format,
either `crtstuff.c' or `libgcc2.c' traverses these lists at startup
time and exit time.  Constructors are called in reverse order of the
list; destructors in forward order.

   The best way to handle static constructors works only for object file
formats which provide arbitrarily-named sections.  A section is set
aside for a list of constructors, and another for a list of destructors.
Traditionally these are called `.ctors' and `.dtors'.  Each object file
that defines an initialization function also puts a word in the
constructor section to point to that function.  The linker accumulates
all these words into one contiguous `.ctors' section.  Termination
functions are handled similarly.

   To use this method, you need appropriate definitions of the macros
`ASM_OUTPUT_CONSTRUCTOR' and `ASM_OUTPUT_DESTRUCTOR'.  Usually you can
get them by including `svr4.h'.

   When arbitrary sections are available, there are two variants,
depending upon how the code in `crtstuff.c' is called.  On systems that
support an "init" section which is executed at program startup, parts
of `crtstuff.c' are compiled into that section.  The program is linked
by the `gcc' driver like this:

     ld -o OUTPUT_FILE crtbegin.o ... crtend.o -lgcc

   The head of a function (`__do_global_ctors') appears in the init
section of `crtbegin.o'; the remainder of the function appears in the
init section of `crtend.o'.  The linker will pull these two parts of
the section together, making a whole function.  If any of the user's
object files linked into the middle of it contribute code, then that
code will be executed as part of the body of `__do_global_ctors'.

   To use this variant, you must define the `INIT_SECTION_ASM_OP' macro
properly.

   If no init section is available, do not define
`INIT_SECTION_ASM_OP'.  Then `__do_global_ctors' is built into the text
section like all other functions, and resides in `libgcc.a'.  When GCC
compiles any function called `main', it inserts a procedure call to
`__main' as the first executable code after the function prologue.  The
`__main' function, also defined in `libgcc2.c', simply calls
`__do_global_ctors'.

   In file formats that don't support arbitrary sections, there are
again two variants.  In the simplest variant, the GNU linker (GNU `ld')
and an `a.out' format must be used.  In this case,
`ASM_OUTPUT_CONSTRUCTOR' is defined to produce a `.stabs' entry of type
`N_SETT', referencing the name `__CTOR_LIST__', and with the address of
the void function containing the initialization code as its value.  The
GNU linker recognizes this as a request to add the value to a "set";
the values are accumulated, and are eventually placed in the executable
as a vector in the format described above, with a leading (ignored)
count and a trailing zero element.  `ASM_OUTPUT_DESTRUCTOR' is handled
similarly.  Since no init section is available, the absence of
`INIT_SECTION_ASM_OP' causes the compilation of `main' to call `__main'
as above, starting the initialization process.

   The last variant uses neither arbitrary sections nor the GNU linker.
This is preferable when you want to do dynamic linking and when using
file formats which the GNU linker does not support, such as `ECOFF'.  In
this case, `ASM_OUTPUT_CONSTRUCTOR' does not produce an `N_SETT'
symbol; initialization and termination functions are recognized simply
by their names.  This requires an extra program in the linkage step,
called `collect2'.  This program pretends to be the linker, for use
with GNU CC; it does its job by running the ordinary linker, but also
arranges to include the vectors of initialization and termination
functions.  These functions are called via `__main' as described above.

   Choosing among these configuration options has been simplified by a
set of operating-system-dependent files in the `config' subdirectory.
These files define all of the relevant parameters.  Usually it is
sufficient to include one into your specific machine-dependent
configuration file.  These files are:

`aoutos.h'
     For operating systems using the `a.out' format.

`next.h'
     For operating systems using the `MachO' format.

`svr3.h'
     For System V Release 3 and similar systems using `COFF' format.

`svr4.h'
     For System V Release 4 and similar systems using `ELF' format.

`vms.h'
     For the VMS operating system.

   The following section describes the specific macros that control and
customize the handling of initialization and termination functions.


File: gcc.info,  Node: Macros for Initialization,  Next: Instruction Output,  Prev: Initialization,  Up: Assembler Format

Macros Controlling Initialization Routines
------------------------------------------

   Here are the macros that control how the compiler handles
initialization and termination functions:

`INIT_SECTION_ASM_OP'
     If defined, a C string constant for the assembler operation to
     identify the following data as initialization code.  If not
     defined, GNU CC will assume such a section does not exist.  When
     you are using special sections for initialization and termination
     functions, this macro also controls how `crtstuff.c' and
     `libgcc2.c' arrange to run the initialization functions.

`HAS_INIT_SECTION'
     If defined, `main' will not call `__main' as described above.
     This macro should be defined for systems that control the contents
     of the init section on a symbol-by-symbol basis, such as OSF/1,
     and should not be defined explicitly for systems that support
     `INIT_SECTION_ASM_OP'.

`LD_INIT_SWITCH'
     If defined, a C string constant for a switch that tells the linker
     that the following symbol is an initialization routine.

`LD_FINI_SWITCH'
     If defined, a C string constant for a switch that tells the linker
     that the following symbol is a finalization routine.

`INVOKE__main'
     If defined, `main' will call `__main' despite the presence of
     `INIT_SECTION_ASM_OP'.  This macro should be defined for systems
     where the init section is not actually run automatically, but is
     still useful for collecting the lists of constructors and
     destructors.

