Libris Britannia 4/science library(b).zip/science library(b)/DJGPP/GCC257DC.ZIP/docs/gcc/gcc.i20

GNU Info File | 1993-12-15 | 49KB | 831 lines

This is Info file gcc.info, produced by Makeinfo-1.54 from the input
file gcc.texi.
   This file documents the use and the internals of the GNU compiler.
   Published by the Free Software Foundation 675 Massachusetts Avenue
Cambridge, MA 02139 USA
   Copyright (C) 1988, 1989, 1992, 1993 Free Software Foundation, Inc.
   Permission is granted to make and distribute verbatim copies of this
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versions, except that the sections entitled "GNU General Public
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File: gcc.info,  Node: Condition Code,  Next: Costs,  Prev: Addressing Modes,  Up: Target Macros
Condition Code Status
=====================
   The file `conditions.h' defines a variable `cc_status' to describe
how the condition code was computed (in case the interpretation of the
condition code depends on the instruction that it was set by).  This
variable contains the RTL expressions on which the condition code is
currently based, and several standard flags.
   Sometimes additional machine-specific flags must be defined in the
machine description header file.  It can also add additional
machine-specific information by defining `CC_STATUS_MDEP'.
`CC_STATUS_MDEP'
     C code for a data type which is used for declaring the `mdep'
     component of `cc_status'.  It defaults to `int'.
     This macro is not used on machines that do not use `cc0'.
`CC_STATUS_MDEP_INIT'
     A C expression to initialize the `mdep' field to "empty".  The
     default definition does nothing, since most machines don't use the
     field anyway.  If you want to use the field, you should probably
     define this macro to initialize it.
     This macro is not used on machines that do not use `cc0'.
`NOTICE_UPDATE_CC (EXP, INSN)'
     A C compound statement to set the components of `cc_status'
     appropriately for an insn INSN whose body is EXP.  It is this
     macro's responsibility to recognize insns that set the condition
     code as a byproduct of other activity as well as those that
     explicitly set `(cc0)'.
     This macro is not used on machines that do not use `cc0'.
     If there are insns that do not set the condition code but do alter
     other machine registers, this macro must check to see whether they
     invalidate the expressions that the condition code is recorded as
     reflecting.  For example, on the 68000, insns that store in address
     registers do not set the condition code, which means that usually
     `NOTICE_UPDATE_CC' can leave `cc_status' unaltered for such insns.
     But suppose that the previous insn set the condition code based
     on location `a4@(102)' and the current insn stores a new value in
     `a4'.  Although the condition code is not changed by this, it will
     no longer be true that it reflects the contents of `a4@(102)'.
     Therefore, `NOTICE_UPDATE_CC' must alter `cc_status' in this case
     to say that nothing is known about the condition code value.
     The definition of `NOTICE_UPDATE_CC' must be prepared to deal with
     the results of peephole optimization: insns whose patterns are
     `parallel' RTXs containing various `reg', `mem' or constants which
     are just the operands.  The RTL structure of these insns is not
     sufficient to indicate what the insns actually do.  What
     `NOTICE_UPDATE_CC' should do when it sees one is just to run
     `CC_STATUS_INIT'.
     A possible definition of `NOTICE_UPDATE_CC' is to call a function
     that looks at an attribute (*note Insn Attributes::.) named, for
     example, `cc'.  This avoids having detailed information about
     patterns in two places, the `md' file and in `NOTICE_UPDATE_CC'.
`EXTRA_CC_MODES'
     A list of names to be used for additional modes for condition code
     values in registers (*note Jump Patterns::.).  These names are
     added to `enum machine_mode' and all have class `MODE_CC'.  By
     convention, they should start with `CC' and end with `mode'.
     You should only define this macro if your machine does not use
     `cc0' and only if additional modes are required.
`EXTRA_CC_NAMES'
     A list of C strings giving the names for the modes listed in
     `EXTRA_CC_MODES'.  For example, the Sparc defines this macro and
     `EXTRA_CC_MODES' as
          #define EXTRA_CC_MODES CC_NOOVmode, CCFPmode
          #define EXTRA_CC_NAMES "CC_NOOV", "CCFP"
     This macro is not required if `EXTRA_CC_MODES' is not defined.
