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* gccint: (gccint).            Internals of the GNU Compiler Collection.
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File: gccint.info,  Node: Type Layout,  Next: Escape Sequences,  Prev: Storage Layout,  Up: Target Macros

Layout of Source Language Data Types
====================================

   These macros define the sizes and other characteristics of the
standard basic data types used in programs being compiled.  Unlike the
macros in the previous section, these apply to specific features of C
and related languages, rather than to fundamental aspects of storage
layout.

`INT_TYPE_SIZE'
     A C expression for the size in bits of the type `int' on the
     target machine.  If you don't define this, the default is one word.

`SHORT_TYPE_SIZE'
     A C expression for the size in bits of the type `short' on the
     target machine.  If you don't define this, the default is half a
     word.  (If this would be less than one storage unit, it is rounded
     up to one unit.)

`LONG_TYPE_SIZE'
     A C expression for the size in bits of the type `long' on the
     target machine.  If you don't define this, the default is one word.

`ADA_LONG_TYPE_SIZE'
     On some machines, the size used for the Ada equivalent of the type
     `long' by a native Ada compiler differs from that used by C.  In
     that situation, define this macro to be a C expression to be used
     for the size of that type.  If you don't define this, the default
     is the value of `LONG_TYPE_SIZE'.

`MAX_LONG_TYPE_SIZE'
     Maximum number for the size in bits of the type `long' on the
     target machine.  If this is undefined, the default is
     `LONG_TYPE_SIZE'.  Otherwise, it is the constant value that is the
     largest value that `LONG_TYPE_SIZE' can have at run-time.  This is
     used in `cpp'.

`LONG_LONG_TYPE_SIZE'
     A C expression for the size in bits of the type `long long' on the
     target machine.  If you don't define this, the default is two
     words.  If you want to support GNU Ada on your machine, the value
     of this macro must be at least 64.

`CHAR_TYPE_SIZE'
     A C expression for the size in bits of the type `char' on the
     target machine.  If you don't define this, the default is
     `BITS_PER_UNIT'.

`MAX_CHAR_TYPE_SIZE'
     Maximum number for the size in bits of the type `char' on the
     target machine.  If this is undefined, the default is
     `CHAR_TYPE_SIZE'.  Otherwise, it is the constant value that is the
     largest value that `CHAR_TYPE_SIZE' can have at run-time.  This is
     used in `cpp'.

`BOOL_TYPE_SIZE'
     A C expression for the size in bits of the C++ type `bool' and C99
     type `_Bool' on the target machine.  If you don't define this, and
     you probably shouldn't, the default is `CHAR_TYPE_SIZE'.

`FLOAT_TYPE_SIZE'
     A C expression for the size in bits of the type `float' on the
     target machine.  If you don't define this, the default is one word.

`DOUBLE_TYPE_SIZE'
     A C expression for the size in bits of the type `double' on the
     target machine.  If you don't define this, the default is two
     words.

`LONG_DOUBLE_TYPE_SIZE'
     A C expression for the size in bits of the type `long double' on
     the target machine.  If you don't define this, the default is two
     words.

     Maximum number for the size in bits of the type `long double' on
     the target machine.  If this is undefined, the default is
     `LONG_DOUBLE_TYPE_SIZE'.  Otherwise, it is the constant value that
     is the largest value that `LONG_DOUBLE_TYPE_SIZE' can have at
     run-time.  This is used in `cpp'.

     Define this macro to be 1 if the target machine uses 80-bit
     floating-point values with 128-bit size and alignment.  This is
     used in `real.c'.

`WIDEST_HARDWARE_FP_SIZE'
     A C expression for the size in bits of the widest floating-point
     format supported by the hardware.  If you define this macro, you
     must specify a value less than or equal to the value of
     `LONG_DOUBLE_TYPE_SIZE'.  If you do not define this macro, the
     value of `LONG_DOUBLE_TYPE_SIZE' is the default.

`DEFAULT_SIGNED_CHAR'
     An expression whose value is 1 or 0, according to whether the type
     `char' should be signed or unsigned by default.  The user can
     always override this default with the options `-fsigned-char' and
     `-funsigned-char'.

`DEFAULT_SHORT_ENUMS'
     A C expression to determine whether to give an `enum' type only as
     many bytes as it takes to represent the range of possible values
     of that type.  A nonzero value means to do that; a zero value
     means all `enum' types should be allocated like `int'.

     If you don't define the macro, the default is 0.

`SIZE_TYPE'
     A C expression for a string describing the name of the data type
     to use for size values.  The typedef name `size_t' is defined
     using the contents of the string.

     The string can contain more than one keyword.  If so, separate
     them with spaces, and write first any length keyword, then
     `unsigned' if appropriate, and finally `int'.  The string must
     exactly match one of the data type names defined in the function
     `init_decl_processing' in the file `c-decl.c'.  You may not omit
     `int' or change the order--that would cause the compiler to crash
     on startup.

