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* gcc: (gcc).                  The GNU Compiler Collection.
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   This file documents the use and the internals of the GNU compiler.


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File: gcc.info,  Node: Function Entry,  Next: Profiling,  Prev: Caller Saves,  Up: Stack and Calling

Function Entry and Exit
-----------------------

   This section describes the macros that output function entry
("prologue") and exit ("epilogue") code.

`FUNCTION_PROLOGUE (FILE, SIZE)'
     A C compound statement that outputs the assembler code for entry
     to a function.  The prologue is responsible for setting up the
     stack frame, initializing the frame pointer register, saving
     registers that must be saved, and allocating SIZE additional bytes
     of storage for the local variables.  SIZE is an integer.  FILE is
     a stdio stream to which the assembler code should be output.

     The label for the beginning of the function need not be output by
     this macro.  That has already been done when the macro is run.

     To determine which registers to save, the macro can refer to the
     array `regs_ever_live': element R is nonzero if hard register R is
     used anywhere within the function.  This implies the function
     prologue should save register R, provided it is not one of the
     call-used registers.  (`FUNCTION_EPILOGUE' must likewise use
     `regs_ever_live'.)

     On machines that have "register windows", the function entry code
     does not save on the stack the registers that are in the windows,
     even if they are supposed to be preserved by function calls;
     instead it takes appropriate steps to "push" the register stack,
     if any non-call-used registers are used in the function.

     On machines where functions may or may not have frame-pointers, the
     function entry code must vary accordingly; it must set up the frame
     pointer if one is wanted, and not otherwise.  To determine whether
     a frame pointer is in wanted, the macro can refer to the variable
     `frame_pointer_needed'.  The variable's value will be 1 at run
     time in a function that needs a frame pointer.  *Note
     Elimination::.

     The function entry code is responsible for allocating any stack
     space required for the function.  This stack space consists of the
     regions listed below.  In most cases, these regions are allocated
     in the order listed, with the last listed region closest to the
     top of the stack (the lowest address if `STACK_GROWS_DOWNWARD' is
     defined, and the highest address if it is not defined).  You can
     use a different order for a machine if doing so is more convenient
     or required for compatibility reasons.  Except in cases where
     required by standard or by a debugger, there is no reason why the
     stack layout used by GCC need agree with that used by other
     compilers for a machine.

        * A region of `current_function_pretend_args_size' bytes of
          uninitialized space just underneath the first argument
          arriving on the stack.  (This may not be at the very start of
          the allocated stack region if the calling sequence has pushed
          anything else since pushing the stack arguments.  But
          usually, on such machines, nothing else has been pushed yet,
          because the function prologue itself does all the pushing.)
          This region is used on machines where an argument may be
          passed partly in registers and partly in memory, and, in some
          cases to support the features in `<varargs.h>' and
          `<stdarg.h>'.

        * An area of memory used to save certain registers used by the
          function.  The size of this area, which may also include
          space for such things as the return address and pointers to
          previous stack frames, is machine-specific and usually
          depends on which registers have been used in the function.
          Machines with register windows often do not require a save
          area.

        * A region of at least SIZE bytes, possibly rounded up to an
          allocation boundary, to contain the local variables of the
          function.  On some machines, this region and the save area
          may occur in the opposite order, with the save area closer to
          the top of the stack.

        * Optionally, when `ACCUMULATE_OUTGOING_ARGS' is defined, a
          region of `current_function_outgoing_args_size' bytes to be
          used for outgoing argument lists of the function.  *Note
          Stack Arguments::.

     Normally, it is necessary for the macros `FUNCTION_PROLOGUE' and
     `FUNCTION_EPILOGUE' to treat leaf functions specially.  The C
     variable `current_function_is_leaf' is nonzero for such a function.

`EXIT_IGNORE_STACK'
     Define this macro as a C expression that is nonzero if the return
     instruction or the function epilogue ignores the value of the stack
     pointer; in other words, if it is safe to delete an instruction to
     adjust the stack pointer before a return from the function.

     Note that this macro's value is relevant only for functions for
     which frame pointers are maintained.  It is never safe to delete a
     final stack adjustment in a function that has no frame pointer,
     and the compiler knows this regardless of `EXIT_IGNORE_STACK'.

`EPILOGUE_USES (REGNO)'
     Define this macro as a C expression that is nonzero for registers
     that are used by the epilogue or the `return' pattern.  The stack
     and frame pointer registers are already be assumed to be used as
     needed.

`FUNCTION_EPILOGUE (FILE, SIZE)'
     A C compound statement that outputs the assembler code for exit
     from a function.  The epilogue is responsible for restoring the
     saved registers and stack pointer to their values when the
     function was called, and returning control to the caller.  This
     macro takes the same arguments as the macro `FUNCTION_PROLOGUE',
     and the registers to restore are determined from `regs_ever_live'
     and `CALL_USED_REGISTERS' in the same way.

