OS/2 Professional/OS2PRO194.ISO/os2/prgramer/unix/info/gcc.i16

GNU Info File | 1993-06-12 | 46KB | 755 lines

This is Info file gcc.info, produced by Makeinfo-1.47 from the input
file gcc.tex.
   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
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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 `stdargs.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, in the case that `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 `FUNCTION_PROLOGUE' and
     `FUNCTION_EPILOGUE' to treat leaf functions specially.  The C
     variable `leaf_function' 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'.
`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 at run time
     in a function that needs a frame pointer.
     Normally, it is necessary for `FUNCTION_PROLOGUE' and
     `FUNCTION_EPILOGUE' to treat leaf functions specially.  The C
     variable `leaf_function' 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 `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'.
File: gcc.info,  Node: Profiling,  Prev: Function Entry,  Up: Stack and Calling
Generating 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'. Before
     calling, the assembler code must load the address of a counter
     variable into a register where `mcount' expects to find the
     address.  The name of this variable is `LP' followed by the number
     LABELNO, so you would generate the name using `LP%d' in a
     `fprintf'.
     The details of how the address should be passed to `mcount' are
     determined by your operating system environment, not by GNU CC.  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.
`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.  This code should 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.
`BLOCK_PROFILER (FILE, BLOCKNO)'
     A C statement or compound statement to increment the count
     associated with the basic block number BLOCKNO.  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.
File: gcc.info,  Node: Varargs,  Next: Trampolines,  Prev: Stack and Calling,  Up: Target Macros
Implementing the Varargs Macros
===============================
   GNU CC 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
   ANSI `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 ANSI 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 ANSI
     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'.
     Regardless of what code is generated for the call to
     `__builtin_saveregs', it appears at the beginning of the function,
     not where the call to `__builtin_saveregs' is written.  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 ()'
     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.
`__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 (ARGS)'
     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'.
     The argument ARGS is a `tree_list' containing the arguments that
     were passed to `__builtin_saveregs'.
     If this macro is not defined, the compiler will output an ordinary
     call to the library function `__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 obtain after processing of 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.
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 GNU CC 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.
`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.
`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.
   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 GNU CC.  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
==================================
`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'.
`TARGET_EDOM'
     The value of `EDOM' on the target machine, as a C integer constant
     expression.  If you don't define this macro, GNU CC 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 GNU CC should generate calls to the System V
     (and ANSI C) library functions `memcpy' 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 `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.
File: gcc.info,  Node: Addressing Modes,  Next: Condition Code,  Prev: Library Calls,  Up: Target Macros
Addressing Modes
================
`HAVE_POST_INCREMENT'
     Define this macro if the machine supports post-increment
     addressing.
`HAVE_PRE_INCREMENT'
`HAVE_POST_DECREMENT'
`HAVE_PRE_DECREMENT'
     Similar for other kinds of addressing.
`CONSTANT_ADDRESS_P (X)'
     A C expression that is 1 if the RTX X is a constant which is a
     valid address.  On most machines, this can be defined as
     `CONSTANT_P (X)', but a few machines are more restrictive in which
     constant addresses are supported.
     `CONSTANT_P' accepts integer-values expressions whose values are
     not explicitly known, such as `symbol_ref', `label_ref', and
     `high' expressions and `const' arithmetic expressions, in addition
     to `const_int' and `const_double' expressions.
`MAX_REGS_PER_ADDRESS'
     A number, the maximum number of registers that can appear in a
     valid memory address.  Note that it is up to you to specify a
     value equal to the maximum number that `GO_IF_LEGITIMATE_ADDRESS'
     would ever accept.
`GO_IF_LEGITIMATE_ADDRESS (MODE, X, LABEL)'
     A C compound statement with a conditional `goto LABEL;' executed
     if X (an RTX) is a legitimate memory address on the target machine
     for a memory operand of mode MODE.
     It usually pays to define several simpler macros to serve as
     subroutines for this one.  Otherwise it may be too complicated to
     understand.
     This macro must exist in two variants: a strict variant and a
     non-strict one.  The strict variant is used in the reload pass.  It
     must be defined so that any pseudo-register that has not been
     allocated a hard register is considered a memory reference.  In
     contexts where some kind of register is required, a pseudo-register
     with no hard register must be rejected.
     The non-strict variant is used in other passes.  It must be
     defined to accept all pseudo-registers in every context where some
     kind of register is required.
     Compiler source files that want to use the strict variant of this
     macro define the macro `REG_OK_STRICT'.  You should use an `#ifdef
     REG_OK_STRICT' conditional to define the strict variant in that
     case and the non-strict variant otherwise.
