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

   Published by the Free Software Foundation
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File: gccint.info,  Node: 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: gccint.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 `TARGET_ASM_FUNCTION_PROLOGUE' and
     `TARGET_ASM_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: gccint.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'.

`DECLARE_LIBRARY_RENAMES'
     This macro, if defined, should expand to a piece of C code that
     will get expanded when compiling functions for libgcc.a.  It can
     be used to provide alternate names for gcc's internal library
     functions if there are ABI-mandated names that the compiler should
     provide.

`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 `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 `libgcc.a' accept their arguments.  It is useful on
     machines where floating and fixed point arguments are passed
     differently, such as the i860.

`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: gccint.info,  Node: Addressing Modes,  Next: Condition Code,  Prev: Library Calls,  Up: Target Macros

Addressing Modes
================

   This is about addressing modes.

`HAVE_PRE_INCREMENT'
`HAVE_PRE_DECREMENT'
`HAVE_POST_INCREMENT'
`HAVE_POST_DECREMENT'
     A C expression that is nonzero if the machine supports
     pre-increment, pre-decrement, post-increment, or post-decrement
     addressing respectively.

`HAVE_PRE_MODIFY_DISP'
`HAVE_POST_MODIFY_DISP'
     A C expression that is nonzero if the machine supports pre- or
     post-address side-effect generation involving constants other than
     the size of the memory operand.

`HAVE_PRE_MODIFY_REG'
`HAVE_POST_MODIFY_REG'
     A C expression that is nonzero if the machine supports pre- or
     post-address side-effect generation involving a register
     displacement.

`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.

     Subroutines to check for acceptable registers for various purposes
     (one for base registers, one for index registers, and so on) are
     typically among the subroutines used to define
     `GO_IF_LEGITIMATE_ADDRESS'.  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 target hook `TARGET_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
     `TARGET_STRIP_NAME_ENCODING' to access the original name string.

     You can check the information stored here into the `symbol_ref' in
     the definitions of the macros `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_MODE_OK_FOR_BASE_P (X, MODE)'
     A C expression that is just like `REG_OK_FOR_BASE_P', except that
     that expression may examine the mode of the memory reference in
     MODE.  You should define this macro if the mode of the memory
     reference affects whether a register may be used as a base
     register.  If you define this macro, the compiler will use it
     instead of `REG_OK_FOR_BASE_P'.

`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.

`FIND_BASE_TERM (X)'
     A C expression to determine the base term of address X.  This
     macro is used in only one place: `find_base_term' in alias.c.

     It is always safe for this macro to not be defined.  It exists so
     that alias analysis can understand machine-dependent addresses.

     The typical use of this macro is to handle addresses containing a
     label_ref or symbol_ref within an UNSPEC.

`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.

`LEGITIMIZE_RELOAD_ADDRESS (X, MODE, OPNUM, TYPE, IND_LEVELS, WIN)'
     A C compound statement that attempts to replace X, which is an
     address that needs reloading, with a valid memory address for an
     operand of mode MODE.  WIN will be a C statement label elsewhere
     in the code.  It is not necessary to define this macro, but it
     might be useful for performance reasons.

     For example, on the i386, it is sometimes possible to use a single
     reload register instead of two by reloading a sum of two pseudo
     registers into a register.  On the other hand, for number of RISC
     processors offsets are limited so that often an intermediate
     address needs to be generated in order to address a stack slot.
     By defining `LEGITIMIZE_RELOAD_ADDRESS' appropriately, the
     intermediate addresses generated for adjacent some stack slots can
     be made identical, and thus be shared.

     _Note_: This macro should be used with caution.  It is necessary
     to know something of how reload works in order to effectively use
     this, and it is quite easy to produce macros that build in too
     much knowledge of reload internals.

     _Note_: This macro must be able to reload an address created by a
     previous invocation of this macro.  If it fails to handle such
     addresses then the compiler may generate incorrect code or abort.

     The macro definition should use `push_reload' to indicate parts
     that need reloading; OPNUM, TYPE and IND_LEVELS are usually
     suitable to be passed unaltered to `push_reload'.