     On some machines, there is a single instruction that does all the
     work of returning from the function.  On these machines, give that
     instruction the name `return' and do not define the macro
     `FUNCTION_EPILOGUE' at all.

     Do not define a pattern named `return' if you want the
     `FUNCTION_EPILOGUE' to be used.  If you want the target switches
     to control whether return instructions or epilogues are used,
     define a `return' pattern with a validity condition that tests the
     target switches appropriately.  If the `return' pattern's validity
     condition is false, epilogues will be used.

     On machines where functions may or may not have frame-pointers, the
     function exit code must vary accordingly.  Sometimes the code for
     these two cases is completely different.  To determine whether a
     frame pointer is wanted, the macro can refer to the variable
     `frame_pointer_needed'.  The variable's value will be 1 when
     compiling a function that needs a frame pointer.

     Normally, `FUNCTION_PROLOGUE' and `FUNCTION_EPILOGUE' must treat
     leaf functions specially.  The C variable
     `current_function_is_leaf' is nonzero for such a function.  *Note
     Leaf Functions::.

     On some machines, some functions pop their arguments on exit while
     others leave that for the caller to do.  For example, the 68020
     when given `-mrtd' pops arguments in functions that take a fixed
     number of arguments.

     Your definition of the macro `RETURN_POPS_ARGS' decides which
     functions pop their own arguments.  `FUNCTION_EPILOGUE' needs to
     know what was decided.  The variable that is called
     `current_function_pops_args' is the number of bytes of its
     arguments that a function should pop.  *Note Scalar Return::.

`DELAY_SLOTS_FOR_EPILOGUE'
     Define this macro if the function epilogue contains delay slots to
     which instructions from the rest of the function can be "moved".
     The definition should be a C expression whose value is an integer
     representing the number of delay slots there.

`ELIGIBLE_FOR_EPILOGUE_DELAY (INSN, N)'
     A C expression that returns 1 if INSN can be placed in delay slot
     number N of the epilogue.

     The argument N is an integer which identifies the delay slot now
     being considered (since different slots may have different rules of
     eligibility).  It is never negative and is always less than the
     number of epilogue delay slots (what `DELAY_SLOTS_FOR_EPILOGUE'
     returns).  If you reject a particular insn for a given delay slot,
     in principle, it may be reconsidered for a subsequent delay slot.
     Also, other insns may (at least in principle) be considered for
     the so far unfilled delay slot.

     The insns accepted to fill the epilogue delay slots are put in an
     RTL list made with `insn_list' objects, stored in the variable
     `current_function_epilogue_delay_list'.  The insn for the first
     delay slot comes first in the list.  Your definition of the macro
     `FUNCTION_EPILOGUE' should fill the delay slots by outputting the
     insns in this list, usually by calling `final_scan_insn'.

     You need not define this macro if you did not define
     `DELAY_SLOTS_FOR_EPILOGUE'.

`ASM_OUTPUT_MI_THUNK (FILE, THUNK_FNDECL, DELTA, FUNCTION)'
     A C compound statement that outputs the assembler code for a thunk
     function, used to implement C++ virtual function calls with
     multiple inheritance.  The thunk acts as a wrapper around a
     virtual function, adjusting the implicit object parameter before
     handing control off to the real function.

     First, emit code to add the integer DELTA to the location that
     contains the incoming first argument.  Assume that this argument
     contains a pointer, and is the one used to pass the `this' pointer
     in C++.  This is the incoming argument _before_ the function
     prologue, e.g. `%o0' on a sparc.  The addition must preserve the
     values of all other incoming arguments.

     After the addition, emit code to jump to FUNCTION, which is a
     `FUNCTION_DECL'.  This is a direct pure jump, not a call, and does
     not touch the return address.  Hence returning from FUNCTION will
     return to whoever called the current `thunk'.

     The effect must be as if FUNCTION had been called directly with
     the adjusted first argument.  This macro is responsible for
     emitting all of the code for a thunk function; `FUNCTION_PROLOGUE'
     and `FUNCTION_EPILOGUE' are not invoked.

     The THUNK_FNDECL is redundant.  (DELTA and FUNCTION have already
     been extracted from it.)  It might possibly be useful on some
     targets, but probably not.

     If you do not define this macro, the target-independent code in
     the C++ front end will generate a less efficient heavyweight thunk
     that calls FUNCTION instead of jumping to it.  The generic
     approach does not support varargs.