`SELECT_CC_MODE (OP, X, Y)'
     Returns a mode from class `MODE_CC' to be used when comparison
     operation code OP is applied to rtx X and Y.  For example, on the
     Sparc, `SELECT_CC_MODE' is defined as (see *note Jump Patterns::.
     for a description of the reason for this definition)
          #define SELECT_CC_MODE(OP,X,Y) \
            (GET_MODE_CLASS (GET_MODE (X)) == MODE_FLOAT          \
             ? ((OP == EQ || OP == NE) ? CCFPmode : CCFPEmode)    \
             : ((GET_CODE (X) == PLUS || GET_CODE (X) == MINUS    \
                 || GET_CODE (X) == NEG) \
                ? CC_NOOVmode : CCmode))
     This macro is not required if `EXTRA_CC_MODES' is not defined.
File: gcc.info,  Node: Costs,  Next: Sections,  Prev: Condition Code,  Up: Target Macros
Describing Relative Costs of Operations
=======================================
   These macros let you describe the relative speed of various
operations on the target machine.
`CONST_COSTS (X, CODE, OUTER_CODE)'
     A part of a C `switch' statement that describes the relative costs
     of constant RTL expressions.  It must contain `case' labels for
     expression codes `const_int', `const', `symbol_ref', `label_ref'
     and `const_double'.  Each case must ultimately reach a `return'
     statement to return the relative cost of the use of that kind of
     constant value in an expression.  The cost may depend on the
     precise value of the constant, which is available for examination
     in X, and the rtx code of the expression in which it is contained,
     found in OUTER_CODE.
     CODE is the expression code--redundant, since it can be obtained
     with `GET_CODE (X)'.
`RTX_COSTS (X, CODE, OUTER_CODE)'
     Like `CONST_COSTS' but applies to nonconstant RTL expressions.
     This can be used, for example, to indicate how costly a multiply
     instruction is.  In writing this macro, you can use the construct
     `COSTS_N_INSNS (N)' to specify a cost equal to N fast
     instructions.  OUTER_CODE is the code of the expression in which X
     is contained.
     This macro is optional; do not define it if the default cost
     assumptions are adequate for the target machine.
`ADDRESS_COST (ADDRESS)'
     An expression giving the cost of an addressing mode that contains
     ADDRESS.  If not defined, the cost is computed from the ADDRESS
     expression and the `CONST_COSTS' values.
     For most CISC machines, the default cost is a good approximation
     of the true cost of the addressing mode.  However, on RISC
     machines, all instructions normally have the same length and
     execution time.  Hence all addresses will have equal costs.
     In cases where more than one form of an address is known, the form
     with the lowest cost will be used.  If multiple forms have the
     same, lowest, cost, the one that is the most complex will be used.
     For example, suppose an address that is equal to the sum of a
     register and a constant is used twice in the same basic block.
     When this macro is not defined, the address will be computed in a
     register and memory references will be indirect through that
     register.  On machines where the cost of the addressing mode
     containing the sum is no higher than that of a simple indirect
     reference, this will produce an additional instruction and
     possibly require an additional register.  Proper specification of
     this macro eliminates this overhead for such machines.
     Similar use of this macro is made in strength reduction of loops.
     ADDRESS need not be valid as an address.  In such a case, the cost
     is not relevant and can be any value; invalid addresses need not be
     assigned a different cost.
     On machines where an address involving more than one register is as
     cheap as an address computation involving only one register,
     defining `ADDRESS_COST' to reflect this can cause two registers to
     be live over a region of code where only one would have been if
     `ADDRESS_COST' were not defined in that manner.  This effect should
     be considered in the definition of this macro.  Equivalent costs
     should probably only be given to addresses with different numbers
     of registers on machines with lots of registers.
     This macro will normally either not be defined or be defined as a
     constant.
`REGISTER_MOVE_COST (FROM, TO)'
     A C expression for the cost of moving data from a register in class
     FROM to one in class TO.  The classes are expressed using the
     enumeration values such as `GENERAL_REGS'.  A value of 4 is the
     default; other values are interpreted relative to that.
     It is not required that the cost always equal 2 when FROM is the
     same as TO; on some machines it is expensive to move between
     registers if they are not general registers.
