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* gccint: (gccint).            Internals of the GNU Compiler Collection.
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   This file documents the internals of the GNU compilers.


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File: gccint.info,  Node: Label Output,  Next: Initialization,  Prev: Uninitialized Data,  Up: Assembler Format

Output and Generation of Labels
-------------------------------

   This is about outputting labels.

`ASM_OUTPUT_LABEL (STREAM, NAME)'
     A C statement (sans semicolon) to output to the stdio stream
     STREAM the assembler definition of a label named NAME.  Use the
     expression `assemble_name (STREAM, NAME)' to output the name
     itself; before and after that, output the additional assembler
     syntax for defining the name, and a newline.

`ASM_DECLARE_FUNCTION_NAME (STREAM, NAME, DECL)'
     A C statement (sans semicolon) to output to the stdio stream
     STREAM any text necessary for declaring the name NAME of a
     function which is being defined.  This macro is responsible for
     outputting the label definition (perhaps using
     `ASM_OUTPUT_LABEL').  The argument DECL is the `FUNCTION_DECL'
     tree node representing the function.

     If this macro is not defined, then the function name is defined in
     the usual manner as a label (by means of `ASM_OUTPUT_LABEL').

`ASM_DECLARE_FUNCTION_SIZE (STREAM, NAME, DECL)'
     A C statement (sans semicolon) to output to the stdio stream
     STREAM any text necessary for declaring the size of a function
     which is being defined.  The argument NAME is the name of the
     function.  The argument DECL is the `FUNCTION_DECL' tree node
     representing the function.

     If this macro is not defined, then the function size is not
     defined.

`ASM_DECLARE_OBJECT_NAME (STREAM, NAME, DECL)'
     A C statement (sans semicolon) to output to the stdio stream
     STREAM any text necessary for declaring the name NAME of an
     initialized variable which is being defined.  This macro must
     output the label definition (perhaps using `ASM_OUTPUT_LABEL').
     The argument DECL is the `VAR_DECL' tree node representing the
     variable.

     If this macro is not defined, then the variable name is defined in
     the usual manner as a label (by means of `ASM_OUTPUT_LABEL').

`ASM_DECLARE_REGISTER_GLOBAL (STREAM, DECL, REGNO, NAME)'
     A C statement (sans semicolon) to output to the stdio stream
     STREAM any text necessary for claiming a register REGNO for a
     global variable DECL with name NAME.

     If you don't define this macro, that is equivalent to defining it
     to do nothing.

`ASM_FINISH_DECLARE_OBJECT (STREAM, DECL, TOPLEVEL, ATEND)'
     A C statement (sans semicolon) to finish up declaring a variable
     name once the compiler has processed its initializer fully and
     thus has had a chance to determine the size of an array when
     controlled by an initializer.  This is used on systems where it's
     necessary to declare something about the size of the object.

     If you don't define this macro, that is equivalent to defining it
     to do nothing.

`ASM_GLOBALIZE_LABEL (STREAM, NAME)'
     A C statement (sans semicolon) to output to the stdio stream
     STREAM some commands that will make the label NAME global; that
     is, available for reference from other files.  Use the expression
     `assemble_name (STREAM, NAME)' to output the name itself; before
     and after that, output the additional assembler syntax for making
     that name global, and a newline.

`ASM_WEAKEN_LABEL (STREAM, NAME)'
     A C statement (sans semicolon) to output to the stdio stream
     STREAM some commands that will make the label NAME weak; that is,
     available for reference from other files but only used if no other
     definition is available.  Use the expression `assemble_name
     (STREAM, NAME)' to output the name itself; before and after that,
     output the additional assembler syntax for making that name weak,
     and a newline.

     If you don't define this macro or `ASM_WEAKEN_DECL', GCC will not
     support weak symbols and you should not define the `SUPPORTS_WEAK'
     macro.

`ASM_WEAKEN_DECL (STREAM, DECL, NAME, VALUE)'
     Combines (and replaces) the function of `ASM_WEAKEN_LABEL' and
     `ASM_OUTPUT_WEAK_ALIAS', allowing access to the associated function
     or variable decl.  If VALUE is not `NULL', this C statement should
     output to the stdio stream STREAM assembler code which defines
     (equates) the weak symbol NAME to have the value VALUE.  If VALUE
     is `NULL', it should output commands to make NAME weak.