     If you don't define this macro, the default is `"long unsigned
     int"'.

`PTRDIFF_TYPE'
     A C expression for a string describing the name of the data type
     to use for the result of subtracting two pointers.  The typedef
     name `ptrdiff_t' is defined using the contents of the string.  See
     `SIZE_TYPE' above for more information.

     If you don't define this macro, the default is `"long int"'.

`WCHAR_TYPE'
     A C expression for a string describing the name of the data type
     to use for wide characters.  The typedef name `wchar_t' is defined
     using the contents of the string.  See `SIZE_TYPE' above for more
     information.

     If you don't define this macro, the default is `"int"'.

`WCHAR_TYPE_SIZE'
     A C expression for the size in bits of the data type for wide
     characters.  This is used in `cpp', which cannot make use of
     `WCHAR_TYPE'.

`MAX_WCHAR_TYPE_SIZE'
     Maximum number for the size in bits of the data type for wide
     characters.  If this is undefined, the default is
     `WCHAR_TYPE_SIZE'.  Otherwise, it is the constant value that is the
     largest value that `WCHAR_TYPE_SIZE' can have at run-time.  This is
     used in `cpp'.

`GCOV_TYPE_SIZE'
     A C expression for the size in bits of the type used for gcov
     counters on the target machine.  If you don't define this, the
     default is one `LONG_TYPE_SIZE' in case it is greater or equal to
     64-bit and `LONG_LONG_TYPE_SIZE' otherwise.  You may want to
     re-define the type to ensure atomicity for counters in
     multithreaded programs.

`WINT_TYPE'
     A C expression for a string describing the name of the data type to
     use for wide characters passed to `printf' and returned from
     `getwc'.  The typedef name `wint_t' is defined using the contents
     of the string.  See `SIZE_TYPE' above for more information.

     If you don't define this macro, the default is `"unsigned int"'.

`INTMAX_TYPE'
     A C expression for a string describing the name of the data type
     that can represent any value of any standard or extended signed
     integer type.  The typedef name `intmax_t' is defined using the
     contents of the string.  See `SIZE_TYPE' above for more
     information.

     If you don't define this macro, the default is the first of
     `"int"', `"long int"', or `"long long int"' that has as much
     precision as `long long int'.

`UINTMAX_TYPE'
     A C expression for a string describing the name of the data type
     that can represent any value of any standard or extended unsigned
     integer type.  The typedef name `uintmax_t' is defined using the
     contents of the string.  See `SIZE_TYPE' above for more
     information.

     If you don't define this macro, the default is the first of
     `"unsigned int"', `"long unsigned int"', or `"long long unsigned
     int"' that has as much precision as `long long unsigned int'.

`TARGET_PTRMEMFUNC_VBIT_LOCATION'
     The C++ compiler represents a pointer-to-member-function with a
     struct that looks like:

            struct {
              union {
                void (*fn)();
                ptrdiff_t vtable_index;
              };
              ptrdiff_t delta;
            };

     The C++ compiler must use one bit to indicate whether the function
     that will be called through a pointer-to-member-function is
     virtual.  Normally, we assume that the low-order bit of a function
     pointer must always be zero.  Then, by ensuring that the
     vtable_index is odd, we can distinguish which variant of the union
     is in use.  But, on some platforms function pointers can be odd,
     and so this doesn't work.  In that case, we use the low-order bit
     of the `delta' field, and shift the remainder of the `delta' field
     to the left.

     GCC will automatically make the right selection about where to
     store this bit using the `FUNCTION_BOUNDARY' setting for your
     platform.  However, some platforms such as ARM/Thumb have
     `FUNCTION_BOUNDARY' set such that functions always start at even
     addresses, but the lowest bit of pointers to functions indicate
     whether the function at that address is in ARM or Thumb mode.  If
     this is the case of your architecture, you should define this
     macro to `ptrmemfunc_vbit_in_delta'.

     In general, you should not have to define this macro.  On
     architectures in which function addresses are always even,
     according to `FUNCTION_BOUNDARY', GCC will automatically define
     this macro to `ptrmemfunc_vbit_in_pfn'.

`TARGET_VTABLE_USES_DESCRIPTORS'
     Normally, the C++ compiler uses function pointers in vtables.  This
     macro allows the target to change to use "function descriptors"
     instead.  Function descriptors are found on targets for whom a
     function pointer is actually a small data structure.  Normally the
     data structure consists of the actual code address plus a data
     pointer to which the function's data is relative.

     If vtables are used, the value of this macro should be the number
     of words that the function descriptor occupies.


