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INFO-DIR-SECTION Programming
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* gccint: (gccint).            Internals of the GNU Compiler Collection.
END-INFO-DIR-ENTRY
   This file documents the internals of the GNU compilers.

   Published by the Free Software Foundation
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File: gccint.info,  Node: File Framework,  Next: Data Output,  Up: Assembler Format

The Overall Framework of an Assembler File
------------------------------------------

   This describes the overall framework of an assembly file.

 - Target Hook: void TARGET_ASM_FILE_START ()
     Output to `asm_out_file' any text which the assembler expects to
     find at the beginning of a file.  The default behavior is
     controlled by two flags, documented below.  Unless your target's
     assembler is quite unusual, if you override the default, you
     should call `default_file_start' at some point in your target
     hook.  This lets other target files rely on these variables.

 - Target Hook: bool TARGET_ASM_FILE_START_APP_OFF
     If this flag is true, the text of the macro `ASM_APP_OFF' will be
     printed as the very first line in the assembly file, unless
     `-fverbose-asm' is in effect.  (If that macro has been defined to
     the empty string, this variable has no effect.)  With the normal
     definition of `ASM_APP_OFF', the effect is to notify the GNU
     assembler that it need not bother stripping comments or extra
     whitespace from its input.  This allows it to work a bit faster.

     The default is false.  You should not set it to true unless you
     have verified that your port does not generate any extra
     whitespace or comments that will cause GAS to issue errors in
     NO_APP mode.

 - Target Hook: bool TARGET_ASM_FILE_START_FILE_DIRECTIVE
     If this flag is true, `output_file_directive' will be called for
     the primary source file, immediately after printing `ASM_APP_OFF'
     (if that is enabled).  Most ELF assemblers expect this to be done.
     The default is false.

 - Target Hook: void TARGET_ASM_FILE_END ()
     Output to `asm_out_file' any text which the assembler expects to
     find at the end of a file.  The default is to output nothing.

 - Function: void file_end_indicate_exec_stack ()
     Some systems use a common convention, the `.note.GNU-stack'
     special section, to indicate whether or not an object file relies
     on the stack being executable.  If your system uses this
     convention, you should define `TARGET_ASM_FILE_END' to this
     function.  If you need to do other things in that hook, have your
     hook function call this function.

 - Macro: ASM_COMMENT_START
     A C string constant describing how to begin a comment in the target
     assembler language.  The compiler assumes that the comment will
     end at the end of the line.

 - Macro: ASM_APP_ON
     A C string constant for text to be output before each `asm'
     statement or group of consecutive ones.  Normally this is
     `"#APP"', which is a comment that has no effect on most assemblers
     but tells the GNU assembler that it must check the lines that
     follow for all valid assembler constructs.

 - Macro: ASM_APP_OFF
     A C string constant for text to be output after each `asm'
     statement or group of consecutive ones.  Normally this is
     `"#NO_APP"', which tells the GNU assembler to resume making the
     time-saving assumptions that are valid for ordinary compiler
     output.

 - Macro: ASM_OUTPUT_SOURCE_FILENAME (STREAM, NAME)
     A C statement to output COFF information or DWARF debugging
     information which indicates that filename NAME is the current
     source file to the stdio stream STREAM.

     This macro need not be defined if the standard form of output for
     the file format in use is appropriate.

 - Macro: OUTPUT_QUOTED_STRING (STREAM, STRING)
     A C statement to output the string STRING to the stdio stream
     STREAM.  If you do not call the function `output_quoted_string' in
     your config files, GCC will only call it to output filenames to
     the assembler source.  So you can use it to canonicalize the format
     of the filename using this macro.

 - Macro: ASM_OUTPUT_SOURCE_LINE (STREAM, LINE, COUNTER)
     A C statement to output DBX or SDB debugging information before
     code for line number LINE of the current source file to the stdio
     stream STREAM. COUNTER is the number of time the macro was
     invoked, including the current invocation; it is intended to
     generate unique labels in the assembly output.

     This macro need not be defined if the standard form of debugging
     information for the debugger in use is appropriate.