`SUPPORTS_WEAK'
     A C expression which evaluates to true if the target supports weak
     symbols.

     If you don't define this macro, `defaults.h' provides a default
     definition.  If either `ASM_WEAKEN_LABEL' or `ASM_WEAKEN_DECL' is
     defined, the default definition is `1'; otherwise, it is `0'.
     Define this macro if you want to control weak symbol support with
     a compiler flag such as `-melf'.

`MAKE_DECL_ONE_ONLY'
     A C statement (sans semicolon) to mark DECL to be emitted as a
     public symbol such that extra copies in multiple translation units
     will be discarded by the linker.  Define this macro if your object
     file format provides support for this concept, such as the `COMDAT'
     section flags in the Microsoft Windows PE/COFF format, and this
     support requires changes to DECL, such as putting it in a separate
     section.

`SUPPORTS_ONE_ONLY'
     A C expression which evaluates to true if the target supports
     one-only semantics.

     If you don't define this macro, `varasm.c' provides a default
     definition.  If `MAKE_DECL_ONE_ONLY' is defined, the default
     definition is `1'; otherwise, it is `0'.  Define this macro if you
     want to control one-only symbol support with a compiler flag, or if
     setting the `DECL_ONE_ONLY' flag is enough to mark a declaration to
     be emitted as one-only.

`ASM_OUTPUT_EXTERNAL (STREAM, DECL, NAME)'
     A C statement (sans semicolon) to output to the stdio stream
     STREAM any text necessary for declaring the name of an external
     symbol named NAME which is referenced in this compilation but not
     defined.  The value of DECL is the tree node for the declaration.

     This macro need not be defined if it does not need to output
     anything.  The GNU assembler and most Unix assemblers don't
     require anything.

`ASM_OUTPUT_EXTERNAL_LIBCALL (STREAM, SYMREF)'
     A C statement (sans semicolon) to output on STREAM an assembler
     pseudo-op to declare a library function name external.  The name
     of the library function is given by SYMREF, which has type `rtx'
     and is a `symbol_ref'.

     This macro need not be defined if it does not need to output
     anything.  The GNU assembler and most Unix assemblers don't
     require anything.

`ASM_OUTPUT_LABELREF (STREAM, NAME)'
     A C statement (sans semicolon) to output to the stdio stream
     STREAM a reference in assembler syntax to a label named NAME.
     This should add `_' to the front of the name, if that is customary
     on your operating system, as it is in most Berkeley Unix systems.
     This macro is used in `assemble_name'.

`ASM_OUTPUT_SYMBOL_REF (STREAM, SYM)'
     A C statement (sans semicolon) to output a reference to
     `SYMBOL_REF' SYM.  If not defined, `assemble_name' will be used to
     output the name of the symbol.  This macro may be used to modify
     the way a symbol is referenced depending on information encoded by
     `ENCODE_SECTION_INFO'.

`ASM_OUTPUT_LABEL_REF (STREAM, BUF)'
     A C statement (sans semicolon) to output a reference to BUF, the
     result of ASM_GENERATE_INTERNAL_LABEL.  If not defined,
     `assemble_name' will be used to output the name of the symbol.
     This macro is not used by `output_asm_label', or the `%l'
     specifier that calls it; the intention is that this macro should
     be set when it is necessary to output a label differently when its
     address is being taken.

`ASM_OUTPUT_INTERNAL_LABEL (STREAM, PREFIX, NUM)'
     A C statement to output to the stdio stream STREAM a label whose
     name is made from the string PREFIX and the number NUM.

     It is absolutely essential that these labels be distinct from the
     labels used for user-level functions and variables.  Otherwise,
     certain programs will have name conflicts with internal labels.

     It is desirable to exclude internal labels from the symbol table
     of the object file.  Most assemblers have a naming convention for
     labels that should be excluded; on many systems, the letter `L' at
     the beginning of a label has this effect.  You should find out what
     convention your system uses, and follow it.