File: gcc.info,  Node: Profiling,  Next: Inlining,  Prev: Function Entry,  Up: Stack and Calling

Generating Code for Profiling
-----------------------------

   These macros will help you generate code for profiling.

`FUNCTION_PROFILER (FILE, LABELNO)'
     A C statement or compound statement to output to FILE some
     assembler code to call the profiling subroutine `mcount'.

     The details of how `mcount' expects to be called are determined by
     your operating system environment, not by GCC.  To figure them out,
     compile a small program for profiling using the system's installed
     C compiler and look at the assembler code that results.

     Older implementations of `mcount' expect the address of a counter
     variable to be loaded into some register.  The name of this
     variable is `LP' followed by the number LABELNO, so you would
     generate the name using `LP%d' in a `fprintf'.

`PROFILE_HOOK'
     A C statement or compound statement to output to FILE some assembly
     code to call the profiling subroutine `mcount' even the target does
     not support profiling.

`NO_PROFILE_COUNTERS'
     Define this macro if the `mcount' subroutine on your system does
     not need a counter variable allocated for each function.  This is
     true for almost all modern implementations.  If you define this
     macro, you must not use the LABELNO argument to
     `FUNCTION_PROFILER'.

`PROFILE_BEFORE_PROLOGUE'
     Define this macro if the code for function profiling should come
     before the function prologue.  Normally, the profiling code comes
     after.

`FUNCTION_BLOCK_PROFILER (FILE, LABELNO)'
     A C statement or compound statement to output to FILE some
     assembler code to initialize basic-block profiling for the current
     object module.  The global compile flag `profile_block_flag'
     distinguishes two profile modes.

    `profile_block_flag != 2'
          Output code to call the subroutine `__bb_init_func' once per
          object module, passing it as its sole argument the address of
          a block allocated in the object module.

          The name of the block is a local symbol made with this
          statement:

               ASM_GENERATE_INTERNAL_LABEL (BUFFER, "LPBX", 0);

          Of course, since you are writing the definition of
          `ASM_GENERATE_INTERNAL_LABEL' as well as that of this macro,
          you can take a short cut in the definition of this macro and
          use the name that you know will result.

          The first word of this block is a flag which will be nonzero
          if the object module has already been initialized.  So test
          this word first, and do not call `__bb_init_func' if the flag
          is nonzero.  LABELNO contains a unique number which may be
          used to generate a label as a branch destination when
          `__bb_init_func' will not be called.

          Described in assembler language, the code to be output looks
          like:

                 cmp (LPBX0),0
                 bne local_label
                 parameter1 <- LPBX0
                 call __bb_init_func
               local_label:

    `profile_block_flag == 2'
          Output code to call the subroutine `__bb_init_trace_func' and
          pass two parameters to it.  The first parameter is the same as
          for `__bb_init_func'.  The second parameter is the number of
          the first basic block of the function as given by LABELNO.
          Note that `__bb_init_trace_func' has to be called, even if
          the object module has been initialized already.

          Described in assembler language, the code to be output looks
          like:
               parameter1 <- LPBX0
               parameter2 <- LABELNO
               call __bb_init_trace_func

`BLOCK_PROFILER (FILE, BLOCKNO)'
     A C statement or compound statement to output to FILE some
     assembler code to increment the count associated with the basic
     block number BLOCKNO.  The global compile flag
     `profile_block_flag' distinguishes two profile modes.

    `profile_block_flag != 2'
          Output code to increment the counter directly.  Basic blocks
          are numbered separately from zero within each compilation.
          The count associated with block number BLOCKNO is at index
          BLOCKNO in a vector of words; the name of this array is a
          local symbol made with this statement:

               ASM_GENERATE_INTERNAL_LABEL (BUFFER, "LPBX", 2);

          Of course, since you are writing the definition of
          `ASM_GENERATE_INTERNAL_LABEL' as well as that of this macro,
          you can take a short cut in the definition of this macro and
          use the name that you know will result.

          Described in assembler language, the code to be output looks
          like:

               inc (LPBX2+4*BLOCKNO)

    `profile_block_flag == 2'
          Output code to initialize the global structure `__bb' and
          call the function `__bb_trace_func', which will increment the
          counter.

          `__bb' consists of two words.  In the first word, the current
          basic block number, as given by BLOCKNO, has to be stored.  In
          the second word, the address of a block allocated in the
          object module has to be stored.  The address is given by the
          label created with this statement:

               ASM_GENERATE_INTERNAL_LABEL (BUFFER, "LPBX", 0);

          Described in assembler language, the code to be output looks
          like:
               move BLOCKNO -> (__bb)
               move LPBX0 -> (__bb+4)
               call __bb_trace_func

`FUNCTION_BLOCK_PROFILER_EXIT (FILE)'
     A C statement or compound statement to output to FILE assembler
     code to call function `__bb_trace_ret'.  The assembler code should
     only be output if the global compile flag `profile_block_flag' ==
     2.  This macro has to be used at every place where code for
     returning from a function is generated (e.g. `FUNCTION_EPILOGUE').
     Although you have to write the definition of `FUNCTION_EPILOGUE'
     as well, you have to define this macro to tell the compiler, that
     the proper call to `__bb_trace_ret' is produced.