`ASM_OUTPUT_CONSTRUCTOR (STREAM, NAME)'
     Define this macro as a C statement to output on the stream STREAM
     the assembler code to arrange to call the function named NAME at
     initialization time.

     Assume that NAME is the name of a C function generated
     automatically by the compiler.  This function takes no arguments.
     Use the function `assemble_name' to output the name NAME; this
     performs any system-specific syntactic transformations such as
     adding an underscore.

     If you don't define this macro, nothing special is output to
     arrange to call the function.  This is correct when the function
     will be called in some other manner--for example, by means of the
     `collect2' program, which looks through the symbol table to find
     these functions by their names.

`ASM_OUTPUT_DESTRUCTOR (STREAM, NAME)'
     This is like `ASM_OUTPUT_CONSTRUCTOR' but used for termination
     functions rather than initialization functions.

   If your system uses `collect2' as the means of processing
constructors, then that program normally uses `nm' to scan an object
file for constructor functions to be called.  On certain kinds of
systems, you can define these macros to make `collect2' work faster
(and, in some cases, make it work at all):

`OBJECT_FORMAT_COFF'
     Define this macro if the system uses COFF (Common Object File
     Format) object files, so that `collect2' can assume this format
     and scan object files directly for dynamic constructor/destructor
     functions.

`OBJECT_FORMAT_ROSE'
     Define this macro if the system uses ROSE format object files, so
     that `collect2' can assume this format and scan object files
     directly for dynamic constructor/destructor functions.

     These macros are effective only in a native compiler; `collect2' as
     part of a cross compiler always uses `nm' for the target machine.

`REAL_NM_FILE_NAME'
     Define this macro as a C string constant containing the file name
     to use to execute `nm'.  The default is to search the path
     normally for `nm'.

     If your system supports shared libraries and has a program to list
     the dynamic dependencies of a given library or executable, you can
     define these macros to enable support for running initialization
     and termination functions in shared libraries:

`LDD_SUFFIX'
     Define this macro to a C string constant containing the name of the
     program which lists dynamic dependencies, like `"ldd"' under SunOS
     4.

`PARSE_LDD_OUTPUT (PTR)'
     Define this macro to be C code that extracts filenames from the
     output of the program denoted by `LDD_SUFFIX'.  PTR is a variable
     of type `char *' that points to the beginning of a line of output
     from `LDD_SUFFIX'.  If the line lists a dynamic dependency, the
     code must advance PTR to the beginning of the filename on that
     line.  Otherwise, it must set PTR to `NULL'.


File: gcc.info,  Node: Instruction Output,  Next: Dispatch Tables,  Prev: Macros for Initialization,  Up: Assembler Format

Output of Assembler Instructions
--------------------------------

   This describes assembler instruction output.

`REGISTER_NAMES'
     A C initializer containing the assembler's names for the machine
     registers, each one as a C string constant.  This is what
     translates register numbers in the compiler into assembler
     language.

`ADDITIONAL_REGISTER_NAMES'
     If defined, a C initializer for an array of structures containing
     a name and a register number.  This macro defines additional names
     for hard registers, thus allowing the `asm' option in declarations
     to refer to registers using alternate names.

`ASM_OUTPUT_OPCODE (STREAM, PTR)'
     Define this macro if you are using an unusual assembler that
     requires different names for the machine instructions.

     The definition is a C statement or statements which output an
     assembler instruction opcode to the stdio stream STREAM.  The
     macro-operand PTR is a variable of type `char *' which points to
     the opcode name in its "internal" form--the form that is written
     in the machine description.  The definition should output the
     opcode name to STREAM, performing any translation you desire, and
     increment the variable PTR to point at the end of the opcode so
     that it will not be output twice.

     In fact, your macro definition may process less than the entire
     opcode name, or more than the opcode name; but if you want to
     process text that includes `%'-sequences to substitute operands,
     you must take care of the substitution yourself.  Just be sure to
     increment PTR over whatever text should not be output normally.

     If you need to look at the operand values, they can be found as the
     elements of `recog_operand'.

     If the macro definition does nothing, the instruction is output in
     the usual way.

`FINAL_PRESCAN_INSN (INSN, OPVEC, NOPERANDS)'
     If defined, a C statement to be executed just prior to the output
     of assembler code for INSN, to modify the extracted operands so
     they will be output differently.

     Here the argument OPVEC is the vector containing the operands
     extracted from INSN, and NOPERANDS is the number of elements of
     the vector which contain meaningful data for this insn.  The
     contents of this vector are what will be used to convert the insn
     template into assembler code, so you can change the assembler
     output by changing the contents of the vector.

     This macro is useful when various assembler syntaxes share a single
     file of instruction patterns; by defining this macro differently,
     you can cause a large class of instructions to be output
     differently (such as with rearranged operands).  Naturally,
     variations in assembler syntax affecting individual insn patterns
     ought to be handled by writing conditional output routines in
     those patterns.

     If this macro is not defined, it is equivalent to a null statement.

`FINAL_PRESCAN_LABEL'
     If defined, `FINAL_PRESCAN_INSN' will be called on each
     `CODE_LABEL'.  In that case, OPVEC will be a null pointer and
     NOPERANDS will be zero.

`PRINT_OPERAND (STREAM, X, CODE)'
     A C compound statement to output to stdio stream STREAM the
     assembler syntax for an instruction operand X.  X is an RTL
     expression.

