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GNU Info File | 1994-02-07 | 44.1 KB | 986 lines

This is Info file gcc.info, produced by Makeinfo-1.49 from the input file gcc.texi. This file documents the use and the internals of the GNU compiler. Copyright (C) 1988, 1989, 1992 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the sections entitled "GNU General Public License" and "Protect Your Freedom--Fight `Look And Feel'" are included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the sections entitled "GNU General Public License" and "Protect Your Freedom--Fight `Look And Feel'", and this permission notice, may be included in translations approved by the Free Software Foundation instead of in the original English. 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 2 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 `LEGITIMIZE_ADDRESS', and `PRINT_OPERAND_ADDRESS' as well. 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', usually `double'. `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 ")" 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_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_LABELREF_AS_INT (FILE, LABEL)' Define this macro for systems that use the program `collect2'. The definition should be a C statement to output a word containing a reference to the label LABEL. `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 same 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. This string will be used as the 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. File: gcc.info, Node: Macros for Initialization, Next: Instruction Output, Prev: Initialization, Up: Assembler Format Macros Controlling Initialization Routines ------------------------------------------ Here are the macros that control how the compiler handles initialization and termination functions: `INIT_SECTION_ASM_OP' If defined, a C string constant for the assembler operation to identify the following data as initialization code. If not defined, GNU CC will assume such a section does not exist. When you are using special sections for initialization and termination functions, this macro also controls how `crtstuff.c' and `libgcc2.c' arrange to run the initialization functions. `ASM_OUTPUT_CONSTRUCTOR (STREAM, NAME)' Define this macro as a C statement to output on the stream STREAM the assembler code to arrange to call the function named NAME at initialization time. Assume that NAME is the name of a C function generated automatically by the compiler. This function takes no arguments. Use the function `assemble_name' to output the name NAME; this performs any system-specific syntactic transformations such as adding an underscore. If you don't define this macro, nothing special is output to arrange to call the function. This is correct when the function will be called in some other manner--for example, by means of the `collect2' program, which looks through the symbol table to find these functions by their names. If you want to use `collect2', then you need to arrange for it to be built and installed and used on your system. `ASM_OUTPUT_DESTRUCTOR (STREAM, NAME)' This is like `ASM_OUTPUT_CONSTRUCTOR' but used for termination functions rather than initialization functions. If your system uses `collect2' as the means of processing constructors, then that program normally uses `nm' to scan an object file for constructor functions to be called. On certain kinds of systems, you can define these macros to make `collect2' work faster (and, in some cases, make it work at all): `OBJECT_FORMAT_COFF' Define this macro if the system uses COFF (Common Object File Format) object files, so that `collect2' can assume this format and scan object files directly for dynamic constructor/destructor functions. `OBJECT_FORMAT_ROSE' Define this macro if the system uses ROSE format object files, so that `collect2' can assume this format and scan object files directly for dynamic constructor/destructor functions. These macros are effective only in a native compiler; `collect2' as part of a cross compiler always uses `nm'. `REAL_NM_FILE_NAME' Define this macro as a C string constant containing the file name to use to execute `nm'. The default is to search the path normally for `nm'.