`MACHINE_STATE_SAVE (ID)'
     A C statement or compound statement to save all registers, which
     may be clobbered by a function call, including condition codes.
     The `asm' statement will be mostly likely needed to handle this
     task.  Local labels in the assembler code can be concatenated with
     the string ID, to obtain a unique label name.

     Registers or condition codes clobbered by `FUNCTION_PROLOGUE' or
     `FUNCTION_EPILOGUE' must be saved in the macros
     `FUNCTION_BLOCK_PROFILER', `FUNCTION_BLOCK_PROFILER_EXIT' and
     `BLOCK_PROFILER' prior calling `__bb_init_trace_func',
     `__bb_trace_ret' and `__bb_trace_func' respectively.

`MACHINE_STATE_RESTORE (ID)'
     A C statement or compound statement to restore all registers,
     including condition codes, saved by `MACHINE_STATE_SAVE'.

     Registers or condition codes clobbered by `FUNCTION_PROLOGUE' or
     `FUNCTION_EPILOGUE' must be restored in the macros
     `FUNCTION_BLOCK_PROFILER', `FUNCTION_BLOCK_PROFILER_EXIT' and
     `BLOCK_PROFILER' after calling `__bb_init_trace_func',
     `__bb_trace_ret' and `__bb_trace_func' respectively.

`BLOCK_PROFILER_CODE'
     A C function or functions which are needed in the library to
     support block profiling.

`TARGET_ALLOWS_PROFILING_WITHOUT_FRAME_POINTER'
     On some targets, it is impossible to use profiling when the frame
     pointer has been omitted.  For example, on x86 GNU/Linux systems,
     the `mcount' routine provided by the GNU C Library finds the
     address of the routine that called the routine that called `mcount'
     by looking in the immediate caller's stack frame.  If the immediate
     caller has no frame pointer, this lookup will fail.

     By default, GCC assumes that the target does allow profiling when
     the frame pointer is omitted.  This macro should be defined to a C
     expression that evaluates to `false' if the target does not allow
     profiling when the frame pointer is omitted.


File: gcc.info,  Node: Inlining,  Next: Tail Calling,  Prev: Profiling,  Up: Stack and Calling

Permitting inlining of functions with attributes
------------------------------------------------

   By default if a function has a target specific attribute attached to
it, it will not be inlined.  This behaviour can be overridden if the
target defines the `FUNCTION_ATTRIBUTE_INLINABLE_P' macro.  This macro
takes one argument, a `DECL' describing the function.  It should return
nonzero if the function can be inlined, otherwise it should return 0.


File: gcc.info,  Node: Tail Calling,  Prev: Inlining,  Up: Stack and Calling

Permitting tail calls to functions
----------------------------------

`FUNCTION_OK_FOR_SIBCALL (DECL)'
     A C expression that evaluates to true if it is ok to perform a
     sibling call to DECL.

     It is not uncommon for limitations of calling conventions to
     prevent tail calls to functions outside the current unit of
     translation, or during PIC compilation.  Use this macro to enforce
     these restrictions, as the `sibcall' md pattern can not fail, or
     fall over to a "normal" call.


File: gcc.info,  Node: Varargs,  Next: Trampolines,  Prev: Stack and Calling,  Up: Target Macros

Implementing the Varargs Macros
===============================

   GCC comes with an implementation of `<varargs.h>' and `<stdarg.h>'
that work without change on machines that pass arguments on the stack.
Other machines require their own implementations of varargs, and the
two machine independent header files must have conditionals to include
it.

   ISO `<stdarg.h>' differs from traditional `<varargs.h>' mainly in
the calling convention for `va_start'.  The traditional implementation
takes just one argument, which is the variable in which to store the
argument pointer.  The ISO implementation of `va_start' takes an
additional second argument.  The user is supposed to write the last
named argument of the function here.

   However, `va_start' should not use this argument.  The way to find
the end of the named arguments is with the built-in functions described
below.

`__builtin_saveregs ()'
     Use this built-in function to save the argument registers in
     memory so that the varargs mechanism can access them.  Both ISO
     and traditional versions of `va_start' must use
     `__builtin_saveregs', unless you use `SETUP_INCOMING_VARARGS' (see
     below) instead.