     The usual definition of this macro is as follows:

          fprintf (STREAM, "L%s%d:\n", PREFIX, NUM)

`ASM_OUTPUT_DEBUG_LABEL (STREAM, PREFIX, NUM)'
     A C statement to output to the stdio stream STREAM a debug info
     label whose name is made from the string PREFIX and the number
     NUM.  This is useful for VLIW targets, where debug info labels may
     need to be treated differently than branch target labels.  On some
     systems, branch target labels must be at the beginning of
     instruction bundles, but debug info labels can occur in the middle
     of instruction bundles.

     If this macro is not defined, then `ASM_OUTPUT_INTERNAL_LABEL'
     will be used.

`ASM_OUTPUT_ALTERNATE_LABEL_NAME (STREAM, STRING)'
     A C statement to output to the stdio stream STREAM the string
     STRING.

     The default definition of this macro is as follows:

          fprintf (STREAM, "%s:\n", LABEL_ALTERNATE_NAME (INSN))

`ASM_GENERATE_INTERNAL_LABEL (STRING, PREFIX, NUM)'
     A C statement to store into the string STRING a label whose name
     is made from the string PREFIX and the number NUM.

     This string, when output subsequently by `assemble_name', should
     produce the output that `ASM_OUTPUT_INTERNAL_LABEL' would produce
     with the same PREFIX and NUM.

     If the string begins with `*', then `assemble_name' will output
     the rest of the string unchanged.  It is often convenient for
     `ASM_GENERATE_INTERNAL_LABEL' to use `*' in this way.  If the
     string doesn't start with `*', then `ASM_OUTPUT_LABELREF' gets to
     output the string, and may change it.  (Of course,
     `ASM_OUTPUT_LABELREF' is also part of your machine description, so
     you should know what it does on your machine.)

`ASM_FORMAT_PRIVATE_NAME (OUTVAR, NAME, NUMBER)'
     A C expression to assign to OUTVAR (which is a variable of type
     `char *') a newly allocated string made from the string NAME and
     the number NUMBER, with some suitable punctuation added.  Use
     `alloca' to get space for the string.

     The string will be used as an argument to `ASM_OUTPUT_LABELREF' to
     produce an assembler label for an internal static variable whose
     name is NAME.  Therefore, the string must be such as to result in
     valid assembler code.  The argument NUMBER is different each time
     this macro is executed; it prevents conflicts between
     similarly-named internal static variables in different scopes.

     Ideally this string should not be a valid C identifier, to prevent
     any conflict with the user's own symbols.  Most assemblers allow
     periods or percent signs in assembler symbols; putting at least
     one of these between the name and the number will suffice.

`ASM_OUTPUT_DEF (STREAM, NAME, VALUE)'
     A C statement to output to the stdio stream STREAM assembler code
     which defines (equates) the symbol NAME to have the value VALUE.

     If `SET_ASM_OP' is defined, a default definition is provided which
     is correct for most systems.

`ASM_OUTPUT_DEF_FROM_DECLS (STREAM, DECL_OF_NAME, DECL_OF_VALUE)'
     A C statement to output to the stdio stream STREAM assembler code
     which defines (equates) the symbol whose tree node is DECL_OF_NAME
     to have the value of the tree node DECL_OF_VALUE.  This macro will
     be used in preference to `ASM_OUTPUT_DEF' if it is defined and if
     the tree nodes are available.

`ASM_OUTPUT_DEFINE_LABEL_DIFFERENCE_SYMBOL (STREAM, SYMBOL, HIGH, LOW)'
     A C statement to output to the stdio stream STREAM assembler code
     which defines (equates) the symbol SYMBOL to have a value equal to
     the difference of the two symbols HIGH and LOW, i.e. HIGH minus
     LOW.  GCC guarantees that the symbols HIGH and LOW are already
     known by the assembler so that the difference resolves into a
     constant.

     If `SET_ASM_OP' is defined, a default definition is provided which
     is correct for most systems.

`ASM_OUTPUT_WEAK_ALIAS (STREAM, NAME, VALUE)'
     A C statement to output to the stdio stream STREAM assembler code
     which defines (equates) the weak symbol NAME to have the value
     VALUE.  If VALUE is `NULL', it defines NAME as an undefined weak
     symbol.