File: gccint.info,  Node: Escape Sequences,  Next: Registers,  Prev: Type Layout,  Up: Target Macros

Target Character Escape Sequences
=================================

   By default, GCC assumes that the C character escape sequences take on
their ASCII values for the target.  If this is not correct, you must
explicitly define all of the macros below.

`TARGET_BELL'
     A C constant expression for the integer value for escape sequence
     `\a'.

`TARGET_ESC'
     A C constant expression for the integer value of the target escape
     character.  As an extension, GCC evaluates the escape sequences
     `\e' and `\E' to this.

`TARGET_BS'
`TARGET_TAB'
`TARGET_NEWLINE'
     C constant expressions for the integer values for escape sequences
     `\b', `\t' and `\n'.

`TARGET_VT'
`TARGET_FF'
`TARGET_CR'
     C constant expressions for the integer values for escape sequences
     `\v', `\f' and `\r'.


File: gccint.info,  Node: Registers,  Next: Register Classes,  Prev: Escape Sequences,  Up: Target Macros

Register Usage
==============

   This section explains how to describe what registers the target
machine has, and how (in general) they can be used.

   The description of which registers a specific instruction can use is
done with register classes; see *Note Register Classes::.  For
information on using registers to access a stack frame, see *Note Frame
Registers::.  For passing values in registers, see *Note Register
Arguments::.  For returning values in registers, see *Note Scalar
Return::.

* Menu:

* Register Basics::		Number and kinds of registers.
* Allocation Order::		Order in which registers are allocated.
* Values in Registers::		What kinds of values each reg can hold.
* Leaf Functions::		Renumbering registers for leaf functions.
* Stack Registers::		Handling a register stack such as 80387.


File: gccint.info,  Node: Register Basics,  Next: Allocation Order,  Up: Registers

Basic Characteristics of Registers
----------------------------------

   Registers have various characteristics.

`FIRST_PSEUDO_REGISTER'
     Number of hardware registers known to the compiler.  They receive
     numbers 0 through `FIRST_PSEUDO_REGISTER-1'; thus, the first
     pseudo register's number really is assigned the number
     `FIRST_PSEUDO_REGISTER'.

`FIXED_REGISTERS'
     An initializer that says which registers are used for fixed
     purposes all throughout the compiled code and are therefore not
     available for general allocation.  These would include the stack
     pointer, the frame pointer (except on machines where that can be
     used as a general register when no frame pointer is needed), the
     program counter on machines where that is considered one of the
     addressable registers, and any other numbered register with a
     standard use.

     This information is expressed as a sequence of numbers, separated
     by commas and surrounded by braces.  The Nth number is 1 if
     register N is fixed, 0 otherwise.

     The table initialized from this macro, and the table initialized by
     the following one, may be overridden at run time either
     automatically, by the actions of the macro
     `CONDITIONAL_REGISTER_USAGE', or by the user with the command
     options `-ffixed-REG', `-fcall-used-REG' and `-fcall-saved-REG'.

`CALL_USED_REGISTERS'
     Like `FIXED_REGISTERS' but has 1 for each register that is
     clobbered (in general) by function calls as well as for fixed
     registers.  This macro therefore identifies the registers that are
     not available for general allocation of values that must live
     across function calls.

     If a register has 0 in `CALL_USED_REGISTERS', the compiler
     automatically saves it on function entry and restores it on
     function exit, if the register is used within the function.

`CALL_REALLY_USED_REGISTERS'
     Like `CALL_USED_REGISTERS' except this macro doesn't require that
     the entire set of `FIXED_REGISTERS' be included.
     (`CALL_USED_REGISTERS' must be a superset of `FIXED_REGISTERS').
     This macro is optional.  If not specified, it defaults to the value
     of `CALL_USED_REGISTERS'.

`HARD_REGNO_CALL_PART_CLOBBERED (REGNO, MODE)'
     A C expression that is nonzero if it is not permissible to store a
     value of mode MODE in hard register number REGNO across a call
     without some part of it being clobbered.  For most machines this
     macro need not be defined.  It is only required for machines that
     do not preserve the entire contents of a register across a call.

`CONDITIONAL_REGISTER_USAGE'
     Zero or more C statements that may conditionally modify five
     variables `fixed_regs', `call_used_regs', `global_regs',
     `reg_names', and `reg_class_contents', to take into account any
     dependence of these register sets on target flags.  The first three
     of these are of type `char []' (interpreted as Boolean vectors).
     `global_regs' is a `const char *[]', and `reg_class_contents' is a
     `HARD_REG_SET'.  Before the macro is called, `fixed_regs',
     `call_used_regs', `reg_class_contents', and `reg_names' have been
     initialized from `FIXED_REGISTERS', `CALL_USED_REGISTERS',
     `REG_CLASS_CONTENTS', and `REGISTER_NAMES', respectively.
     `global_regs' has been cleared, and any `-ffixed-REG',
     `-fcall-used-REG' and `-fcall-saved-REG' command options have been
     applied.