     Typically among the subroutines used to define
     `GO_IF_LEGITIMATE_ADDRESS' are subroutines to check for acceptable
     registers for various purposes (one for base registers, one for
     index registers, and so on).  Then only these subroutine macros
     need have two variants; the higher levels of macros may be the same
     whether strict or not.
     Normally, constant addresses which are the sum of a `symbol_ref'
     and an integer are stored inside a `const' RTX to mark them as
     constant.  Therefore, there is no need to recognize such sums
     specifically as legitimate addresses.  Normally you would simply
     recognize any `const' as legitimate.
     Usually `PRINT_OPERAND_ADDRESS' is not prepared to handle constant
     sums that are not marked with  `const'.  It assumes that a naked
     `plus' indicates indexing.  If so, then you *must* reject such
     naked constant sums as illegitimate addresses, so that none of
     them will be given to `PRINT_OPERAND_ADDRESS'.
     On some machines, whether a symbolic address is legitimate depends
     on the section that the address refers to.  On these machines,
     define the macro `ENCODE_SECTION_INFO' to store the information
     into the `symbol_ref', and then check for it here.  When you see a
     `const', you will have to look inside it to find the `symbol_ref'
     in order to determine the section.  *Note Assembler Format::.
     The best way to modify the name string is by adding text to the
     beginning, with suitable punctuation to prevent any ambiguity.
     Allocate the new name in `saveable_obstack'.  You will have to
     modify `ASM_OUTPUT_LABELREF' to remove and decode the added text
     and output the name accordingly, and define `STRIP_NAME_ENCODING'
     to access the original name string.
     You can check the information stored here into the `symbol_ref' in
     the definitions of `GO_IF_LEGITIMATE_ADDRESS' and
     `PRINT_OPERAND_ADDRESS'.
`REG_OK_FOR_BASE_P (X)'
     A C expression that is nonzero if X (assumed to be a `reg' RTX) is
     valid for use as a base register.  For hard registers, it should
     always accept those which the hardware permits and reject the
     others.  Whether the macro accepts or rejects pseudo registers
     must be controlled by `REG_OK_STRICT' as described above.  This
     usually requires two variant definitions, of which `REG_OK_STRICT'
     controls the one actually used.
`REG_OK_FOR_INDEX_P (X)'
     A C expression that is nonzero if X (assumed to be a `reg' RTX) is
     valid for use as an index register.
     The difference between an index register and a base register is
     that the index register may be scaled.  If an address involves the
     sum of two registers, neither one of them scaled, then either one
     may be labeled the "base" and the other the "index"; but whichever
     labeling is used must fit the machine's constraints of which
     registers may serve in each capacity.  The compiler will try both
     labelings, looking for one that is valid, and will reload one or
     both registers only if neither labeling works.
`LEGITIMIZE_ADDRESS (X, OLDX, MODE, WIN)'
     A C compound statement that attempts to replace X with a valid
     memory address for an operand of mode MODE.  WIN will be a C
     statement label elsewhere in the code; the macro definition may use
          GO_IF_LEGITIMATE_ADDRESS (MODE, X, WIN);
     to avoid further processing if the address has become legitimate.
     X will always be the result of a call to `break_out_memory_refs',
     and OLDX will be the operand that was given to that function to
     produce X.
     The code generated by this macro should not alter the substructure
     of X.  If it transforms X into a more legitimate form, it should
     assign X (which will always be a C variable) a new value.
     It is not necessary for this macro to come up with a legitimate
     address.  The compiler has standard ways of doing so in all cases.
      In fact, it is safe for this macro to do nothing.  But often a
     machine-dependent strategy can generate better code.
`GO_IF_MODE_DEPENDENT_ADDRESS (ADDR, LABEL)'
     A C statement or compound statement with a conditional `goto
     LABEL;' executed if memory address X (an RTX) can have different
     meanings depending on the machine mode of the memory reference it
     is used for or if the address is valid for some modes but not
     others.
     Autoincrement and autodecrement addresses typically have
     mode-dependent effects because the amount of the increment or
     decrement is the size of the operand being addressed.  Some
     machines have other mode-dependent addresses.  Many RISC machines
     have no mode-dependent addresses.
     You may assume that ADDR is a valid address for the machine.
`LEGITIMATE_CONSTANT_P (X)'
     A C expression that is nonzero if X is a legitimate constant for
     an immediate operand on the target machine.  You can assume that X
     satisfies `CONSTANT_P', so you need not check this.  In fact, `1'
     is a suitable definition for this macro on machines where anything
     `CONSTANT_P' is valid.
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.