     Define this macro if the target only supports weak aliases; define
     `ASM_OUTPUT_DEF' instead if possible.

`OBJC_GEN_METHOD_LABEL (BUF, IS_INST, CLASS_NAME, CAT_NAME, SEL_NAME)'
     Define this macro to override the default assembler names used for
     Objective-C methods.

     The default name is a unique method number followed by the name of
     the class (e.g. `_1_Foo').  For methods in categories, the name of
     the category is also included in the assembler name (e.g.
     `_1_Foo_Bar').

     These names are safe on most systems, but make debugging difficult
     since the method's selector is not present in the name.
     Therefore, particular systems define other ways of computing names.

     BUF is an expression of type `char *' which gives you a buffer in
     which to store the name; its length is as long as CLASS_NAME,
     CAT_NAME and SEL_NAME put together, plus 50 characters extra.

     The argument IS_INST specifies whether the method is an instance
     method or a class method; CLASS_NAME is the name of the class;
     CAT_NAME is the name of the category (or `NULL' if the method is
     not in a category); and SEL_NAME is the name of the selector.

     On systems where the assembler can handle quoted names, you can
     use this macro to provide more human-readable names.

`ASM_DECLARE_CLASS_REFERENCE (STREAM, NAME)'
     A C statement (sans semicolon) to output to the stdio stream
     STREAM commands to declare that the label NAME is an Objective-C
     class reference.  This is only needed for targets whose linkers
     have special support for NeXT-style runtimes.

`ASM_DECLARE_UNRESOLVED_REFERENCE (STREAM, NAME)'
     A C statement (sans semicolon) to output to the stdio stream
     STREAM commands to declare that the label NAME is an unresolved
     Objective-C class reference.  This is only needed for targets
     whose linkers have special support for NeXT-style runtimes.


File: gccint.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.

   This method will be chosen as the default by `target-def.h' if
`TARGET_ASM_NAMED_SECTION' is defined.  A target that does not support
arbitrary sections, but does support special designated constructor and
destructor sections may define `CTORS_SECTION_ASM_OP' and
`DTORS_SECTION_ASM_OP' to achieve the same effect.

   When arbitrary sections are available, there are two variants,
depending upon how the code in `crtstuff.c' is called.  On systems that
support a ".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 crti.o crtbegin.o ... -lgcc crtend.o crtn.o

   The prologue of a function (`__init') appears in the `.init' section
of `crti.o'; the epilogue appears in `crtn.o'.  Likewise for the
function `__fini' in the ".fini" section.  Normally these files are
provided by the operating system or by the GNU C library, but are
provided by GCC for a few targets.

   The objects `crtbegin.o' and `crtend.o' are (for most targets)
compiled from `crtstuff.c'.  They contain, among other things, code
fragments within the `.init' and `.fini' sections that branch to
routines in the `.text' section.  The linker will pull all parts of a
section together, which results in a complete `__init' function that
invokes the routines we need at startup.

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

   If no init section is available, when GCC compiles any function
called `main' (or more accurately, any function designated as a program
entry point by the language front end calling `expand_main_function'),
it inserts a procedure call to `__main' as the first executable code
after the function prologue.  The `__main' function is defined in
`libgcc2.c' and runs the global constructors.

   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,
`TARGET_ASM_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.  `TARGET_ASM_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, `TARGET_HAVE_CTORS_DTORS' is false, 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 GCC; 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.  In order to use this method,
`use_collect2' must be defined in the target in `config.gcc'.

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


File: gccint.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, including spacing, for the
     assembler operation to identify the following data as
     initialization code.  If not defined, GCC 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 start-up code
     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.

`COLLECT_SHARED_INIT_FUNC (STREAM, FUNC)'
     If defined, a C statement that will write a function that can be
     automatically called when a shared library is loaded.  The function
     should call FUNC, which takes no arguments.  If not defined, and
     the object format requires an explicit initialization function,
     then a function called `_GLOBAL__DI' will be generated.

     This function and the following one are used by collect2 when
     linking a shared library that needs constructors or destructors,
     or has DWARF2 exception tables embedded in the code.