     You need not define this macro if it has no work to do.

     If the usage of an entire class of registers depends on the target
     flags, you may indicate this to GCC by using this macro to modify
     `fixed_regs' and `call_used_regs' to 1 for each of the registers
     in the classes which should not be used by GCC.  Also define the
     macro `REG_CLASS_FROM_LETTER' to return `NO_REGS' if it is called
     with a letter for a class that shouldn't be used.

     (However, if this class is not included in `GENERAL_REGS' and all
     of the insn patterns whose constraints permit this class are
     controlled by target switches, then GCC will automatically avoid
     using these registers when the target switches are opposed to
     them.)

`NON_SAVING_SETJMP'
     If this macro is defined and has a nonzero value, it means that
     `setjmp' and related functions fail to save the registers, or that
     `longjmp' fails to restore them.  To compensate, the compiler
     avoids putting variables in registers in functions that use
     `setjmp'.

`INCOMING_REGNO (OUT)'
     Define this macro if the target machine has register windows.
     This C expression returns the register number as seen by the
     called function corresponding to the register number OUT as seen
     by the calling function.  Return OUT if register number OUT is not
     an outbound register.

`OUTGOING_REGNO (IN)'
     Define this macro if the target machine has register windows.
     This C expression returns the register number as seen by the
     calling function corresponding to the register number IN as seen
     by the called function.  Return IN if register number IN is not an
     inbound register.

`LOCAL_REGNO (REGNO)'
     Define this macro if the target machine has register windows.
     This C expression returns true if the register is call-saved but
     is in the register window.  Unlike most call-saved registers, such
     registers need not be explicitly restored on function exit or
     during non-local gotos.


File: gccint.info,  Node: Allocation Order,  Next: Values in Registers,  Prev: Register Basics,  Up: Registers

Order of Allocation of Registers
--------------------------------

   Registers are allocated in order.

`REG_ALLOC_ORDER'
     If defined, an initializer for a vector of integers, containing the
     numbers of hard registers in the order in which GCC should prefer
     to use them (from most preferred to least).

     If this macro is not defined, registers are used lowest numbered
     first (all else being equal).

     One use of this macro is on machines where the highest numbered
     registers must always be saved and the save-multiple-registers
     instruction supports only sequences of consecutive registers.  On
     such machines, define `REG_ALLOC_ORDER' to be an initializer that
     lists the highest numbered allocable register first.

`ORDER_REGS_FOR_LOCAL_ALLOC'
     A C statement (sans semicolon) to choose the order in which to
     allocate hard registers for pseudo-registers local to a basic
     block.

     Store the desired register order in the array `reg_alloc_order'.
     Element 0 should be the register to allocate first; element 1, the
     next register; and so on.

     The macro body should not assume anything about the contents of
     `reg_alloc_order' before execution of the macro.

     On most machines, it is not necessary to define this macro.


File: gccint.info,  Node: Values in Registers,  Next: Leaf Functions,  Prev: Allocation Order,  Up: Registers

How Values Fit in Registers
---------------------------

   This section discusses the macros that describe which kinds of values
(specifically, which machine modes) each register can hold, and how many
consecutive registers are needed for a given mode.

`HARD_REGNO_NREGS (REGNO, MODE)'
     A C expression for the number of consecutive hard registers,
     starting at register number REGNO, required to hold a value of mode
     MODE.

     On a machine where all registers are exactly one word, a suitable
     definition of this macro is

          #define HARD_REGNO_NREGS(REGNO, MODE)            \
             ((GET_MODE_SIZE (MODE) + UNITS_PER_WORD - 1)  \
              / UNITS_PER_WORD)

`HARD_REGNO_MODE_OK (REGNO, MODE)'
     A C expression that is nonzero if it is permissible to store a
     value of mode MODE in hard register number REGNO (or in several
     registers starting with that one).  For a machine where all
     registers are equivalent, a suitable definition is

          #define HARD_REGNO_MODE_OK(REGNO, MODE) 1

     You need not include code to check for the numbers of fixed
     registers, because the allocation mechanism considers them to be
     always occupied.

     On some machines, double-precision values must be kept in even/odd
     register pairs.  You can implement that by defining this macro to
     reject odd register numbers for such modes.

     The minimum requirement for a mode to be OK in a register is that
     the `movMODE' instruction pattern support moves between the
     register and other hard register in the same class and that moving
     a value into the register and back out not alter it.