     On some machines, `__builtin_saveregs' is open-coded under the
     control of the macro `EXPAND_BUILTIN_SAVEREGS'.  On other machines,
     it calls a routine written in assembler language, found in
     `libgcc2.c'.

     Code generated for the call to `__builtin_saveregs' appears at the
     beginning of the function, as opposed to where the call to
     `__builtin_saveregs' is written, regardless of what the code is.
     This is because the registers must be saved before the function
     starts to use them for its own purposes.

`__builtin_args_info (CATEGORY)'
     Use this built-in function to find the first anonymous arguments in
     registers.

     In general, a machine may have several categories of registers
     used for arguments, each for a particular category of data types.
     (For example, on some machines, floating-point registers are used
     for floating-point arguments while other arguments are passed in
     the general registers.)  To make non-varargs functions use the
     proper calling convention, you have defined the `CUMULATIVE_ARGS'
     data type to record how many registers in each category have been
     used so far

     `__builtin_args_info' accesses the same data structure of type
     `CUMULATIVE_ARGS' after the ordinary argument layout is finished
     with it, with CATEGORY specifying which word to access.  Thus, the
     value indicates the first unused register in a given category.

     Normally, you would use `__builtin_args_info' in the implementation
     of `va_start', accessing each category just once and storing the
     value in the `va_list' object.  This is because `va_list' will
     have to update the values, and there is no way to alter the values
     accessed by `__builtin_args_info'.

`__builtin_next_arg (LASTARG)'
     This is the equivalent of `__builtin_args_info', for stack
     arguments.  It returns the address of the first anonymous stack
     argument, as type `void *'.  If `ARGS_GROW_DOWNWARD', it returns
     the address of the location above the first anonymous stack
     argument.  Use it in `va_start' to initialize the pointer for
     fetching arguments from the stack.  Also use it in `va_start' to
     verify that the second parameter LASTARG is the last named argument
     of the current function.

`__builtin_classify_type (OBJECT)'
     Since each machine has its own conventions for which data types are
     passed in which kind of register, your implementation of `va_arg'
     has to embody these conventions.  The easiest way to categorize the
     specified data type is to use `__builtin_classify_type' together
     with `sizeof' and `__alignof__'.

     `__builtin_classify_type' ignores the value of OBJECT, considering
     only its data type.  It returns an integer describing what kind of
     type that is--integer, floating, pointer, structure, and so on.

     The file `typeclass.h' defines an enumeration that you can use to
     interpret the values of `__builtin_classify_type'.

   These machine description macros help implement varargs:

`EXPAND_BUILTIN_SAVEREGS ()'
     If defined, is a C expression that produces the machine-specific
     code for a call to `__builtin_saveregs'.  This code will be moved
     to the very beginning of the function, before any parameter access
     are made.  The return value of this function should be an RTX that
     contains the value to use as the return of `__builtin_saveregs'.

`SETUP_INCOMING_VARARGS (ARGS_SO_FAR, MODE, TYPE, PRETEND_ARGS_SIZE, SECOND_TIME)'
     This macro offers an alternative to using `__builtin_saveregs' and
     defining the macro `EXPAND_BUILTIN_SAVEREGS'.  Use it to store the
     anonymous register arguments into the stack so that all the
     arguments appear to have been passed consecutively on the stack.
     Once this is done, you can use the standard implementation of
     varargs that works for machines that pass all their arguments on
     the stack.

     The argument ARGS_SO_FAR is the `CUMULATIVE_ARGS' data structure,
     containing the values that are obtained after processing the named
     arguments.  The arguments MODE and TYPE describe the last named
     argument--its machine mode and its data type as a tree node.

     The macro implementation should do two things: first, push onto the
     stack all the argument registers _not_ used for the named
     arguments, and second, store the size of the data thus pushed into
     the `int'-valued variable whose name is supplied as the argument
     PRETEND_ARGS_SIZE.  The value that you store here will serve as
     additional offset for setting up the stack frame.

     Because you must generate code to push the anonymous arguments at
     compile time without knowing their data types,
     `SETUP_INCOMING_VARARGS' is only useful on machines that have just
     a single category of argument register and use it uniformly for
     all data types.

     If the argument SECOND_TIME is nonzero, it means that the
     arguments of the function are being analyzed for the second time.
     This happens for an inline function, which is not actually
     compiled until the end of the source file.  The macro
     `SETUP_INCOMING_VARARGS' should not generate any instructions in
     this case.

`STRICT_ARGUMENT_NAMING'
     Define this macro to be a nonzero value if the location where a
     function argument is passed depends on whether or not it is a
     named argument.