 - Macro: ASM_OUTPUT_IDENT (STREAM, STRING)
     A C statement to output something to the assembler file to handle a
     `#ident' directive containing the text STRING.  If this macro is
     not defined, nothing is output for a `#ident' directive.

 - Target Hook: void TARGET_ASM_NAMED_SECTION (const char *NAME,
          unsigned int FLAGS, unsigned int ALIGN)
     Output assembly directives to switch to section NAME.  The section
     should have attributes as specified by FLAGS, which is a bit mask
     of the `SECTION_*' flags defined in `output.h'.  If ALIGN is
     nonzero, it contains an alignment in bytes to be used for the
     section, otherwise some target default should be used.  Only
     targets that must specify an alignment within the section
     directive need pay attention to ALIGN - we will still use
     `ASM_OUTPUT_ALIGN'.

 - Target Hook: bool TARGET_HAVE_NAMED_SECTIONS
     This flag is true if the target supports
     `TARGET_ASM_NAMED_SECTION'.

 - Target Hook: unsigned int TARGET_SECTION_TYPE_FLAGS (tree DECL,
          const char *NAME, int RELOC)
     Choose a set of section attributes for use by
     `TARGET_ASM_NAMED_SECTION' based on a variable or function decl, a
     section name, and whether or not the declaration's initializer may
     contain runtime relocations.  DECL may be  null, in which case
     read-write data should be assumed.

     The default version if this function handles choosing code vs data,
     read-only vs read-write data, and `flag_pic'.  You should only
     need to override this if your target has special flags that might
     be set via `__attribute__'.


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

Output of Data
--------------

 - Target Hook: const char * TARGET_ASM_BYTE_OP
 - Target Hook: const char * TARGET_ASM_ALIGNED_HI_OP
 - Target Hook: const char * TARGET_ASM_ALIGNED_SI_OP
 - Target Hook: const char * TARGET_ASM_ALIGNED_DI_OP
 - Target Hook: const char * TARGET_ASM_ALIGNED_TI_OP
 - Target Hook: const char * TARGET_ASM_UNALIGNED_HI_OP
 - Target Hook: const char * TARGET_ASM_UNALIGNED_SI_OP
 - Target Hook: const char * TARGET_ASM_UNALIGNED_DI_OP
 - Target Hook: const char * TARGET_ASM_UNALIGNED_TI_OP
     These hooks specify assembly directives for creating certain kinds
     of integer object.  The `TARGET_ASM_BYTE_OP' directive creates a
     byte-sized object, the `TARGET_ASM_ALIGNED_HI_OP' one creates an
     aligned two-byte object, and so on.  Any of the hooks may be
     `NULL', indicating that no suitable directive is available.

     The compiler will print these strings at the start of a new line,
     followed immediately by the object's initial value.  In most cases,
     the string should contain a tab, a pseudo-op, and then another tab.

 - Target Hook: bool TARGET_ASM_INTEGER (rtx X, unsigned int SIZE, int
          ALIGNED_P)
     The `assemble_integer' function uses this hook to output an
     integer object.  X is the object's value, SIZE is its size in
     bytes and ALIGNED_P indicates whether it is aligned.  The function
     should return `true' if it was able to output the object.  If it
     returns false, `assemble_integer' will try to split the object
     into smaller parts.

     The default implementation of this hook will use the
     `TARGET_ASM_BYTE_OP' family of strings, returning `false' when the
     relevant string is `NULL'.

 - Macro: OUTPUT_ADDR_CONST_EXTRA (STREAM, X, FAIL)
     A C statement to recognize RTX patterns that `output_addr_const'
     can't deal with, and output assembly code to STREAM corresponding
     to the pattern X.  This may be used to allow machine-dependent
     `UNSPEC's to appear within constants.

     If `OUTPUT_ADDR_CONST_EXTRA' fails to recognize a pattern, it must
     `goto fail', so that a standard error message is printed.  If it
     prints an error message itself, by calling, for example,
     `output_operand_lossage', it may just complete normally.