     CODE is a value that can be used to specify one of several ways of
     printing the operand.  It is used when identical operands must be
     printed differently depending on the context.  CODE comes from the
     `%' specification that was used to request printing of the
     operand.  If the specification was just `%DIGIT' then CODE is 0;
     if the specification was `%LTR DIGIT' then CODE is the ASCII code
     for LTR.

     If X is a register, this macro should print the register's name.
     The names can be found in an array `reg_names' whose type is `char
     *[]'.  `reg_names' is initialized from `REGISTER_NAMES'.

     When the machine description has a specification `%PUNCT' (a `%'
     followed by a punctuation character), this macro is called with a
     null pointer for X and the punctuation character for CODE.

`PRINT_OPERAND_PUNCT_VALID_P (CODE)'
     A C expression which evaluates to true if CODE is a valid
     punctuation character for use in the `PRINT_OPERAND' macro.  If
     `PRINT_OPERAND_PUNCT_VALID_P' is not defined, it means that no
     punctuation characters (except for the standard one, `%') are used
     in this way.

`PRINT_OPERAND_ADDRESS (STREAM, X)'
     A C compound statement to output to stdio stream STREAM the
     assembler syntax for an instruction operand that is a memory
     reference whose address is X.  X is an RTL expression.

     On some machines, the syntax for a symbolic address depends on the
     section that the address refers to.  On these machines, define the
     macro `ENCODE_SECTION_INFO' to store the information into the
     `symbol_ref', and then check for it here.  *Note Assembler
     Format::.

`DBR_OUTPUT_SEQEND(FILE)'
     A C statement, to be executed after all slot-filler instructions
     have been output.  If necessary, call `dbr_sequence_length' to
     determine the number of slots filled in a sequence (zero if not
     currently outputting a sequence), to decide how many no-ops to
     output, or whatever.

     Don't define this macro if it has nothing to do, but it is helpful
     in reading assembly output if the extent of the delay sequence is
     made explicit (e.g. with white space).

     Note that output routines for instructions with delay slots must be
     prepared to deal with not being output as part of a sequence (i.e.
     when the scheduling pass is not run, or when no slot fillers could
     be found.)  The variable `final_sequence' is null when not
     processing a sequence, otherwise it contains the `sequence' rtx
     being output.

`REGISTER_PREFIX'
`LOCAL_LABEL_PREFIX'
`USER_LABEL_PREFIX'
`IMMEDIATE_PREFIX'
     If defined, C string expressions to be used for the `%R', `%L',
     `%U', and `%I' options of `asm_fprintf' (see `final.c').  These
     are useful when a single `md' file must support multiple assembler
     formats.  In that case, the various `tm.h' files can define these
     macros differently.

`ASSEMBLER_DIALECT'
     If your target supports multiple dialects of assembler language
     (such as different opcodes), define this macro as a C expression
     that gives the numeric index of the assembler language dialect to
     use, with zero as the first variant.

     If this macro is defined, you may use constructs of the form
     `{option0|option1|option2...}' in the output templates of patterns
     (*note Output Template::.) or in the first argument of
     `asm_fprintf'.  This construct outputs `option0', `option1' or
     `option2', etc., if the value of `ASSEMBLER_DIALECT' is zero, one
     or two, etc.  Any special characters within these strings retain
     their usual meaning.

     If you do not define this macro, the characters `{', `|' and `}'
     do not have any special meaning when used in templates or operands
     to `asm_fprintf'.

     Define the macros `REGISTER_PREFIX', `LOCAL_LABEL_PREFIX',
     `USER_LABEL_PREFIX' and `IMMEDIATE_PREFIX' if you can express the
     variations in assembler language syntax with that mechanism.
     Define `ASSEMBLER_DIALECT' and use the `{option0|option1}' syntax
     if the syntax variant are larger and involve such things as
     different opcodes or operand order.

`ASM_OUTPUT_REG_PUSH (STREAM, REGNO)'
     A C expression to output to STREAM some assembler code which will
     push hard register number REGNO onto the stack.  The code need not
     be optimal, since this macro is used only when profiling.

`ASM_OUTPUT_REG_POP (STREAM, REGNO)'
     A C expression to output to STREAM some assembler code which will
     pop hard register number REGNO off of the stack.  The code need
     not be optimal, since this macro is used only when profiling.


File: gcc.info,  Node: Dispatch Tables,  Next: Exception Region Output,  Prev: Instruction Output,  Up: Assembler Format

Output of Dispatch Tables
-------------------------

   This concerns dispatch tables.

`ASM_OUTPUT_ADDR_DIFF_ELT (STREAM, VALUE, REL)'
     A C statement to output to the stdio stream STREAM an assembler
     pseudo-instruction to generate a difference between two labels.
     VALUE and REL are the numbers of two internal labels.  The
     definitions of these labels are output using
     `ASM_OUTPUT_INTERNAL_LABEL', and they must be printed in the same
     way here.  For example,

          fprintf (STREAM, "\t.word L%d-L%d\n",
                   VALUE, REL)

     You must provide this macro on machines where the addresses in a
     dispatch table are relative to the table's own address.  If
     defined, GNU CC will also use this macro on all machines when
     producing PIC.

`ASM_OUTPUT_ADDR_VEC_ELT (STREAM, VALUE)'
     This macro should be provided on machines where the addresses in a
     dispatch table are absolute.