     This macro controls how the NAMED argument to `FUNCTION_ARG' is
     set for varargs and stdarg functions.  If this macro returns a
     nonzero value, the NAMED argument is always true for named
     arguments, and false for unnamed arguments.  If it returns a value
     of zero, but `SETUP_INCOMING_VARARGS' is defined, then all
     arguments are treated as named.  Otherwise, all named arguments
     except the last are treated as named.

     You need not define this macro if it always returns zero.

`PRETEND_OUTGOING_VARARGS_NAMED'
     If you need to conditionally change ABIs so that one works with
     `SETUP_INCOMING_VARARGS', but the other works like neither
     `SETUP_INCOMING_VARARGS' nor `STRICT_ARGUMENT_NAMING' was defined,
     then define this macro to return nonzero if
     `SETUP_INCOMING_VARARGS' is used, zero otherwise.  Otherwise, you
     should not define this macro.


File: gcc.info,  Node: Trampolines,  Next: Library Calls,  Prev: Varargs,  Up: Target Macros

Trampolines for Nested Functions
================================

   A "trampoline" is a small piece of code that is created at run time
when the address of a nested function is taken.  It normally resides on
the stack, in the stack frame of the containing function.  These macros
tell GCC how to generate code to allocate and initialize a trampoline.

   The instructions in the trampoline must do two things: load a
constant address into the static chain register, and jump to the real
address of the nested function.  On CISC machines such as the m68k,
this requires two instructions, a move immediate and a jump.  Then the
two addresses exist in the trampoline as word-long immediate operands.
On RISC machines, it is often necessary to load each address into a
register in two parts.  Then pieces of each address form separate
immediate operands.

   The code generated to initialize the trampoline must store the
variable parts--the static chain value and the function address--into
the immediate operands of the instructions.  On a CISC machine, this is
simply a matter of copying each address to a memory reference at the
proper offset from the start of the trampoline.  On a RISC machine, it
may be necessary to take out pieces of the address and store them
separately.

`TRAMPOLINE_TEMPLATE (FILE)'
     A C statement to output, on the stream FILE, assembler code for a
     block of data that contains the constant parts of a trampoline.
     This code should not include a label--the label is taken care of
     automatically.

     If you do not define this macro, it means no template is needed
     for the target.  Do not define this macro on systems where the
     block move code to copy the trampoline into place would be larger
     than the code to generate it on the spot.

`TRAMPOLINE_SECTION'
     The name of a subroutine to switch to the section in which the
     trampoline template is to be placed (*note Sections::).  The
     default is a value of `readonly_data_section', which places the
     trampoline in the section containing read-only data.

`TRAMPOLINE_SIZE'
     A C expression for the size in bytes of the trampoline, as an
     integer.

`TRAMPOLINE_ALIGNMENT'
     Alignment required for trampolines, in bits.

     If you don't define this macro, the value of `BIGGEST_ALIGNMENT'
     is used for aligning trampolines.

`INITIALIZE_TRAMPOLINE (ADDR, FNADDR, STATIC_CHAIN)'
     A C statement to initialize the variable parts of a trampoline.
     ADDR is an RTX for the address of the trampoline; FNADDR is an RTX
     for the address of the nested function; STATIC_CHAIN is an RTX for
     the static chain value that should be passed to the function when
     it is called.

`TRAMPOLINE_ADJUST_ADDRESS (ADDR)'
     A C statement that should perform any machine-specific adjustment
     in the address of the trampoline.  Its argument contains the
     address that was passed to `INITIALIZE_TRAMPOLINE'.  In case the
     address to be used for a function call should be different from
     the address in which the template was stored, the different
     address should be assigned to ADDR.  If this macro is not defined,
     ADDR will be used for function calls.

`ALLOCATE_TRAMPOLINE (FP)'
     A C expression to allocate run-time space for a trampoline.  The
     expression value should be an RTX representing a memory reference
     to the space for the trampoline.

     If this macro is not defined, by default the trampoline is
     allocated as a stack slot.  This default is right for most
     machines.  The exceptions are machines where it is impossible to
     execute instructions in the stack area.  On such machines, you may
     have to implement a separate stack, using this macro in
     conjunction with `FUNCTION_PROLOGUE' and `FUNCTION_EPILOGUE'.

     FP points to a data structure, a `struct function', which
     describes the compilation status of the immediate containing
     function of the function which the trampoline is for.  Normally
     (when `ALLOCATE_TRAMPOLINE' is not defined), the stack slot for the
     trampoline is in the stack frame of this containing function.
     Other allocation strategies probably must do something analogous
     with this information.

   Implementing trampolines is difficult on many machines because they
have separate instruction and data caches.  Writing into a stack
location fails to clear the memory in the instruction cache, so when
the program jumps to that location, it executes the old contents.