 - Macro: ASM_OUTPUT_ASCII (STREAM, PTR, LEN)
     A C statement to output to the stdio stream STREAM an assembler
     instruction to assemble a string constant containing the LEN bytes
     at PTR.  PTR will be a C expression of type `char *' and LEN a C
     expression of type `int'.

     If the assembler has a `.ascii' pseudo-op as found in the Berkeley
     Unix assembler, do not define the macro `ASM_OUTPUT_ASCII'.

 - Macro: ASM_OUTPUT_FDESC (STREAM, DECL, N)
     A C statement to output word N of a function descriptor for DECL.
     This must be defined if `TARGET_VTABLE_USES_DESCRIPTORS' is
     defined, and is otherwise unused.

 - Macro: CONSTANT_POOL_BEFORE_FUNCTION
     You may define this macro as a C expression.  You should define the
     expression to have a nonzero value if GCC should output the
     constant pool for a function before the code for the function, or
     a zero value if GCC should output the constant pool after the
     function.  If you do not define this macro, the usual case, GCC
     will output the constant pool before the function.

 - Macro: ASM_OUTPUT_POOL_PROLOGUE (FILE, FUNNAME, FUNDECL, SIZE)
     A C statement to output assembler commands to define the start of
     the constant pool for a function.  FUNNAME is a string giving the
     name of the function.  Should the return type of the function be
     required, it can be obtained via FUNDECL.  SIZE is the size, in
     bytes, of the constant pool that will be written immediately after
     this call.

     If no constant-pool prefix is required, the usual case, this macro
     need not be defined.

 - Macro: ASM_OUTPUT_SPECIAL_POOL_ENTRY (FILE, X, MODE, ALIGN, LABELNO,
          JUMPTO)
     A C statement (with or without semicolon) to output a constant in
     the constant pool, if it needs special treatment.  (This macro
     need not do anything for RTL expressions that can be output
     normally.)

     The argument FILE is the standard I/O stream to output the
     assembler code on.  X is the RTL expression for the constant to
     output, and MODE is the machine mode (in case X is a `const_int').
     ALIGN is the required alignment for the value X; you should
     output an assembler directive to force this much alignment.

     The argument LABELNO is a number to use in an internal label for
     the address of this pool entry.  The definition of this macro is
     responsible for outputting the label definition at the proper
     place.  Here is how to do this:

          `(*targetm.asm_out.internal_label)' (FILE, "LC", LABELNO);

     When you output a pool entry specially, you should end with a
     `goto' to the label JUMPTO.  This will prevent the same pool entry
     from being output a second time in the usual manner.

     You need not define this macro if it would do nothing.

 - Macro: ASM_OUTPUT_POOL_EPILOGUE (FILE FUNNAME FUNDECL SIZE)
     A C statement to output assembler commands to at the end of the
     constant pool for a function.  FUNNAME is a string giving the name
     of the function.  Should the return type of the function be
     required, you can obtain it via FUNDECL.  SIZE is the size, in
     bytes, of the constant pool that GCC wrote immediately before this
     call.

     If no constant-pool epilogue is required, the usual case, you need
     not define this macro.

 - Macro: IS_ASM_LOGICAL_LINE_SEPARATOR (C)
     Define this macro as a C expression which is nonzero if C is used
     as a logical line separator by the assembler.

     If you do not define this macro, the default is that only the
     character `;' is treated as a logical line separator.

 - Target Hook: const char * TARGET_ASM_OPEN_PAREN
 - Target Hook: const char * TARGET_ASM_CLOSE_PAREN
     These target hooks are C string constants, describing the syntax
     in the assembler for grouping arithmetic expressions.  If not
     overridden, they default to normal parentheses, which is correct
     for most assemblers.

   These macros are provided by `real.h' for writing the definitions of
`ASM_OUTPUT_DOUBLE' and the like:

 - Macro: REAL_VALUE_TO_TARGET_SINGLE (X, L)
 - Macro: REAL_VALUE_TO_TARGET_DOUBLE (X, L)
 - Macro: REAL_VALUE_TO_TARGET_LONG_DOUBLE (X, L)
     These translate X, of type `REAL_VALUE_TYPE', to the target's
     floating point representation, and store its bit pattern in the
     variable L.  For `REAL_VALUE_TO_TARGET_SINGLE', this variable
     should be a simple `long int'.  For the others, it should be an
     array of `long int'.  The number of elements in this array is
     determined by the size of the desired target floating point data
     type: 32 bits of it go in each `long int' array element.  Each
     array element holds 32 bits of the result, even if `long int' is
     wider than 32 bits on the host machine.