     The code generated by this macro must 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.  This
     also applies to parts that you change indirectly by calling
     `push_reload'.

     The macro definition may use `strict_memory_address_p' to test if
     the address has become legitimate.

     If you want to change only a part of X, one standard way of doing
     this is to use `copy_rtx'.  Note, however, that is unshares only a
     single level of rtl.  Thus, if the part to be changed is not at the
     top level, you'll need to replace first the top level.  It is not
     necessary for this macro to come up with a legitimate address;
     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: gccint.info,  Node: Condition Code,  Next: Costs,  Prev: Addressing Modes,  Up: Target Macros

Condition Code Status
=====================

   This describes the 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'
     Condition codes are represented in registers by machine modes of
     class `MODE_CC'.  By default, there is just one mode, `CCmode',
     with this class.  If you need more such modes, create a file named
     `MACHINE-modes.def' in your `config/MACHINE' directory (*note
     Anatomy of a Target Back End: Back End.), containing a list of
     these modes.  Each entry in the list should be a call to the macro
     `CC'.  This macro takes one argument, which is the name of the
     mode: it should begin with `CC'.  Do not put quotation marks
     around the name, or include the trailing `mode'; these are
     automatically added.  There should not be anything else in the file
     except comments.

     A sample `MACHINE-modes.def' file might look like this:

          CC (CC_NOOV)   /* Comparison only valid if there was no overflow. */
          CC (CCFP)      /* Floating point comparison that cannot trap. */
          CC (CCFPE)     /* Floating point comparison that may trap. */

     When you create this file, the macro `EXTRA_CC_MODES' is
     automatically defined by `configure', with value `1'.

`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))

     You need not define this macro if `EXTRA_CC_MODES' is not defined.

`CANONICALIZE_COMPARISON (CODE, OP0, OP1)'
     On some machines not all possible comparisons are defined, but you
     can convert an invalid comparison into a valid one.  For example,
     the Alpha does not have a `GT' comparison, but you can use an `LT'
     comparison instead and swap the order of the operands.

     On such machines, define this macro to be a C statement to do any
     required conversions.  CODE is the initial comparison code and OP0
     and OP1 are the left and right operands of the comparison,
     respectively.  You should modify CODE, OP0, and OP1 as required.

     GCC will not assume that the comparison resulting from this macro
     is valid but will see if the resulting insn matches a pattern in
     the `md' file.

     You need not define this macro if it would never change the
     comparison code or operands.

`REVERSIBLE_CC_MODE (MODE)'
     A C expression whose value is one if it is always safe to reverse a
     comparison whose mode is MODE.  If `SELECT_CC_MODE' can ever
     return MODE for a floating-point inequality comparison, then
     `REVERSIBLE_CC_MODE (MODE)' must be zero.

     You need not define this macro if it would always returns zero or
     if the floating-point format is anything other than
     `IEEE_FLOAT_FORMAT'.  For example, here is the definition used on
     the SPARC, where floating-point inequality comparisons are always
     given `CCFPEmode':

          #define REVERSIBLE_CC_MODE(MODE)  ((MODE) != CCFPEmode)

     A C expression whose value is reversed condition code of the CODE
     for comparison done in CC_MODE MODE.  The macro is used only in
     case `REVERSIBLE_CC_MODE (MODE)' is nonzero.  Define this macro in
     case machine has some non-standard way how to reverse certain
     conditionals.  For instance in case all floating point conditions
     are non-trapping, compiler may freely convert unordered compares
     to ordered one.  Then definition may look like:

          #define REVERSE_CONDITION(CODE, MODE) \
             ((MODE) != CCFPmode ? reverse_condition (CODE) \
              : reverse_condition_maybe_unordered (CODE))

`REVERSE_CONDEXEC_PREDICATES_P (CODE1, CODE2)'
     A C expression that returns true if the conditional execution
     predicate CODE1 is the inverse of CODE2 and vice versa.  Define
     this to return 0 if the target has conditional execution
     predicates that cannot be reversed safely.  If no expansion is
     specified, this macro is defined as follows:

          #define REVERSE_CONDEXEC_PREDICATES_P (x, y) \
             ((x) == reverse_condition (y))