   Here are two possible solutions.  One is to clear the relevant parts
of the instruction cache whenever a trampoline is set up.  The other is
to make all trampolines identical, by having them jump to a standard
subroutine.  The former technique makes trampoline execution faster; the
latter makes initialization faster.

   To clear the instruction cache when a trampoline is initialized,
define the following macros which describe the shape of the cache.

`INSN_CACHE_SIZE'
     The total size in bytes of the cache.

`INSN_CACHE_LINE_WIDTH'
     The length in bytes of each cache line.  The cache is divided into
     cache lines which are disjoint slots, each holding a contiguous
     chunk of data fetched from memory.  Each time data is brought into
     the cache, an entire line is read at once.  The data loaded into a
     cache line is always aligned on a boundary equal to the line size.

`INSN_CACHE_DEPTH'
     The number of alternative cache lines that can hold any particular
     memory location.

   Alternatively, if the machine has system calls or instructions to
clear the instruction cache directly, you can define the following
macro.

`CLEAR_INSN_CACHE (BEG, END)'
     If defined, expands to a C expression clearing the _instruction
     cache_ in the specified interval.  If it is not defined, and the
     macro `INSN_CACHE_SIZE' is defined, some generic code is generated
     to clear the cache.  The definition of this macro would typically
     be a series of `asm' statements.  Both BEG and END are both pointer
     expressions.

   To use a standard subroutine, define the following macro.  In
addition, you must make sure that the instructions in a trampoline fill
an entire cache line with identical instructions, or else ensure that
the beginning of the trampoline code is always aligned at the same
point in its cache line.  Look in `m68k.h' as a guide.

`TRANSFER_FROM_TRAMPOLINE'
     Define this macro if trampolines need a special subroutine to do
     their work.  The macro should expand to a series of `asm'
     statements which will be compiled with GCC.  They go in a library
     function named `__transfer_from_trampoline'.

     If you need to avoid executing the ordinary prologue code of a
     compiled C function when you jump to the subroutine, you can do so
     by placing a special label of your own in the assembler code.  Use
     one `asm' statement to generate an assembler label, and another to
     make the label global.  Then trampolines can use that label to
     jump directly to your special assembler code.


File: gcc.info,  Node: Library Calls,  Next: Addressing Modes,  Prev: Trampolines,  Up: Target Macros

Implicit Calls to Library Routines
==================================

   Here is an explanation of implicit calls to library routines.

`MULSI3_LIBCALL'
     A C string constant giving the name of the function to call for
     multiplication of one signed full-word by another.  If you do not
     define this macro, the default name is used, which is `__mulsi3',
     a function defined in `libgcc.a'.

`DIVSI3_LIBCALL'
     A C string constant giving the name of the function to call for
     division of one signed full-word by another.  If you do not define
     this macro, the default name is used, which is `__divsi3', a
     function defined in `libgcc.a'.

`UDIVSI3_LIBCALL'
     A C string constant giving the name of the function to call for
     division of one unsigned full-word by another.  If you do not
     define this macro, the default name is used, which is `__udivsi3',
     a function defined in `libgcc.a'.

`MODSI3_LIBCALL'
     A C string constant giving the name of the function to call for the
     remainder in division of one signed full-word by another.  If you
     do not define this macro, the default name is used, which is
     `__modsi3', a function defined in `libgcc.a'.

`UMODSI3_LIBCALL'
     A C string constant giving the name of the function to call for the
     remainder in division of one unsigned full-word by another.  If
     you do not define this macro, the default name is used, which is
     `__umodsi3', a function defined in `libgcc.a'.

`MULDI3_LIBCALL'
     A C string constant giving the name of the function to call for
     multiplication of one signed double-word by another.  If you do not
     define this macro, the default name is used, which is `__muldi3',
     a function defined in `libgcc.a'.

`DIVDI3_LIBCALL'
     A C string constant giving the name of the function to call for
     division of one signed double-word by another.  If you do not
     define this macro, the default name is used, which is `__divdi3', a
     function defined in `libgcc.a'.

`UDIVDI3_LIBCALL'
     A C string constant giving the name of the function to call for
     division of one unsigned full-word by another.  If you do not
     define this macro, the default name is used, which is `__udivdi3',
     a function defined in `libgcc.a'.

`MODDI3_LIBCALL'
     A C string constant giving the name of the function to call for the
     remainder in division of one signed double-word by another.  If
     you do not define this macro, the default name is used, which is
     `__moddi3', a function defined in `libgcc.a'.

`UMODDI3_LIBCALL'
     A C string constant giving the name of the function to call for the
     remainder in division of one unsigned full-word by another.  If
     you do not define this macro, the default name is used, which is
     `__umoddi3', a function defined in `libgcc.a'.