     Since the same instruction used to move `word_mode' will work for
     all narrower integer modes, it is not necessary on any machine for
     `HARD_REGNO_MODE_OK' to distinguish between these modes, provided
     you define patterns `movhi', etc., to take advantage of this.  This
     is useful because of the interaction between `HARD_REGNO_MODE_OK'
     and `MODES_TIEABLE_P'; it is very desirable for all integer modes
     to be tieable.

     Many machines have special registers for floating point arithmetic.
     Often people assume that floating point machine modes are allowed
     only in floating point registers.  This is not true.  Any
     registers that can hold integers can safely _hold_ a floating
     point machine mode, whether or not floating arithmetic can be done
     on it in those registers.  Integer move instructions can be used
     to move the values.

     On some machines, though, the converse is true: fixed-point machine
     modes may not go in floating registers.  This is true if the
     floating registers normalize any value stored in them, because
     storing a non-floating value there would garble it.  In this case,
     `HARD_REGNO_MODE_OK' should reject fixed-point machine modes in
     floating registers.  But if the floating registers do not
     automatically normalize, if you can store any bit pattern in one
     and retrieve it unchanged without a trap, then any machine mode
     may go in a floating register, so you can define this macro to say
     so.

     The primary significance of special floating registers is rather
     that they are the registers acceptable in floating point arithmetic
     instructions.  However, this is of no concern to
     `HARD_REGNO_MODE_OK'.  You handle it by writing the proper
     constraints for those instructions.

     On some machines, the floating registers are especially slow to
     access, so that it is better to store a value in a stack frame
     than in such a register if floating point arithmetic is not being
     done.  As long as the floating registers are not in class
     `GENERAL_REGS', they will not be used unless some pattern's
     constraint asks for one.

`MODES_TIEABLE_P (MODE1, MODE2)'
     A C expression that is nonzero if a value of mode MODE1 is
     accessible in mode MODE2 without copying.

     If `HARD_REGNO_MODE_OK (R, MODE1)' and `HARD_REGNO_MODE_OK (R,
     MODE2)' are always the same for any R, then `MODES_TIEABLE_P
     (MODE1, MODE2)' should be nonzero.  If they differ for any R, you
     should define this macro to return zero unless some other
     mechanism ensures the accessibility of the value in a narrower
     mode.

     You should define this macro to return nonzero in as many cases as
     possible since doing so will allow GCC to perform better register
     allocation.

`AVOID_CCMODE_COPIES'
     Define this macro if the compiler should avoid copies to/from
     `CCmode' registers.  You should only define this macro if support
     for copying to/from `CCmode' is incomplete.


File: gccint.info,  Node: Leaf Functions,  Next: Stack Registers,  Prev: Values in Registers,  Up: Registers

Handling Leaf Functions
-----------------------

   On some machines, a leaf function (i.e., one which makes no calls)
can run more efficiently if it does not make its own register window.
Often this means it is required to receive its arguments in the
registers where they are passed by the caller, instead of the registers
where they would normally arrive.

   The special treatment for leaf functions generally applies only when
other conditions are met; for example, often they may use only those
registers for its own variables and temporaries.  We use the term "leaf
function" to mean a function that is suitable for this special
handling, so that functions with no calls are not necessarily "leaf
functions".

   GCC assigns register numbers before it knows whether the function is
suitable for leaf function treatment.  So it needs to renumber the
registers in order to output a leaf function.  The following macros
accomplish this.

`LEAF_REGISTERS'
     Name of a char vector, indexed by hard register number, which
     contains 1 for a register that is allowable in a candidate for leaf
     function treatment.

     If leaf function treatment involves renumbering the registers,
     then the registers marked here should be the ones before
     renumbering--those that GCC would ordinarily allocate.  The
     registers which will actually be used in the assembler code, after
     renumbering, should not be marked with 1 in this vector.

     Define this macro only if the target machine offers a way to
     optimize the treatment of leaf functions.

`LEAF_REG_REMAP (REGNO)'
     A C expression whose value is the register number to which REGNO
     should be renumbered, when a function is treated as a leaf
     function.

     If REGNO is a register number which should not appear in a leaf
     function before renumbering, then the expression should yield -1,
     which will cause the compiler to abort.

     Define this macro only if the target machine offers a way to
     optimize the treatment of leaf functions, and registers need to be
     renumbered to do this.

   `TARGET_ASM_FUNCTION_PROLOGUE' and `TARGET_ASM_FUNCTION_EPILOGUE'
must usually treat leaf functions specially.  They can test the C
variable `current_function_is_leaf' which is nonzero for leaf
functions.  `current_function_is_leaf' is set prior to local register
allocation and is valid for the remaining compiler passes.  They can
also test the C variable `current_function_uses_only_leaf_regs' which
is nonzero for leaf functions which only use leaf registers.
`current_function_uses_only_leaf_regs' is valid after reload and is
only useful if `LEAF_REGISTERS' is defined.