     The array element values are designed so that you can print them
     out using `fprintf' in the order they should appear in the target
     machine's memory.


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

Output of Uninitialized Variables
---------------------------------

   Each of the macros in this section is used to do the whole job of
outputting a single uninitialized variable.

 - Macro: ASM_OUTPUT_COMMON (STREAM, NAME, SIZE, ROUNDED)
     A C statement (sans semicolon) to output to the stdio stream
     STREAM the assembler definition of a common-label named NAME whose
     size is SIZE bytes.  The variable ROUNDED is the size rounded up
     to whatever alignment the caller wants.

     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.

     This macro controls how the assembler definitions of uninitialized
     common global variables are output.

 - Macro: ASM_OUTPUT_ALIGNED_COMMON (STREAM, NAME, SIZE, ALIGNMENT)
     Like `ASM_OUTPUT_COMMON' except takes the required alignment as a
     separate, explicit argument.  If you define this macro, it is used
     in place of `ASM_OUTPUT_COMMON', and gives you more flexibility in
     handling the required alignment of the variable.  The alignment is
     specified as the number of bits.

 - Macro: ASM_OUTPUT_ALIGNED_DECL_COMMON (STREAM, DECL, NAME, SIZE,
          ALIGNMENT)
     Like `ASM_OUTPUT_ALIGNED_COMMON' except that DECL of the variable
     to be output, if there is one, or `NULL_TREE' if there is no
     corresponding variable.  If you define this macro, GCC will use it
     in place of both `ASM_OUTPUT_COMMON' and
     `ASM_OUTPUT_ALIGNED_COMMON'.  Define this macro when you need to
     see the variable's decl in order to chose what to output.

 - Macro: ASM_OUTPUT_SHARED_COMMON (STREAM, NAME, SIZE, ROUNDED)
     If defined, it is similar to `ASM_OUTPUT_COMMON', except that it
     is used when NAME is shared.  If not defined, `ASM_OUTPUT_COMMON'
     will be used.

 - Macro: ASM_OUTPUT_BSS (STREAM, DECL, NAME, SIZE, ROUNDED)
     A C statement (sans semicolon) to output to the stdio stream
     STREAM the assembler definition of uninitialized global DECL named
     NAME whose size is SIZE bytes.  The variable ROUNDED is the size
     rounded up to whatever alignment the caller wants.

     Try to use function `asm_output_bss' defined in `varasm.c' when
     defining this macro.  If unable, 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.

     This macro controls how the assembler definitions of uninitialized
     global variables are output.  This macro exists to properly
     support languages like C++ which do not have `common' data.
     However, this macro currently is not defined for all targets.  If
     this macro and `ASM_OUTPUT_ALIGNED_BSS' are not defined then
     `ASM_OUTPUT_COMMON' or `ASM_OUTPUT_ALIGNED_COMMON' or
     `ASM_OUTPUT_ALIGNED_DECL_COMMON' is used.

 - Macro: ASM_OUTPUT_ALIGNED_BSS (STREAM, DECL, NAME, SIZE, ALIGNMENT)
     Like `ASM_OUTPUT_BSS' except takes the required alignment as a
     separate, explicit argument.  If you define this macro, it is used
     in place of `ASM_OUTPUT_BSS', and gives you more flexibility in
     handling the required alignment of the variable.  The alignment is
     specified as the number of bits.

     Try to use function `asm_output_aligned_bss' defined in file
     `varasm.c' when defining this macro.

 - Macro: ASM_OUTPUT_SHARED_BSS (STREAM, DECL, NAME, SIZE, ROUNDED)
     If defined, it is similar to `ASM_OUTPUT_BSS', except that it is
     used when NAME is shared.  If not defined, `ASM_OUTPUT_BSS' will
     be used.