     If reload sees an insn consisting of a single `set' between two
     hard registers, and if `REGISTER_MOVE_COST' applied to their
     classes returns a value of 2, reload does not check to ensure that
     the constraints of the insn are met.  Setting a cost of other than
     2 will allow reload to verify that the constraints are met.  You
     should do this if the `movM' pattern's constraints do not allow
     such copying.
`MEMORY_MOVE_COST (M)'
     A C expression for the cost of moving data of mode M between a
     register and memory.  A value of 2 is the default; this cost is
     relative to those in `REGISTER_MOVE_COST'.
     If moving between registers and memory is more expensive than
     between two registers, you should define this macro to express the
     relative cost.
`BRANCH_COST'
     A C expression for the cost of a branch instruction.  A value of 1
     is the default; other values are interpreted relative to that.
   Here are additional macros which do not specify precise relative
costs, but only that certain actions are more expensive than GNU CC
would ordinarily expect.
`SLOW_BYTE_ACCESS'
     Define this macro as a C expression which is nonzero if accessing
     less than a word of memory (i.e. a `char' or a `short') is no
     faster than accessing a word of memory, i.e., if such access
     require more than one instruction or if there is no difference in
     cost between byte and (aligned) word loads.
     When this macro is not defined, the compiler will access a field by
     finding the smallest containing object; when it is defined, a
     fullword load will be used if alignment permits.  Unless bytes
     accesses are faster than word accesses, using word accesses is
     preferable since it may eliminate subsequent memory access if
     subsequent accesses occur to other fields in the same word of the
     structure, but to different bytes.
`SLOW_ZERO_EXTEND'
     Define this macro if zero-extension (of a `char' or `short' to an
     `int') can be done faster if the destination is a register that is
     known to be zero.
     If you define this macro, you must have instruction patterns that
     recognize RTL structures like this:
          (set (strict_low_part (subreg:QI (reg:SI ...) 0)) ...)
     and likewise for `HImode'.
`SLOW_UNALIGNED_ACCESS'
     Define this macro to be the value 1 if unaligned accesses have a
     cost many times greater than aligned accesses, for example if they
     are emulated in a trap handler.
     When this macro is non-zero, the compiler will act as if
     `STRICT_ALIGNMENT' were non-zero when generating code for block
     moves.  This can cause significantly more instructions to be
     produced.  Therefore, do not set this macro non-zero if unaligned
     accesses only add a cycle or two to the time for a memory access.
     If the value of this macro is always zero, it need not be defined.
`DONT_REDUCE_ADDR'
     Define this macro to inhibit strength reduction of memory
     addresses.  (On some machines, such strength reduction seems to do
     harm rather than good.)
`MOVE_RATIO'
     The number of scalar move insns which should be generated instead
     of a string move insn or a library call.  Increasing the value
     will always make code faster, but eventually incurs high cost in
     increased code size.
     If you don't define this, a reasonable default is used.
`NO_FUNCTION_CSE'
     Define this macro if it is as good or better to call a constant
     function address than to call an address kept in a register.
`NO_RECURSIVE_FUNCTION_CSE'
     Define this macro if it is as good or better for a function to call
     itself with an explicit address than to call an address kept in a
     register.
`ADJUST_COST (INSN, LINK, DEP_INSN, COST)'
     A C statement (sans semicolon) to update the integer variable COST
     based on the relationship between INSN that is dependent on
     DEP_INSN through the dependence LINK.  The default is to make no
     adjustment to COST.  This can be used for example to specify to
     the scheduler that an output- or anti-dependence does not incur
     the same cost as a data-dependence.
File: gcc.info,  Node: Sections,  Next: PIC,  Prev: Costs,  Up: Target Macros
Dividing the Output into Sections (Texts, Data, ...)
====================================================
   An object file is divided into sections containing different types of
data.  In the most common case, there are three sections: the "text
section", which holds instructions and read-only data; the "data
section", which holds initialized writable data; and the "bss section",
which holds uninitialized data.  Some systems have other kinds of
sections.
   The compiler must tell the assembler when to switch sections.  These
macros control what commands to output to tell the assembler this.  You
can also define additional sections.