File: gccint.info,  Node: Stack Registers,  Prev: Leaf Functions,  Up: Registers

Registers That Form a Stack
---------------------------

   There are special features to handle computers where some of the
"registers" form a stack, as in the 80387 coprocessor for the 80386.
Stack registers are normally written by pushing onto the stack, and are
numbered relative to the top of the stack.

   Currently, GCC can only handle one group of stack-like registers, and
they must be consecutively numbered.

`STACK_REGS'
     Define this if the machine has any stack-like registers.

`FIRST_STACK_REG'
     The number of the first stack-like register.  This one is the top
     of the stack.

`LAST_STACK_REG'
     The number of the last stack-like register.  This one is the
     bottom of the stack.


File: gccint.info,  Node: Register Classes,  Next: Stack and Calling,  Prev: Registers,  Up: Target Macros

Register Classes
================

   On many machines, the numbered registers are not all equivalent.
For example, certain registers may not be allowed for indexed
addressing; certain registers may not be allowed in some instructions.
These machine restrictions are described to the compiler using
"register classes".

   You define a number of register classes, giving each one a name and
saying which of the registers belong to it.  Then you can specify
register classes that are allowed as operands to particular instruction
patterns.

   In general, each register will belong to several classes.  In fact,
one class must be named `ALL_REGS' and contain all the registers.
Another class must be named `NO_REGS' and contain no registers.  Often
the union of two classes will be another class; however, this is not
required.

   One of the classes must be named `GENERAL_REGS'.  There is nothing
terribly special about the name, but the operand constraint letters `r'
and `g' specify this class.  If `GENERAL_REGS' is the same as
`ALL_REGS', just define it as a macro which expands to `ALL_REGS'.

   Order the classes so that if class X is contained in class Y then X
has a lower class number than Y.

   The way classes other than `GENERAL_REGS' are specified in operand
constraints is through machine-dependent operand constraint letters.
You can define such letters to correspond to various classes, then use
them in operand constraints.

   You should define a class for the union of two classes whenever some
instruction allows both classes.  For example, if an instruction allows
either a floating point (coprocessor) register or a general register
for a certain operand, you should define a class `FLOAT_OR_GENERAL_REGS'
which includes both of them.  Otherwise you will get suboptimal code.

   You must also specify certain redundant information about the
register classes: for each class, which classes contain it and which
ones are contained in it; for each pair of classes, the largest class
contained in their union.

   When a value occupying several consecutive registers is expected in a
certain class, all the registers used must belong to that class.
Therefore, register classes cannot be used to enforce a requirement for
a register pair to start with an even-numbered register.  The way to
specify this requirement is with `HARD_REGNO_MODE_OK'.

   Register classes used for input-operands of bitwise-and or shift
instructions have a special requirement: each such class must have, for
each fixed-point machine mode, a subclass whose registers can transfer
that mode to or from memory.  For example, on some machines, the
operations for single-byte values (`QImode') are limited to certain
registers.  When this is so, each register class that is used in a
bitwise-and or shift instruction must have a subclass consisting of
registers from which single-byte values can be loaded or stored.  This
is so that `PREFERRED_RELOAD_CLASS' can always have a possible value to
return.

`enum reg_class'
     An enumeral type that must be defined with all the register class
     names as enumeral values.  `NO_REGS' must be first.  `ALL_REGS'
     must be the last register class, followed by one more enumeral
     value, `LIM_REG_CLASSES', which is not a register class but rather
     tells how many classes there are.

     Each register class has a number, which is the value of casting
     the class name to type `int'.  The number serves as an index in
     many of the tables described below.

`N_REG_CLASSES'
     The number of distinct register classes, defined as follows:

          #define N_REG_CLASSES (int) LIM_REG_CLASSES

`REG_CLASS_NAMES'
     An initializer containing the names of the register classes as C
     string constants.  These names are used in writing some of the
     debugging dumps.

`REG_CLASS_CONTENTS'
     An initializer containing the contents of the register classes, as
     integers which are bit masks.  The Nth integer specifies the
     contents of class N.  The way the integer MASK is interpreted is
     that register R is in the class if `MASK & (1 << R)' is 1.

     When the machine has more than 32 registers, an integer does not
     suffice.  Then the integers are replaced by sub-initializers,
     braced groupings containing several integers.  Each
     sub-initializer must be suitable as an initializer for the type
     `HARD_REG_SET' which is defined in `hard-reg-set.h'.  In this
     situation, the first integer in each sub-initializer corresponds to
     registers 0 through 31, the second integer to registers 32 through
     63, and so on.

`REGNO_REG_CLASS (REGNO)'
     A C expression whose value is a register class containing hard
     register REGNO.  In general there is more than one such class;
     choose a class which is "minimal", meaning that no smaller class
     also contains the register.