     The definition should be a C statement to output to the stdio
     stream STREAM an assembler pseudo-instruction to generate a
     reference to a label.  VALUE is the number of an internal label
     whose definition is output using `ASM_OUTPUT_INTERNAL_LABEL'.  For
     example,

          fprintf (STREAM, "\t.word L%d\n", VALUE)

`ASM_OUTPUT_CASE_LABEL (STREAM, PREFIX, NUM, TABLE)'
     Define this if the label before a jump-table needs to be output
     specially.  The first three arguments are the same as for
     `ASM_OUTPUT_INTERNAL_LABEL'; the fourth argument is the jump-table
     which follows (a `jump_insn' containing an `addr_vec' or
     `addr_diff_vec').

     This feature is used on system V to output a `swbeg' statement for
     the table.

     If this macro is not defined, these labels are output with
     `ASM_OUTPUT_INTERNAL_LABEL'.

`ASM_OUTPUT_CASE_END (STREAM, NUM, TABLE)'
     Define this if something special must be output at the end of a
     jump-table.  The definition should be a C statement to be executed
     after the assembler code for the table is written.  It should write
     the appropriate code to stdio stream STREAM.  The argument TABLE
     is the jump-table insn, and NUM is the label-number of the
     preceding label.

     If this macro is not defined, nothing special is output at the end
     of the jump-table.


File: gcc.info,  Node: Exception Region Output,  Next: Alignment Output,  Prev: Dispatch Tables,  Up: Assembler Format

Assembler Commands for Exception Regions
----------------------------------------

   This describes commands marking the start and the end of an exception
region.

`ASM_OUTPUT_EH_REGION_BEG ()'
     A C expression to output text to mark the start of an exception
     region.

     This macro need not be defined on most platforms.

`ASM_OUTPUT_EH_REGION_END ()'
     A C expression to output text to mark the end of an exception
     region.

     This macro need not be defined on most platforms.

`EXCEPTION_SECTION ()'
     A C expression to switch to the section in which the main
     exception table is to be placed (*note Sections::.).  The default
     is a section named `.gcc_except_table' on machines that support
     named sections via `ASM_OUTPUT_SECTION_NAME', otherwise if `-fpic'
     or `-fPIC' is in effect, the `data_section', otherwise the
     `readonly_data_section'.

`EH_FRAME_SECTION_ASM_OP'
     If defined, a C string constant for the assembler operation to
     switch to the section for exception handling frame unwind
     information.  If not defined, GNU CC will provide a default
     definition if the target supports named sections.  `crtstuff.c'
     uses this macro to switch to the appropriate section.

     You should define this symbol if your target supports DWARF 2 frame
     unwind information and the default definition does not work.

`OMIT_EH_TABLE ()'
     A C expression that is nonzero if the normal exception table output
     should be omitted.

     This macro need not be defined on most platforms.

`EH_TABLE_LOOKUP ()'
     Alternate runtime support for looking up an exception at runtime
     and finding the associated handler, if the default method won't
     work.

     This macro need not be defined on most platforms.

`DOESNT_NEED_UNWINDER'
     A C expression that decides whether or not the current function
     needs to have a function unwinder generated for it.  See the file
     `except.c' for details on when to define this, and how.

`MASK_RETURN_ADDR'
     An rtx used to mask the return address found via RETURN_ADDR_RTX,
     so that it does not contain any extraneous set bits in it.

`DWARF2_UNWIND_INFO'
     Define this macro to 0 if your target supports DWARF 2 frame unwind
     information, but it does not yet work with exception handling.
     Otherwise, if your target supports this information (if it defines
     `INCOMING_RETURN_ADDR_RTX' and either `UNALIGNED_INT_ASM_OP' or
     `OBJECT_FORMAT_ELF'), GCC will provide a default definition of 1.

     If this macro is defined to 1, the DWARF 2 unwinder will be the
     default exception handling mechanism; otherwise, setjmp/longjmp
     will be used by default.

     If this macro is defined to anything, the DWARF 2 unwinder will be
     used instead of inline unwinders and __unwind_function in the
     non-setjmp case.


File: gcc.info,  Node: Alignment Output,  Prev: Exception Region Output,  Up: Assembler Format

Assembler Commands for Alignment
--------------------------------

   This describes commands for alignment.

`ASM_OUTPUT_ALIGN_CODE (FILE)'
     A C expression to output text to align the location counter in the
     way that is desirable at a point in the code that is reached only
     by jumping.

     This macro need not be defined if you don't want any special
     alignment to be done at such a time.  Most machine descriptions do
     not currently define the macro.

`ASM_OUTPUT_LOOP_ALIGN (FILE)'
     A C expression to output text to align the location counter in the
     way that is desirable at the beginning of a loop.

     This macro need not be defined if you don't want any special
     alignment to be done at such a time.  Most machine descriptions do
     not currently define the macro.

`ASM_OUTPUT_SKIP (STREAM, NBYTES)'
     A C statement to output to the stdio stream STREAM an assembler
     instruction to advance the location counter by NBYTES bytes.
     Those bytes should be zero when loaded.  NBYTES will be a C
     expression of type `int'.

`ASM_NO_SKIP_IN_TEXT'
     Define this macro if `ASM_OUTPUT_SKIP' should not be used in the
     text section because it fails put zeros in the bytes that are
     skipped.  This is true on many Unix systems, where the pseudo-op
     to skip bytes produces no-op instructions rather than zeros when
     used in the text section.

`ASM_OUTPUT_ALIGN (STREAM, POWER)'
     A C statement to output to the stdio stream STREAM an assembler
     command to advance the location counter to a multiple of 2 to the
     POWER bytes.  POWER will be a C expression of type `int'.