`TEXT_SECTION_ASM_OP'
     A C expression whose value is a string containing the assembler
     operation that should precede instructions and read-only data.
     Normally `".text"' is right.
`DATA_SECTION_ASM_OP'
     A C expression whose value is a string containing the assembler
     operation to identify the following data as writable initialized
     data.  Normally `".data"' is right.
`SHARED_SECTION_ASM_OP'
     if defined, a C expression whose value is a string containing the
     assembler operation to identify the following data as shared data.
     If not defined, `DATA_SECTION_ASM_OP' will be used.
`INIT_SECTION_ASM_OP'
     if defined, a C expression whose value is a string containing the
     assembler operation to identify the following data as
     initialization code.  If not defined, GNU CC will assume such a
     section does not exist.
`EXTRA_SECTIONS'
     A list of names for sections other than the standard two, which are
     `in_text' and `in_data'.  You need not define this macro on a
     system with no other sections (that GCC needs to use).
`EXTRA_SECTION_FUNCTIONS'
     One or more functions to be defined in `varasm.c'.  These
     functions should do jobs analogous to those of `text_section' and
     `data_section', for your additional sections.  Do not define this
     macro if you do not define `EXTRA_SECTIONS'.
`READONLY_DATA_SECTION'
     On most machines, read-only variables, constants, and jump tables
     are placed in the text section.  If this is not the case on your
     machine, this macro should be defined to be the name of a function
     (either `data_section' or a function defined in `EXTRA_SECTIONS')
     that switches to the section to be used for read-only items.
     If these items should be placed in the text section, this macro
     should not be defined.
`SELECT_SECTION (EXP, RELOC)'
     A C statement or statements to switch to the appropriate section
     for output of EXP.  You can assume that EXP is either a `VAR_DECL'
     node or a constant of some sort.  RELOC indicates whether the
     initial value of EXP requires link-time relocations.  Select the
     section by calling `text_section' or one of the alternatives for
     other sections.
     Do not define this macro if you put all read-only variables and
     constants in the read-only data section (usually the text section).
`SELECT_RTX_SECTION (MODE, RTX)'
     A C statement or statements to switch to the appropriate section
     for output of RTX in mode MODE.  You can assume that RTX is some
     kind of constant in RTL.  The argument MODE is redundant except in
     the case of a `const_int' rtx.  Select the section by calling
     `text_section' or one of the alternatives for other sections.
     Do not define this macro if you put all constants in the read-only
     data section.
`JUMP_TABLES_IN_TEXT_SECTION'
     Define this macro if jump tables (for `tablejump' insns) should be
     output in the text section, along with the assembler instructions.
     Otherwise, the readonly data section is used.
     This macro is irrelevant if there is no separate readonly data
     section.
`ENCODE_SECTION_INFO (DECL)'
     Define this macro if references to a symbol must be treated
     differently depending on something about the variable or function
     named by the symbol (such as what section it is in).
     The macro definition, if any, is executed immediately after the
     rtl for DECL has been created and stored in `DECL_RTL (DECL)'.
     The value of the rtl will be a `mem' whose address is a
     `symbol_ref'.
     The usual thing for this macro to do is to record a flag in the
     `symbol_ref' (such as `SYMBOL_REF_FLAG') or to store a modified
     name string in the `symbol_ref' (if one bit is not enough
     information).
`STRIP_NAME_ENCODING (VAR, SYM_NAME)'
     Decode SYM_NAME and store the real name part in VAR, sans the
     characters that encode section info.  Define this macro if
     `ENCODE_SECTION_INFO' alters the symbol's name string.
File: gcc.info,  Node: PIC,  Next: Assembler Format,  Prev: Sections,  Up: Target Macros
Position Independent Code
=========================
   This section describes macros that help implement generation of
position independent code.  Simply defining these macros is not enough
to generate valid PIC; you must also add support to the macros
`GO_IF_LEGITIMATE_ADDRESS' and `PRINT_OPERAND_ADDRESS', as well as
`LEGITIMIZE_ADDRESS'.  You must modify the definition of `movsi' to do
something appropriate when the source operand contains a symbolic
address.  You may also need to alter the handling of switch statements
so that they use relative addresses.