`COLLECT_SHARED_FINI_FUNC (STREAM, FUNC)'
     If defined, a C statement that will write a function that can be
     automatically called when a shared library is unloaded.  The
     function should call FUNC, which takes no arguments.  If not
     defined, and the object format requires an explicit finalization
     function, then a function called `_GLOBAL__DD' will be generated.

`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.

`SUPPORTS_INIT_PRIORITY'
     If nonzero, the C++ `init_priority' attribute is supported and the
     compiler should emit instructions to control the order of
     initialization of objects.  If zero, the compiler will issue an
     error message upon encountering an `init_priority' attribute.

 - Target Hook: bool TARGET_HAVE_CTORS_DTORS
     This value is true if the target supports some "native" method of
     collecting constructors and destructors to be run at startup and
     exit.  It is false if we must use `collect2'.

 - Target Hook: void TARGET_ASM_CONSTRUCTOR (rtx SYMBOL, int PRIORITY)
     If defined, a function that outputs assembler code to arrange to
     call the function referenced by SYMBOL at initialization time.

     Assume that SYMBOL is a `SYMBOL_REF' for a function taking no
     arguments and with no return value.  If the target supports
     initialization priorities, PRIORITY is a value between 0 and
     `MAX_INIT_PRIORITY'; otherwise it must be `DEFAULT_INIT_PRIORITY'.

     If this macro is not defined by the target, a suitable default will
     be chosen if (1) the target supports arbitrary section names, (2)
     the target defines `CTORS_SECTION_ASM_OP', or (3) `USE_COLLECT2'
     is not defined.

 - Target Hook: void TARGET_ASM_DESTRUCTOR (rtx SYMBOL, int PRIORITY)
     This is like `TARGET_ASM_CONSTRUCTOR' but used for termination
     functions rather than initialization functions.

   If `TARGET_HAVE_CTORS_DTORS' is true, the initialization routine
generated for the generated object file will have static linkage.

   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: gccint.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_data.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.

`ASM_FPRINTF_EXTENSIONS(FILE, ARGPTR, FORMAT)'
     If defined this macro should expand to a series of `case'
     statements which will be parsed inside the `switch' statement of
     the `asm_fprintf' function.  This allows targets to define extra
     printf formats which may useful when generating their assembler
     statements.  Note that upper case letters are reserved for future
     generic extensions to asm_fprintf, and so are not available to
     target specific code.  The output file is given by the parameter
     FILE.  The varargs input pointer is ARGPTR and the rest of the
     format string, starting the character after the one that is being
     switched upon, is pointed to by FORMAT.

`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', `option2', etc., if the value of
     `ASSEMBLER_DIALECT' is zero, one, two, etc.  Any special characters
     within these strings retain their usual meaning.  If there are
     fewer alternatives within the braces than the value of
     `ASSEMBLER_DIALECT', the construct outputs nothing.

     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: gccint.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, BODY, 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, GCC will also use this macro on all machines when
     producing PIC.  BODY is the body of the `ADDR_DIFF_VEC'; it is
     provided so that the mode and flags can be read.

`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: gccint.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.

`EH_FRAME_SECTION_NAME'
     If defined, a C string constant for the name of the section
     containing exception handling frame unwind information.  If not
     defined, GCC 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.

`EH_FRAME_IN_DATA_SECTION'
     If defined, DWARF 2 frame unwind information will be placed in the
     data section even though the target supports named sections.  This
     might be necessary, for instance, if the system linker does garbage
     collection and sections cannot be marked as not to be collected.

     Do not define this macro unless `TARGET_ASM_NAMED_SECTION' is also
     defined.

`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.

`DWARF_CIE_DATA_ALIGNMENT'
     This macro need only be defined if the target might save registers
     in the function prologue at an offset to the stack pointer that is
     not aligned to `UNITS_PER_WORD'.  The definition should be the
     negative minimum alignment if `STACK_GROWS_DOWNWARD' is defined,
     and the positive minimum alignment otherwise.  *Note SDB and
     DWARF::.  Only applicable if the target supports DWARF 2 frame
     unwind information.