File: gcc.info,  Node: Debugging Info,  Next: Cross-compilation,  Prev: Assembler Format,  Up: Target Macros

Controlling Debugging Information Format
========================================

   This describes how to specify debugging information.

* Menu:

* All Debuggers::      Macros that affect all debugging formats uniformly.
* DBX Options::        Macros enabling specific options in DBX format.
* DBX Hooks::          Hook macros for varying DBX format.
* File Names and DBX:: Macros controlling output of file names in DBX format.
* SDB and DWARF::      Macros for SDB (COFF) and DWARF formats.


File: gcc.info,  Node: All Debuggers,  Next: DBX Options,  Up: Debugging Info

Macros Affecting All Debugging Formats
--------------------------------------

   These macros affect all debugging formats.

`DBX_REGISTER_NUMBER (REGNO)'
     A C expression that returns the DBX register number for the
     compiler register number REGNO.  In simple cases, the value of this
     expression may be REGNO itself.  But sometimes there are some
     registers that the compiler knows about and DBX does not, or vice
     versa.  In such cases, some register may need to have one number in
     the compiler and another for DBX.

     If two registers have consecutive numbers inside GNU CC, and they
     can be used as a pair to hold a multiword value, then they *must*
     have consecutive numbers after renumbering with
     `DBX_REGISTER_NUMBER'.  Otherwise, debuggers will be unable to
     access such a pair, because they expect register pairs to be
     consecutive in their own numbering scheme.

     If you find yourself defining `DBX_REGISTER_NUMBER' in way that
     does not preserve register pairs, then what you must do instead is
     redefine the actual register numbering scheme.

`DEBUGGER_AUTO_OFFSET (X)'
     A C expression that returns the integer offset value for an
     automatic variable having address X (an RTL expression).  The
     default computation assumes that X is based on the frame-pointer
     and gives the offset from the frame-pointer.  This is required for
     targets that produce debugging output for DBX or COFF-style
     debugging output for SDB and allow the frame-pointer to be
     eliminated when the `-g' options is used.

`DEBUGGER_ARG_OFFSET (OFFSET, X)'
     A C expression that returns the integer offset value for an
     argument having address X (an RTL expression).  The nominal offset
     is OFFSET.

`PREFERRED_DEBUGGING_TYPE'
     A C expression that returns the type of debugging output GNU CC
     should produce when the user specifies just `-g'.  Define this if
     you have arranged for GNU CC to support more than one format of
     debugging output.  Currently, the allowable values are `DBX_DEBUG',
     `SDB_DEBUG', `DWARF_DEBUG', `DWARF2_DEBUG', and `XCOFF_DEBUG'.

     When the user specifies `-ggdb', GNU CC normally also uses the
     value of this macro to select the debugging output format, but
     with two exceptions.  If `DWARF2_DEBUGGING_INFO' is defined and
     `LINKER_DOES_NOT_WORK_WITH_DWARF2' is not defined, GNU CC uses the
     value `DWARF2_DEBUG'.  Otherwise, if `DBX_DEBUGGING_INFO' is
     defined, GNU CC uses `DBX_DEBUG'.

     The value of this macro only affects the default debugging output;
     the user can always get a specific type of output by using
     `-gstabs', `-gcoff', `-gdwarf-1', `-gdwarf-2', or `-gxcoff'.


File: gcc.info,  Node: DBX Options,  Next: DBX Hooks,  Prev: All Debuggers,  Up: Debugging Info

Specific Options for DBX Output
-------------------------------

   These are specific options for DBX output.

`DBX_DEBUGGING_INFO'
     Define this macro if GNU CC should produce debugging output for DBX
     in response to the `-g' option.

`XCOFF_DEBUGGING_INFO'
     Define this macro if GNU CC should produce XCOFF format debugging
     output in response to the `-g' option.  This is a variant of DBX
     format.

`DEFAULT_GDB_EXTENSIONS'
     Define this macro to control whether GNU CC should by default
     generate GDB's extended version of DBX debugging information
     (assuming DBX-format debugging information is enabled at all).  If
     you don't define the macro, the default is 1: always generate the
     extended information if there is any occasion to.

`DEBUG_SYMS_TEXT'
     Define this macro if all `.stabs' commands should be output while
     in the text section.

`ASM_STABS_OP'
     A C string constant naming the assembler pseudo op to use instead
     of `.stabs' to define an ordinary debugging symbol.  If you don't
     define this macro, `.stabs' is used.  This macro applies only to
     DBX debugging information format.

`ASM_STABD_OP'
     A C string constant naming the assembler pseudo op to use instead
     of `.stabd' to define a debugging symbol whose value is the current
     location.  If you don't define this macro, `.stabd' is used.  This
     macro applies only to DBX debugging information format.

`ASM_STABN_OP'
     A C string constant naming the assembler pseudo op to use instead
     of `.stabn' to define a debugging symbol with no name.  If you
     don't define this macro, `.stabn' is used.  This macro applies
     only to DBX debugging information format.

`DBX_NO_XREFS'
     Define this macro if DBX on your system does not support the
     construct `xsTAGNAME'.  On some systems, this construct is used to
     describe a forward reference to a structure named TAGNAME.  On
     other systems, this construct is not supported at all.