`PIC_OFFSET_TABLE_REGNUM'
     The register number of the register used to address a table of
     static data addresses in memory.  In some cases this register is
     defined by a processor's "application binary interface" (ABI).
     When this macro is defined, RTL is generated for this register
     once, as with the stack pointer and frame pointer registers.  If
     this macro is not defined, it is up to the machine-dependent files
     to allocate such a register (if necessary).
`FINALIZE_PIC'
     By generating position-independent code, when two different
     programs (A and B) share a common library (libC.a), the text of
     the library can be shared whether or not the library is linked at
     the same address for both programs.  In some of these
     environments, position-independent code requires not only the use
     of different addressing modes, but also special code to enable the
     use of these addressing modes.
     The `FINALIZE_PIC' macro serves as a hook to emit these special
     codes once the function is being compiled into assembly code, but
     not before.  (It is not done before, because in the case of
     compiling an inline function, it would lead to multiple PIC
     prologues being included in functions which used inline functions
     and were compiled to assembly language.)
`LEGITIMATE_PIC_OPERAND_P (X)'
     A C expression that is nonzero if X is a legitimate immediate
     operand on the target machine when generating position independent
     code.  You can assume that X satisfies `CONSTANT_P', so you need
     not check this.  You can also assume FLAG_PIC is true, so you need
     not check it either.  You need not define this macro if all
     constants (including `SYMBOL_REF') can be immediate operands when
     generating position independent code.
File: gcc.info,  Node: Assembler Format,  Next: Debugging Info,  Prev: PIC,  Up: Target Macros
Defining the Output Assembler Language
======================================
   This section describes macros whose principal purpose is to describe
how to write instructions in assembler language-rather than what the
instructions do.
* Menu:
* File Framework::       Structural information for the assembler file.
* Data Output::          Output of constants (numbers, strings, addresses).
* Uninitialized Data::   Output of uninitialized variables.
* Label Output::         Output and generation of labels.
* Initialization::       General principles of initialization
               and termination routines.
* Macros for Initialization::
             Specific macros that control the handling of
               initialization and termination routines.
* Instruction Output::   Output of actual instructions.
* Dispatch Tables::      Output of jump tables.
* Alignment Output::     Pseudo ops for alignment and skipping data.
File: gcc.info,  Node: File Framework,  Next: Data Output,  Up: Assembler Format
The Overall Framework of an Assembler File
------------------------------------------
`ASM_FILE_START (STREAM)'
     A C expression which outputs to the stdio stream STREAM some
     appropriate text to go at the start of an assembler file.
     Normally this macro is defined to output a line containing
     `#NO_APP', which is a comment that has no effect on most
     assemblers but tells the GNU assembler that it can save time by not
     checking for certain assembler constructs.
     On systems that use SDB, it is necessary to output certain
     commands; see `attasm.h'.
`ASM_FILE_END (STREAM)'
     A C expression which outputs to the stdio stream STREAM some
     appropriate text to go at the end of an assembler file.
     If this macro is not defined, the default is to output nothing
     special at the end of the file.  Most systems don't require any
     definition.
     On systems that use SDB, it is necessary to output certain
     commands; see `attasm.h'.
`ASM_IDENTIFY_GCC (FILE)'
     A C statement to output assembler commands which will identify the
     object file as having been compiled with GNU CC (or another GNU
     compiler).
     If you don't define this macro, the string `gcc_compiled.:' is
     output.  This string is calculated to define a symbol which, on
     BSD systems, will never be defined for any other reason.  GDB
     checks for the presence of this symbol when reading the symbol
     table of an executable.
     On non-BSD systems, you must arrange communication with GDB in
     some other fashion.  If GDB is not used on your system, you can
     define this macro with an empty body.
`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.
`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.
`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.
`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.
`ASM_OUTPUT_SOURCE_LINE (STREAM, LINE)'
     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.
     This macro need not be defined if the standard form of debugging
     information for the debugger in use is appropriate.
`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.
`OBJC_PROLOGUE'
     A C statement to output any assembler statements which are
     required to precede any Objective C object definitions or message
     sending.  The statement is executed only when compiling an
     Objective C program.