 - Macro: ASM_OUTPUT_LOCAL (STREAM, NAME, SIZE, ROUNDED)
     A C statement (sans semicolon) to output to the stdio stream
     STREAM the assembler definition of a local-common-label named NAME
     whose size is SIZE bytes.  The variable ROUNDED is the size
     rounded up to whatever alignment the caller wants.

     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.

     This macro controls how the assembler definitions of uninitialized
     static variables are output.

 - Macro: ASM_OUTPUT_ALIGNED_LOCAL (STREAM, NAME, SIZE, ALIGNMENT)
     Like `ASM_OUTPUT_LOCAL' except takes the required alignment as a
     separate, explicit argument.  If you define this macro, it is used
     in place of `ASM_OUTPUT_LOCAL', and gives you more flexibility in
     handling the required alignment of the variable.  The alignment is
     specified as the number of bits.

 - Macro: ASM_OUTPUT_ALIGNED_DECL_LOCAL (STREAM, DECL, NAME, SIZE,
          ALIGNMENT)
     Like `ASM_OUTPUT_ALIGNED_DECL' except that DECL of the variable to
     be output, if there is one, or `NULL_TREE' if there is no
     corresponding variable.  If you define this macro, GCC will use it
     in place of both `ASM_OUTPUT_DECL' and `ASM_OUTPUT_ALIGNED_DECL'.
     Define this macro when you need to see the variable's decl in
     order to chose what to output.

 - Macro: ASM_OUTPUT_SHARED_LOCAL (STREAM, NAME, SIZE, ROUNDED)
     If defined, it is similar to `ASM_OUTPUT_LOCAL', except that it is
     used when NAME is shared.  If not defined, `ASM_OUTPUT_LOCAL' will
     be used.


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

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

   This is about outputting labels.

 - Macro: 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.  A default definition
     of this macro is provided which is correct for most systems.

 - Macro: SIZE_ASM_OP
     A C string containing the appropriate assembler directive to
     specify the size of a symbol, without any arguments.  On systems
     that use ELF, the default (in `config/elfos.h') is `"\t.size\t"';
     on other systems, the default is not to define this macro.

     Define this macro only if it is correct to use the default
     definitions of `ASM_OUTPUT_SIZE_DIRECTIVE' and
     `ASM_OUTPUT_MEASURED_SIZE' for your system.  If you need your own
     custom definitions of those macros, or if you do not need explicit
     symbol sizes at all, do not define this macro.

 - Macro: ASM_OUTPUT_SIZE_DIRECTIVE (STREAM, NAME, SIZE)
     A C statement (sans semicolon) to output to the stdio stream
     STREAM a directive telling the assembler that the size of the
     symbol NAME is SIZE.  SIZE is a `HOST_WIDE_INT'.  If you define
     `SIZE_ASM_OP', a default definition of this macro is provided.

 - Macro: ASM_OUTPUT_MEASURED_SIZE (STREAM, NAME)
     A C statement (sans semicolon) to output to the stdio stream
     STREAM a directive telling the assembler to calculate the size of
     the symbol NAME by subtracting its address from the current
     address.

     If you define `SIZE_ASM_OP', a default definition of this macro is
     provided.  The default assumes that the assembler recognizes a
     special `.' symbol as referring to the current address, and can
     calculate the difference between this and another symbol.  If your
     assembler does not recognize `.' or cannot do calculations with
     it, you will need to redefine `ASM_OUTPUT_MEASURED_SIZE' to use
     some other technique.

 - Macro: TYPE_ASM_OP
     A C string containing the appropriate assembler directive to
     specify the type of a symbol, without any arguments.  On systems
     that use ELF, the default (in `config/elfos.h') is `"\t.type\t"';
     on other systems, the default is not to define this macro.

     Define this macro only if it is correct to use the default
     definition of `ASM_OUTPUT_TYPE_DIRECTIVE' for your system.  If you
     need your own custom definition of this macro, or if you do not
     need explicit symbol types at all, do not define this macro.