`BASE_REG_CLASS'
     A macro whose definition is the name of the class to which a valid
     base register must belong.  A base register is one used in an
     address which is the register value plus a displacement.

`MODE_BASE_REG_CLASS (MODE)'
     This is a variation of the `BASE_REG_CLASS' macro which allows the
     selection of a base register in a mode depenedent manner.  If MODE
     is VOIDmode then it should return the same value as
     `BASE_REG_CLASS'.

`INDEX_REG_CLASS'
     A macro whose definition is the name of the class to which a valid
     index register must belong.  An index register is one used in an
     address where its value is either multiplied by a scale factor or
     added to another register (as well as added to a displacement).

`REG_CLASS_FROM_LETTER (CHAR)'
     A C expression which defines the machine-dependent operand
     constraint letters for register classes.  If CHAR is such a
     letter, the value should be the register class corresponding to
     it.  Otherwise, the value should be `NO_REGS'.  The register
     letter `r', corresponding to class `GENERAL_REGS', will not be
     passed to this macro; you do not need to handle it.

`REGNO_OK_FOR_BASE_P (NUM)'
     A C expression which is nonzero if register number NUM is suitable
     for use as a base register in operand addresses.  It may be either
     a suitable hard register or a pseudo register that has been
     allocated such a hard register.

`REGNO_MODE_OK_FOR_BASE_P (NUM, MODE)'
     A C expression that is just like `REGNO_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 `REGNO_OK_FOR_BASE_P'.

`REGNO_OK_FOR_INDEX_P (NUM)'
     A C expression which is nonzero if register number NUM is suitable
     for use as an index register in operand addresses.  It may be
     either a suitable hard register or a pseudo register that has been
     allocated such a hard 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.

`PREFERRED_RELOAD_CLASS (X, CLASS)'
     A C expression that places additional restrictions on the register
     class to use when it is necessary to copy value X into a register
     in class CLASS.  The value is a register class; perhaps CLASS, or
     perhaps another, smaller class.  On many machines, the following
     definition is safe:

          #define PREFERRED_RELOAD_CLASS(X,CLASS) CLASS

     Sometimes returning a more restrictive class makes better code.
     For example, on the 68000, when X is an integer constant that is
     in range for a `moveq' instruction, the value of this macro is
     always `DATA_REGS' as long as CLASS includes the data registers.
     Requiring a data register guarantees that a `moveq' will be used.

     If X is a `const_double', by returning `NO_REGS' you can force X
     into a memory constant.  This is useful on certain machines where
     immediate floating values cannot be loaded into certain kinds of
     registers.

`PREFERRED_OUTPUT_RELOAD_CLASS (X, CLASS)'
     Like `PREFERRED_RELOAD_CLASS', but for output reloads instead of
     input reloads.  If you don't define this macro, the default is to
     use CLASS, unchanged.

`LIMIT_RELOAD_CLASS (MODE, CLASS)'
     A C expression that places additional restrictions on the register
     class to use when it is necessary to be able to hold a value of
     mode MODE in a reload register for which class CLASS would
     ordinarily be used.

     Unlike `PREFERRED_RELOAD_CLASS', this macro should be used when
     there are certain modes that simply can't go in certain reload
     classes.

     The value is a register class; perhaps CLASS, or perhaps another,
     smaller class.

     Don't define this macro unless the target machine has limitations
     which require the macro to do something nontrivial.

`SECONDARY_RELOAD_CLASS (CLASS, MODE, X)'
`SECONDARY_INPUT_RELOAD_CLASS (CLASS, MODE, X)'
`SECONDARY_OUTPUT_RELOAD_CLASS (CLASS, MODE, X)'
     Many machines have some registers that cannot be copied directly
     to or from memory or even from other types of registers.  An
     example is the `MQ' register, which on most machines, can only be
     copied to or from general registers, but not memory.  Some
     machines allow copying all registers to and from memory, but
     require a scratch register for stores to some memory locations
     (e.g., those with symbolic address on the RT, and those with
     certain symbolic address on the Sparc when compiling PIC).  In
     some cases, both an intermediate and a scratch register are
     required.

     You should define these macros to indicate to the reload phase
     that it may need to allocate at least one register for a reload in
     addition to the register to contain the data.  Specifically, if
     copying X to a register CLASS in MODE requires an intermediate
     register, you should define `SECONDARY_INPUT_RELOAD_CLASS' to
     return the largest register class all of whose registers can be
     used as intermediate registers or scratch registers.

     If copying a register CLASS in MODE to X requires an intermediate
     or scratch register, `SECONDARY_OUTPUT_RELOAD_CLASS' should be
     defined to return the largest register class required.  If the
     requirements for input and output reloads are the same, the macro
     `SECONDARY_RELOAD_CLASS' should be used instead of defining both
     macros identically.