`DBX_CONTIN_LENGTH'
     A symbol name in DBX-format debugging information is normally
     continued (split into two separate `.stabs' directives) when it
     exceeds a certain length (by default, 80 characters).  On some
     operating systems, DBX requires this splitting; on others,
     splitting must not be done.  You can inhibit splitting by defining
     this macro with the value zero.  You can override the default
     splitting-length by defining this macro as an expression for the
     length you desire.

`DBX_CONTIN_CHAR'
     Normally continuation is indicated by adding a `\' character to
     the end of a `.stabs' string when a continuation follows.  To use
     a different character instead, define this macro as a character
     constant for the character you want to use.  Do not define this
     macro if backslash is correct for your system.

`DBX_STATIC_STAB_DATA_SECTION'
     Define this macro if it is necessary to go to the data section
     before outputting the `.stabs' pseudo-op for a non-global static
     variable.

`DBX_TYPE_DECL_STABS_CODE'
     The value to use in the "code" field of the `.stabs' directive for
     a typedef.  The default is `N_LSYM'.

`DBX_STATIC_CONST_VAR_CODE'
     The value to use in the "code" field of the `.stabs' directive for
     a static variable located in the text section.  DBX format does not
     provide any "right" way to do this.  The default is `N_FUN'.

`DBX_REGPARM_STABS_CODE'
     The value to use in the "code" field of the `.stabs' directive for
     a parameter passed in registers.  DBX format does not provide any
     "right" way to do this.  The default is `N_RSYM'.

`DBX_REGPARM_STABS_LETTER'
     The letter to use in DBX symbol data to identify a symbol as a
     parameter passed in registers.  DBX format does not customarily
     provide any way to do this.  The default is `'P''.

`DBX_MEMPARM_STABS_LETTER'
     The letter to use in DBX symbol data to identify a symbol as a
     stack parameter.  The default is `'p''.

`DBX_FUNCTION_FIRST'
     Define this macro if the DBX information for a function and its
     arguments should precede the assembler code for the function.
     Normally, in DBX format, the debugging information entirely
     follows the assembler code.

`DBX_LBRAC_FIRST'
     Define this macro if the `N_LBRAC' symbol for a block should
     precede the debugging information for variables and functions
     defined in that block.  Normally, in DBX format, the `N_LBRAC'
     symbol comes first.

`DBX_BLOCKS_FUNCTION_RELATIVE'
     Define this macro if the value of a symbol describing the scope of
     a block (`N_LBRAC' or `N_RBRAC') should be relative to the start
     of the enclosing function.  Normally, GNU C uses an absolute
     address.

`DBX_USE_BINCL'
     Define this macro if GNU C should generate `N_BINCL' and `N_EINCL'
     stabs for included header files, as on Sun systems.  This macro
     also directs GNU C to output a type number as a pair of a file
     number and a type number within the file.  Normally, GNU C does not
     generate `N_BINCL' or `N_EINCL' stabs, and it outputs a single
     number for a type number.


File: gcc.info,  Node: DBX Hooks,  Next: File Names and DBX,  Prev: DBX Options,  Up: Debugging Info

Open-Ended Hooks for DBX Format
-------------------------------

   These are hooks for DBX format.

`DBX_OUTPUT_LBRAC (STREAM, NAME)'
     Define this macro to say how to output to STREAM the debugging
     information for the start of a scope level for variable names.  The
     argument NAME is the name of an assembler symbol (for use with
     `assemble_name') whose value is the address where the scope begins.

`DBX_OUTPUT_RBRAC (STREAM, NAME)'
     Like `DBX_OUTPUT_LBRAC', but for the end of a scope level.

`DBX_OUTPUT_ENUM (STREAM, TYPE)'
     Define this macro if the target machine requires special handling
     to output an enumeration type.  The definition should be a C
     statement (sans semicolon) to output the appropriate information
     to STREAM for the type TYPE.

`DBX_OUTPUT_FUNCTION_END (STREAM, FUNCTION)'
     Define this macro if the target machine requires special output at
     the end of the debugging information for a function.  The
     definition should be a C statement (sans semicolon) to output the
     appropriate information to STREAM.  FUNCTION is the
     `FUNCTION_DECL' node for the function.

`DBX_OUTPUT_STANDARD_TYPES (SYMS)'
     Define this macro if you need to control the order of output of the
     standard data types at the beginning of compilation.  The argument
     SYMS is a `tree' which is a chain of all the predefined global
     symbols, including names of data types.

     Normally, DBX output starts with definitions of the types for
     integers and characters, followed by all the other predefined
     types of the particular language in no particular order.

     On some machines, it is necessary to output different particular
     types first.  To do this, define `DBX_OUTPUT_STANDARD_TYPES' to
     output those symbols in the necessary order.  Any predefined types
     that you don't explicitly output will be output afterward in no
     particular order.

     Be careful not to define this macro so that it works only for C.
     There are no global variables to access most of the built-in
     types, because another language may have another set of types.
     The way to output a particular type is to look through SYMS to see
     if you can find it.  Here is an example:

          {
            tree decl;
            for (decl = syms; decl; decl = TREE_CHAIN (decl))
              if (!strcmp (IDENTIFIER_POINTER (DECL_NAME (decl)),
                           "long int"))
                dbxout_symbol (decl);
            ...
          }

     This does nothing if the expected type does not exist.

     See the function `init_decl_processing' in `c-decl.c' to find the
     names to use for all the built-in C types.