 - Macro: TYPE_OPERAND_FMT
     A C string which specifies (using `printf' syntax) the format of
     the second operand to `TYPE_ASM_OP'.  On systems that use ELF, the
     default (in `config/elfos.h') is `"@%s"'; on other systems, the
     default is not to define this macro.

     Define this macro only if it is correct to use the default
     definition of `ASM_OUTPUT_TYPE_DIRECTIVE' for your system.  If you
     need your own custom definition of this macro, or if you do not
     need explicit symbol types at all, do not define this macro.

 - Macro: ASM_OUTPUT_TYPE_DIRECTIVE (STREAM, TYPE)
     A C statement (sans semicolon) to output to the stdio stream
     STREAM a directive telling the assembler that the type of the
     symbol NAME is TYPE.  TYPE is a C string; currently, that string
     is always either `"function"' or `"object"', but you should not
     count on this.

     If you define `TYPE_ASM_OP' and `TYPE_OPERAND_FMT', a default
     definition of this macro is provided.

 - Macro: 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').

     You may wish to use `ASM_OUTPUT_TYPE_DIRECTIVE' in the definition
     of this macro.

 - Macro: 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.

     You may wish to use `ASM_OUTPUT_MEASURED_SIZE' in the definition
     of this macro.

 - Macro: 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').

     You may wish to use `ASM_OUTPUT_TYPE_DIRECTIVE' and/or
     `ASM_OUTPUT_SIZE_DIRECTIVE' in the definition of this macro.

 - Macro: ASM_DECLARE_CONSTANT_NAME (STREAM, NAME, EXP, SIZE)
     A C statement (sans semicolon) to output to the stdio stream
     STREAM any text necessary for declaring the name NAME of a
     constant which is being defined.  This macro is responsible for
     outputting the label definition (perhaps using
     `ASM_OUTPUT_LABEL').  The argument EXP is the value of the
     constant, and SIZE is the size of the constant in bytes.  NAME
     will be an internal label.

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

     You may wish to use `ASM_OUTPUT_TYPE_DIRECTIVE' in the definition
     of this macro.

 - Macro: 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.

 - Macro: 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.

     You may wish to use `ASM_OUTPUT_SIZE_DIRECTIVE' and/or
     `ASM_OUTPUT_MEASURED_SIZE' in the definition of this macro.

 - Target Hook: void TARGET_ASM_GLOBALIZE_LABEL (FILE *STREAM, const
          char *NAME)
     This target hook is a function to output to the stdio stream
     STREAM some commands that will make the label NAME global; that
     is, available for reference from other files.

     The default implementation relies on a proper definition of
     `GLOBAL_ASM_OP'.

 - Macro: 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.

 - 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.

 - Macro: 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'.

 - Macro: MAKE_DECL_ONE_ONLY (DECL)
     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.

 - Macro: 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.

 - Target Hook: void TARGET_ASM_ASSEMBLE_VISIBILITY (tree DECL, const
          char *VISIBILITY)
     This target hook is a function to output to ASM_OUT_FILE some
     commands that will make the symbol(s) associated with DECL have
     hidden, protected or internal visibility as specified by
     VISIBILITY.

 - Macro: 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.

 - Macro: 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.

 - Macro: 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'.

 - Macro: 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
     `TARGET_ENCODE_SECTION_INFO'.

 - Macro: 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.

 - Target Hook: void TARGET_ASM_INTERNAL_LABEL (FILE *STREAM, const
          char *PREFIX, unsigned long LABELNO)
     A function to output to the stdio stream STREAM a label whose name
     is made from the string PREFIX and the number LABELNO.

     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 default version of this function utilizes
     ASM_GENERATE_INTERNAL_LABEL.

 - Macro: 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
     `(*targetm.asm_out.internal_label)' will be used.

 - Macro: 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 `(*targetm.asm_out.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.)

 - Macro: 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.

     If this macro is not defined, a default definition will be provided
     which is correct for most systems.

 - Macro: 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.

 - Macro: 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.

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

 - Macro: 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.

 - Macro: 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.

 - Macro: 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.

 - Macro: 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.