 - Target Hook: void TARGET_ASM_EXCEPTION_SECTION ()
     If defined, a function that switches to the section in which the
     main exception table is to be placed (*note Sections::).  The
     default is a function that switches to a section named
     `.gcc_except_table' on machines that support named sections via
     `TARGET_ASM_NAMED_SECTION', otherwise if `-fpic' or `-fPIC' is in
     effect, the `data_section', otherwise the `readonly_data_section'.

 - Target Hook: void TARGET_ASM_EH_FRAME_SECTION ()
     If defined, a function that switches to the section in which the
     DWARF 2 frame unwind information to be placed (*note Sections::).
     The default is a function that outputs a standard GAS section
     directive, if `EH_FRAME_SECTION_NAME' is defined, or else a data
     section directive followed by a synthetic label.


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

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

   This describes commands for alignment.

`JUMP_ALIGN (LABEL)'
     The alignment (log base 2) to put in front of LABEL, which is a
     common destination of jumps and has no fallthru incoming edge.

     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.

     Unless it's necessary to inspect the LABEL parameter, it is better
     to set the variable ALIGN_JUMPS in the target's
     `OVERRIDE_OPTIONS'.  Otherwise, you should try to honor the user's
     selection in ALIGN_JUMPS in a `JUMP_ALIGN' implementation.

`LABEL_ALIGN_AFTER_BARRIER (LABEL)'
     The alignment (log base 2) to put in front of LABEL, which follows
     a `BARRIER'.

     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.

`LABEL_ALIGN_AFTER_BARRIER_MAX_SKIP'
     The maximum number of bytes to skip when applying
     `LABEL_ALIGN_AFTER_BARRIER'.  This works only if
     `ASM_OUTPUT_MAX_SKIP_ALIGN' is defined.

`LOOP_ALIGN (LABEL)'
     The alignment (log base 2) to put in front of LABEL, which follows
     a `NOTE_INSN_LOOP_BEG' note.

     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.

     Unless it's necessary to inspect the LABEL parameter, it is better
     to set the variable `align_loops' in the target's
     `OVERRIDE_OPTIONS'.  Otherwise, you should try to honor the user's
     selection in `align_loops' in a `LOOP_ALIGN' implementation.

`LOOP_ALIGN_MAX_SKIP'
     The maximum number of bytes to skip when applying `LOOP_ALIGN'.
     This works only if `ASM_OUTPUT_MAX_SKIP_ALIGN' is defined.

`LABEL_ALIGN (LABEL)'
     The alignment (log base 2) to put in front of LABEL.  If
     `LABEL_ALIGN_AFTER_BARRIER' / `LOOP_ALIGN' specify a different
     alignment, the maximum of the specified values is used.

     Unless it's necessary to inspect the LABEL parameter, it is better
     to set the variable `align_labels' in the target's
     `OVERRIDE_OPTIONS'.  Otherwise, you should try to honor the user's
     selection in `align_labels' in a `LABEL_ALIGN' implementation.

`LABEL_ALIGN_MAX_SKIP'
     The maximum number of bytes to skip when applying `LABEL_ALIGN'.
     This works only if `ASM_OUTPUT_MAX_SKIP_ALIGN' is defined.

`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 to 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'.

`ASM_OUTPUT_MAX_SKIP_ALIGN (STREAM, POWER, MAX_SKIP)'
     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, but only if MAX_SKIP or fewer bytes are needed to
     satisfy the alignment request.  POWER and MAX_SKIP will be a C
     expression of type `int'.


File: gccint.info,  Node: Debugging Info,  Next: Floating Point,  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.
* VMS Debug::          Macros for VMS debug format.


File: gccint.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 the default macro provided,
     the value of this expression will 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 GCC, 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 GCC should
     produce when the user specifies just `-g'.  Define this if you
     have arranged for GCC to support more than one format of debugging
     output.  Currently, the allowable values are `DBX_DEBUG',
     `SDB_DEBUG', `DWARF_DEBUG', `DWARF2_DEBUG', `XCOFF_DEBUG',
     `VMS_DEBUG', and `VMS_AND_DWARF2_DEBUG'.

     When the user specifies `-ggdb', GCC 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, GCC uses the
     value `DWARF2_DEBUG'.  Otherwise, if `DBX_DEBUGGING_INFO' is
     defined, GCC 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', `-gxcoff', or
     `-gvms'.