     Here is another way of finding a particular type:

          {
            tree decl;
            for (decl = syms; decl; decl = TREE_CHAIN (decl))
              if (TREE_CODE (decl) == TYPE_DECL
                  && (TREE_CODE (TREE_TYPE (decl))
                      == INTEGER_CST)
                  && TYPE_PRECISION (TREE_TYPE (decl)) == 16
                  && TYPE_UNSIGNED (TREE_TYPE (decl)))
          /* This must be `unsigned short'.  */
                dbxout_symbol (decl);
            ...
          }

`NO_DBX_FUNCTION_END'
     Some stabs encapsulation formats (in particular ECOFF), cannot
     handle the `.stabs "",N_FUN,,0,0,Lscope-function-1' gdb dbx
     extention construct.  On those machines, define this macro to turn
     this feature off without disturbing the rest of the gdb extensions.


File: gcc.info,  Node: File Names and DBX,  Next: SDB and DWARF,  Prev: DBX Hooks,  Up: Debugging Info

File Names in DBX Format
------------------------

   This describes file names in DBX format.

`DBX_WORKING_DIRECTORY'
     Define this if DBX wants to have the current directory recorded in
     each object file.

     Note that the working directory is always recorded if GDB
     extensions are enabled.

`DBX_OUTPUT_MAIN_SOURCE_FILENAME (STREAM, NAME)'
     A C statement to output DBX debugging information to the stdio
     stream STREAM which indicates that file NAME is the main source
     file--the file specified as the input file for compilation.  This
     macro is called only once, at the beginning of compilation.

     This macro need not be defined if the standard form of output for
     DBX debugging information is appropriate.

`DBX_OUTPUT_MAIN_SOURCE_DIRECTORY (STREAM, NAME)'
     A C statement to output DBX debugging information to the stdio
     stream STREAM which indicates that the current directory during
     compilation is named NAME.

     This macro need not be defined if the standard form of output for
     DBX debugging information is appropriate.

`DBX_OUTPUT_MAIN_SOURCE_FILE_END (STREAM, NAME)'
     A C statement to output DBX debugging information at the end of
     compilation of the main source file NAME.

     If you don't define this macro, nothing special is output at the
     end of compilation, which is correct for most machines.

`DBX_OUTPUT_SOURCE_FILENAME (STREAM, NAME)'
     A C statement to output DBX debugging information to the stdio
     stream STREAM which indicates that file NAME is the current source
     file.  This output is generated each time input shifts to a
     different source file as a result of `#include', the end of an
     included file, or a `#line' command.

     This macro need not be defined if the standard form of output for
     DBX debugging information is appropriate.


File: gcc.info,  Node: SDB and DWARF,  Prev: File Names and DBX,  Up: Debugging Info

Macros for SDB and DWARF Output
-------------------------------

   Here are macros for SDB and DWARF output.

`SDB_DEBUGGING_INFO'
     Define this macro if GNU CC should produce COFF-style debugging
     output for SDB in response to the `-g' option.

`DWARF_DEBUGGING_INFO'
     Define this macro if GNU CC should produce dwarf format debugging
     output in response to the `-g' option.

`DWARF2_DEBUGGING_INFO'
     Define this macro if GNU CC should produce dwarf version 2 format
     debugging output in response to the `-g' option.

     To support optional call frame debugging information, you must also
     define `INCOMING_RETURN_ADDR_RTX' and either set
     `RTX_FRAME_RELATED_P' on the prologue insns if you use RTL for the
     prologue, or call `dwarf2out_def_cfa' and `dwarf2out_reg_save' as
     appropriate from `FUNCTION_PROLOGUE' if you don't.

`LINKER_DOES_NOT_WORK_WITH_DWARF2'
     Define this macro if the linker does not work with Dwarf version 2.
     Normally, if the user specifies only `-ggdb' GNU CC will use Dwarf
     version 2 if available; this macro disables this.  See the
     description of the `PREFERRED_DEBUGGING_TYPE' macro for more
     details.

`PUT_SDB_...'
     Define these macros to override the assembler syntax for the
     special SDB assembler directives.  See `sdbout.c' for a list of
     these macros and their arguments.  If the standard syntax is used,
     you need not define them yourself.

`SDB_DELIM'
     Some assemblers do not support a semicolon as a delimiter, even
     between SDB assembler directives.  In that case, define this macro
     to be the delimiter to use (usually `\n').  It is not necessary to
     define a new set of `PUT_SDB_OP' macros if this is the only change
     required.

`SDB_GENERATE_FAKE'
     Define this macro to override the usual method of constructing a
     dummy name for anonymous structure and union types.  See
     `sdbout.c' for more information.

`SDB_ALLOW_UNKNOWN_REFERENCES'
     Define this macro to allow references to unknown structure, union,
     or enumeration tags to be emitted.  Standard COFF does not allow
     handling of unknown references, MIPS ECOFF has support for it.

`SDB_ALLOW_FORWARD_REFERENCES'
     Define this macro to allow references to structure, union, or
     enumeration tags that have not yet been seen to be handled.  Some
     assemblers choke if forward tags are used, while some require it.


File: gcc.info,  Node: Cross-compilation,  Next: Misc,  Prev: Debugging Info,  Up: Target Macros

Cross Compilation and Floating Point
====================================

   While all modern machines use 2's complement representation for
integers, there are a variety of representations for floating point
numbers.  This means that in a cross-compiler the representation of
floating point numbers in the compiled program may be different from
that used in the machine doing the compilation.