`INIT_TARGET_OPTABS'
     Define this macro as a C statement that declares additional library
     routines renames existing ones.  `init_optabs' calls this macro
     after initializing all the normal library routines.

`FLOAT_LIB_COMPARE_RETURNS_BOOL'
     Define this macro as a C statement that returns nonzero if a call
     to the floating point comparison library function will return a
     boolean value that indicates the result of the comparison.  It
     should return zero if one of gcc's own libgcc functions is called.

     Most ports don't need to define this macro.

`TARGET_EDOM'
     The value of `EDOM' on the target machine, as a C integer constant
     expression.  If you don't define this macro, GCC does not attempt
     to deposit the value of `EDOM' into `errno' directly.  Look in
     `/usr/include/errno.h' to find the value of `EDOM' on your system.

     If you do not define `TARGET_EDOM', then compiled code reports
     domain errors by calling the library function and letting it
     report the error.  If mathematical functions on your system use
     `matherr' when there is an error, then you should leave
     `TARGET_EDOM' undefined so that `matherr' is used normally.

`GEN_ERRNO_RTX'
     Define this macro as a C expression to create an rtl expression
     that refers to the global "variable" `errno'.  (On certain systems,
     `errno' may not actually be a variable.)  If you don't define this
     macro, a reasonable default is used.

`TARGET_MEM_FUNCTIONS'
     Define this macro if GCC should generate calls to the ISO C (and
     System V) library functions `memcpy', `memmove' and `memset'
     rather than the BSD functions `bcopy' and `bzero'.

`LIBGCC_NEEDS_DOUBLE'
     Define this macro if only `float' arguments cannot be passed to
     library routines (so they must be converted to `double').  This
     macro affects both how library calls are generated and how the
     library routines in `libgcc1.c' accept their arguments.  It is
     useful on machines where floating and fixed point arguments are
     passed differently, such as the i860.

`FLOAT_ARG_TYPE'
     Define this macro to override the type used by the library
     routines to pick up arguments of type `float'.  (By default, they
     use a union of `float' and `int'.)

     The obvious choice would be `float'--but that won't work with
     traditional C compilers that expect all arguments declared as
     `float' to arrive as `double'.  To avoid this conversion, the
     library routines ask for the value as some other type and then
     treat it as a `float'.

     On some systems, no other type will work for this.  For these
     systems, you must use `LIBGCC_NEEDS_DOUBLE' instead, to force
     conversion of the values `double' before they are passed.

`FLOATIFY (PASSED-VALUE)'
     Define this macro to override the way library routines redesignate
     a `float' argument as a `float' instead of the type it was passed
     as.  The default is an expression which takes the `float' field of
     the union.

`FLOAT_VALUE_TYPE'
     Define this macro to override the type used by the library
     routines to return values that ought to have type `float'.  (By
     default, they use `int'.)

     The obvious choice would be `float'--but that won't work with
     traditional C compilers gratuitously convert values declared as
     `float' into `double'.

`INTIFY (FLOAT-VALUE)'
     Define this macro to override the way the value of a
     `float'-returning library routine should be packaged in order to
     return it.  These functions are actually declared to return type
     `FLOAT_VALUE_TYPE' (normally `int').

     These values can't be returned as type `float' because traditional
     C compilers would gratuitously convert the value to a `double'.

     A local variable named `intify' is always available when the macro
     `INTIFY' is used.  It is a union of a `float' field named `f' and
     a field named `i' whose type is `FLOAT_VALUE_TYPE' or `int'.

     If you don't define this macro, the default definition works by
     copying the value through that union.

`nongcc_SI_type'
     Define this macro as the name of the data type corresponding to
     `SImode' in the system's own C compiler.

     You need not define this macro if that type is `long int', as it
     usually is.

`nongcc_word_type'
     Define this macro as the name of the data type corresponding to the
     word_mode in the system's own C compiler.

     You need not define this macro if that type is `long int', as it
     usually is.

`perform_...'
     Define these macros to supply explicit C statements to carry out
     various arithmetic operations on types `float' and `double' in the
     library routines in `libgcc1.c'.  See that file for a full list of
     these macros and their arguments.

     On most machines, you don't need to define any of these macros,
     because the C compiler that comes with the system takes care of
     doing them.

`NEXT_OBJC_RUNTIME'
     Define this macro to generate code for Objective C message sending
     using the calling convention of the NeXT system.  This calling
     convention involves passing the object, the selector and the
     method arguments all at once to the method-lookup library function.

     The default calling convention passes just the object and the
     selector to the lookup function, which returns a pointer to the
     method.