File: gcc.info,  Node: Data Output,  Next: Uninitialized Data,  Prev: File Framework,  Up: Assembler Format
Output of Data
--------------
`ASM_OUTPUT_LONG_DOUBLE (STREAM, VALUE)'
`ASM_OUTPUT_DOUBLE (STREAM, VALUE)'
`ASM_OUTPUT_FLOAT (STREAM, VALUE)'
     A C statement to output to the stdio stream STREAM an assembler
     instruction to assemble a floating-point constant of `TFmode',
     `DFmode' or `SFmode', respectively, whose value is VALUE.  VALUE
     will be a C expression of type `REAL_VALUE_TYPE'.  Macros such as
     `REAL_VALUE_TO_TARGET_DOUBLE' are useful for writing these
     definitions.
`ASM_OUTPUT_QUADRUPLE_INT (STREAM, EXP)'
`ASM_OUTPUT_DOUBLE_INT (STREAM, EXP)'
`ASM_OUTPUT_INT (STREAM, EXP)'
`ASM_OUTPUT_SHORT (STREAM, EXP)'
`ASM_OUTPUT_CHAR (STREAM, EXP)'
     A C statement to output to the stdio stream STREAM an assembler
     instruction to assemble an integer of 16, 8, 4, 2 or 1 bytes,
     respectively, whose value is VALUE.  The argument EXP will be an
     RTL expression which represents a constant value.  Use
     `output_addr_const (STREAM, EXP)' to output this value as an
     assembler expression.
     For sizes larger than `UNITS_PER_WORD', if the action of a macro
     would be identical to repeatedly calling the macro corresponding to
     a size of `UNITS_PER_WORD', once for each word, you need not define
     the macro.
`ASM_OUTPUT_BYTE (STREAM, VALUE)'
     A C statement to output to the stdio stream STREAM an assembler
     instruction to assemble a single byte containing the number VALUE.
`ASM_BYTE_OP'
     A C string constant giving the pseudo-op to use for a sequence of
     single-byte constants.  If this macro is not defined, the default
     is `"byte"'.
`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'.
`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.
`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:
          ASM_OUTPUT_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.
`ASM_OPEN_PAREN'
`ASM_CLOSE_PAREN'
     These macros are defined as C string constant, describing the
     syntax in the assembler for grouping arithmetic expressions.  The
     following definitions are correct for most assemblers:
          #define ASM_OPEN_PAREN "("
          #define ASM_CLOSE_PAREN ")"
   These macros are provided by `real.h' for writing the definitions of
`ASM_OUTPUT_DOUBLE' and the like:
`REAL_VALUE_TO_TARGET_SINGLE (X, L)'
`REAL_VALUE_TO_TARGET_DOUBLE (X, L)'
`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
     array of `long int' whose address is L.  The number of elements in
     the output 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.
`REAL_VALUE_TO_DECIMAL (X, FORMAT, STRING)'
     This macro converts X, of type `REAL_VALUE_TYPE', to a decimal
     number and stores it as a string into STRING.  You must pass, as
     STRING, the address of a long enough block of space to hold the
     result.
     The argument FORMAT is a `printf'-specification that serves as a
     suggestion for how to format the output string.
File: gcc.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.
`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
     global variables are output.
`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.
`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.
`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.
`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.
`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: gcc.info,  Node: Label Output,  Next: Initialization,  Prev: Uninitialized Data,  Up: Assembler Format
Output and Generation of 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_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_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_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_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.
`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.
File: gcc.info,  Node: Initialization,  Next: Macros for Initialization,  Prev: Label Output,  Up: Assembler Format
How Initialization Functions Are Handled
----------------------------------------
   The compiled code for certain languages includes "constructors"
(also called "initialization routines")--functions to initialize data
in the program when the program is started.  These functions need to be
called before the program is "started"--that is to say, before `main'
is called.
   Compiling some languages generates "destructors" (also called
"termination routines") that should be called when the program
terminates.
   To make the initialization and termination functions work, the
compiler must output something in the assembler code to cause those
functions to be called at the appropriate time.  When you port the
compiler to a new system, you need to specify how to do this.
   There are two major ways that GCC currently supports the execution of
initialization and termination functions.  Each way has two variants.
Much of the structure is common to all four variations.
   The linker must build two lists of these functions--a list of
initialization functions, called `__CTOR_LIST__', and a list of
termination functions, called `__DTOR_LIST__'.