   Because different representation systems may offer different amounts
of range and precision, the cross compiler cannot safely use the host
machine's floating point arithmetic.  Therefore, floating point
constants must be represented in the target machine's format.  This
means that the cross compiler cannot use `atof' to parse a floating
point constant; it must have its own special routine to use instead.
Also, constant folding must emulate the target machine's arithmetic (or
must not be done at all).

   The macros in the following table should be defined only if you are
cross compiling between different floating point formats.

   Otherwise, don't define them.  Then default definitions will be set
up which use `double' as the data type, `==' to test for equality, etc.

   You don't need to worry about how many times you use an operand of
any of these macros.  The compiler never uses operands which have side
effects.

`REAL_VALUE_TYPE'
     A macro for the C data type to be used to hold a floating point
     value in the target machine's format.  Typically this would be a
     `struct' containing an array of `int'.

`REAL_VALUES_EQUAL (X, Y)'
     A macro for a C expression which compares for equality the two
     values, X and Y, both of type `REAL_VALUE_TYPE'.

`REAL_VALUES_LESS (X, Y)'
     A macro for a C expression which tests whether X is less than Y,
     both values being of type `REAL_VALUE_TYPE' and interpreted as
     floating point numbers in the target machine's representation.

`REAL_VALUE_LDEXP (X, SCALE)'
     A macro for a C expression which performs the standard library
     function `ldexp', but using the target machine's floating point
     representation.  Both X and the value of the expression have type
     `REAL_VALUE_TYPE'.  The second argument, SCALE, is an integer.

`REAL_VALUE_FIX (X)'
     A macro whose definition is a C expression to convert the
     target-machine floating point value X to a signed integer.  X has
     type `REAL_VALUE_TYPE'.

`REAL_VALUE_UNSIGNED_FIX (X)'
     A macro whose definition is a C expression to convert the
     target-machine floating point value X to an unsigned integer.  X
     has type `REAL_VALUE_TYPE'.

`REAL_VALUE_RNDZINT (X)'
     A macro whose definition is a C expression to round the
     target-machine floating point value X towards zero to an integer
     value (but still as a floating point number).  X has type
     `REAL_VALUE_TYPE', and so does the value.

`REAL_VALUE_UNSIGNED_RNDZINT (X)'
     A macro whose definition is a C expression to round the
     target-machine floating point value X towards zero to an unsigned
     integer value (but still represented as a floating point number).
     X has type `REAL_VALUE_TYPE', and so does the value.

`REAL_VALUE_ATOF (STRING, MODE)'
     A macro for a C expression which converts STRING, an expression of
     type `char *', into a floating point number in the target machine's
     representation for mode MODE.  The value has type
     `REAL_VALUE_TYPE'.

`REAL_INFINITY'
     Define this macro if infinity is a possible floating point value,
     and therefore division by 0 is legitimate.

`REAL_VALUE_ISINF (X)'
     A macro for a C expression which determines whether X, a floating
     point value, is infinity.  The value has type `int'.  By default,
     this is defined to call `isinf'.

`REAL_VALUE_ISNAN (X)'
     A macro for a C expression which determines whether X, a floating
     point value, is a "nan" (not-a-number).  The value has type `int'.
     By default, this is defined to call `isnan'.

   Define the following additional macros if you want to make floating
point constant folding work while cross compiling.  If you don't define
them, cross compilation is still possible, but constant folding will
not happen for floating point values.

`REAL_ARITHMETIC (OUTPUT, CODE, X, Y)'
     A macro for a C statement which calculates an arithmetic operation
     of the two floating point values X and Y, both of type
     `REAL_VALUE_TYPE' in the target machine's representation, to
     produce a result of the same type and representation which is
     stored in OUTPUT (which will be a variable).

     The operation to be performed is specified by CODE, a tree code
     which will always be one of the following: `PLUS_EXPR',
     `MINUS_EXPR', `MULT_EXPR', `RDIV_EXPR', `MAX_EXPR', `MIN_EXPR'.

     The expansion of this macro is responsible for checking for
     overflow.  If overflow happens, the macro expansion should execute
     the statement `return 0;', which indicates the inability to
     perform the arithmetic operation requested.

`REAL_VALUE_NEGATE (X)'
     A macro for a C expression which returns the negative of the
     floating point value X.  Both X and the value of the expression
     have type `REAL_VALUE_TYPE' and are in the target machine's
     floating point representation.

     There is no way for this macro to report overflow, since overflow
     can't happen in the negation operation.

`REAL_VALUE_TRUNCATE (MODE, X)'
     A macro for a C expression which converts the floating point value
     X to mode MODE.

     Both X and the value of the expression are in the target machine's
     floating point representation and have type `REAL_VALUE_TYPE'.
     However, the value should have an appropriate bit pattern to be
     output properly as a floating constant whose precision accords
     with mode MODE.

     There is no way for this macro to report overflow.

`REAL_VALUE_TO_INT (LOW, HIGH, X)'
     A macro for a C expression which converts a floating point value X
     into a double-precision integer which is then stored into LOW and
     HIGH, two variables of type INT.

`REAL_VALUE_FROM_INT (X, LOW, HIGH, MODE)'
     A macro for a C expression which converts a double-precision
     integer found in LOW and HIGH, two variables of type INT, into a
     floating point value which is then stored into X.  The value is in
     the target machine's representation for mode MODE and has the type
     `REAL_VALUE_TYPE'.