   Each list always begins with an ignored function pointer (which may
hold 0, -1, or a count of the function pointers after it, depending on
the environment).  This is followed by a series of zero or more function
pointers to constructors (or destructors), followed by a function
pointer containing zero.
   Depending on the operating system and its executable file format,
either `crtstuff.c' or `libgcc2.c' traverses these lists at startup
time and exit time.  Constructors are called in forward order of the
list; destructors in reverse order.
   The best way to handle static constructors works only for object file
formats which provide arbitrarily-named sections.  A section is set
aside for a list of constructors, and another for a list of destructors.
Traditionally these are called `.ctors' and `.dtors'.  Each object file
that defines an initialization function also puts a word in the
constructor section to point to that function.  The linker accumulates
all these words into one contiguous `.ctors' section.  Termination
functions are handled similarly.
   To use this method, you need appropriate definitions of the macros
`ASM_OUTPUT_CONSTRUCTOR' and `ASM_OUTPUT_DESTRUCTOR'.  Usually you can
get them by including `svr4.h'.
   When arbitrary sections are available, there are two variants,
depending upon how the code in `crtstuff.c' is called.  On systems that
support an "init" section which is executed at program startup, parts
of `crtstuff.c' are compiled into that section.  The program is linked
by the `gcc' driver like this:
     ld -o OUTPUT_FILE crtbegin.o ... crtend.o -lgcc
   The head of a function (`__do_global_ctors') appears in the init
section of `crtbegin.o'; the remainder of the function appears in the
init section of `crtend.o'.  The linker will pull these two parts of
the section together, making a whole function.  If any of the user's
object files linked into the middle of it contribute code, then that
code will be executed as part of the body of `__do_global_ctors'.
   To use this variant, you must define the `INIT_SECTION_ASM_OP' macro
properly.
   If no init section is available, do not define
`INIT_SECTION_ASM_OP'.  Then `__do_global_ctors' is built into the text
section like all other functions, and resides in `libgcc.a'.  When GCC
compiles any function called `main', it inserts a procedure call to
`__main' as the first executable code after the function prologue.  The
`__main' function, also defined in `libgcc2.c', simply calls
`__do_global_ctors'.
   In file formats that don't support arbitrary sections, there are
again two variants.  In the simplest variant, the GNU linker (GNU `ld')
and an `a.out' format must be used.  In this case,
`ASM_OUTPUT_CONSTRUCTOR' is defined to produce a `.stabs' entry of type
`N_SETT', referencing the name `__CTOR_LIST__', and with the address of
the void function containing the initialization code as its value.  The
GNU linker recognizes this as a request to add the value to a "set";
the values are accumulated, and are eventually placed in the executable
as a vector in the format described above, with a leading (ignored)
count and a trailing zero element.  `ASM_OUTPUT_DESTRUCTOR' is handled
similarly.  Since no init section is available, the absence of
`INIT_SECTION_ASM_OP' causes the compilation of `main' to call `__main'
as above, starting the initialization process.
   The last variant uses neither arbitrary sections nor the GNU linker.
This is preferable when you want to do dynamic linking and when using
file formats which the GNU linker does not support, such as `ECOFF'.  In
this case, `ASM_OUTPUT_CONSTRUCTOR' does not produce an `N_SETT'
symbol; initialization and termination functions are recognized simply
by their names.  This requires an extra program in the linkage step,
called `collect2'.  This program pretends to be the linker, for use
with GNU CC; it does its job by running the ordinary linker, but also
arranges to include the vectors of initialization and termination
functions.  These functions are called via `__main' as described above.
   Choosing among these configuration options has been simplified by a
set of operating-system-dependent files in the `config' subdirectory.
These files define all of the relevant parameters.  Usually it is
sufficient to include one into your specific machine-dependent
configuration file.  These files are:
`aoutos.h'
     For operating systems using the `a.out' format.
`next.h'
     For operating systems using the `MachO' format.
`svr3.h'
     For System V Release 3 and similar systems using `COFF' format.
`svr4.h'
     For System V Release 4 and similar systems using `ELF' format.
`vms.h'
     For the VMS operating system.
   The following section describes the specific macros that control and
customize the handling of initialization and termination functions.