     The values returned by these macros are often `GENERAL_REGS'.
     Return `NO_REGS' if no spare register is needed; i.e., if X can be
     directly copied to or from a register of CLASS in MODE without
     requiring a scratch register.  Do not define this macro if it
     would always return `NO_REGS'.

     If a scratch register is required (either with or without an
     intermediate register), you should define patterns for
     `reload_inM' or `reload_outM', as required (*note Standard
     Names::.  These patterns, which will normally be implemented with
     a `define_expand', should be similar to the `movM' patterns,
     except that operand 2 is the scratch register.

     Define constraints for the reload register and scratch register
     that contain a single register class.  If the original reload
     register (whose class is CLASS) can meet the constraint given in
     the pattern, the value returned by these macros is used for the
     class of the scratch register.  Otherwise, two additional reload
     registers are required.  Their classes are obtained from the
     constraints in the insn pattern.

     X might be a pseudo-register or a `subreg' of a pseudo-register,
     which could either be in a hard register or in memory.  Use
     `true_regnum' to find out; it will return -1 if the pseudo is in
     memory and the hard register number if it is in a register.

     These macros should not be used in the case where a particular
     class of registers can only be copied to memory and not to another
     class of registers.  In that case, secondary reload registers are
     not needed and would not be helpful.  Instead, a stack location
     must be used to perform the copy and the `movM' pattern should use
     memory as an intermediate storage.  This case often occurs between
     floating-point and general registers.

`SECONDARY_MEMORY_NEEDED (CLASS1, CLASS2, M)'
     Certain machines have the property that some registers cannot be
     copied to some other registers without using memory.  Define this
     macro on those machines to be a C expression that is nonzero if
     objects of mode M in registers of CLASS1 can only be copied to
     registers of class CLASS2 by storing a register of CLASS1 into
     memory and loading that memory location into a register of CLASS2.

     Do not define this macro if its value would always be zero.

`SECONDARY_MEMORY_NEEDED_RTX (MODE)'
     Normally when `SECONDARY_MEMORY_NEEDED' is defined, the compiler
     allocates a stack slot for a memory location needed for register
     copies.  If this macro is defined, the compiler instead uses the
     memory location defined by this macro.

     Do not define this macro if you do not define
     `SECONDARY_MEMORY_NEEDED'.

`SECONDARY_MEMORY_NEEDED_MODE (MODE)'
     When the compiler needs a secondary memory location to copy
     between two registers of mode MODE, it normally allocates
     sufficient memory to hold a quantity of `BITS_PER_WORD' bits and
     performs the store and load operations in a mode that many bits
     wide and whose class is the same as that of MODE.

     This is right thing to do on most machines because it ensures that
     all bits of the register are copied and prevents accesses to the
     registers in a narrower mode, which some machines prohibit for
     floating-point registers.

     However, this default behavior is not correct on some machines,
     such as the DEC Alpha, that store short integers in floating-point
     registers differently than in integer registers.  On those
     machines, the default widening will not work correctly and you
     must define this macro to suppress that widening in some cases.
     See the file `alpha.h' for details.

     Do not define this macro if you do not define
     `SECONDARY_MEMORY_NEEDED' or if widening MODE to a mode that is
     `BITS_PER_WORD' bits wide is correct for your machine.

`SMALL_REGISTER_CLASSES'
     On some machines, it is risky to let hard registers live across
     arbitrary insns.  Typically, these machines have instructions that
     require values to be in specific registers (like an accumulator),
     and reload will fail if the required hard register is used for
     another purpose across such an insn.

     Define `SMALL_REGISTER_CLASSES' to be an expression with a nonzero
     value on these machines.  When this macro has a nonzero value, the
     compiler will try to minimize the lifetime of hard registers.

     It is always safe to define this macro with a nonzero value, but
     if you unnecessarily define it, you will reduce the amount of
     optimizations that can be performed in some cases.  If you do not
     define this macro with a nonzero value when it is required, the
     compiler will run out of spill registers and print a fatal error
     message.  For most machines, you should not define this macro at
     all.

`CLASS_LIKELY_SPILLED_P (CLASS)'
     A C expression whose value is nonzero if pseudos that have been
     assigned to registers of class CLASS would likely be spilled
     because registers of CLASS are needed for spill registers.

     The default value of this macro returns 1 if CLASS has exactly one
     register and zero otherwise.  On most machines, this default
     should be used.  Only define this macro to some other expression
     if pseudos allocated by `local-alloc.c' end up in memory because
     their hard registers were needed for spill registers.  If this
     macro returns nonzero for those classes, those pseudos will only
     be allocated by `global.c', which knows how to reallocate the
     pseudo to another register.  If there would not be another
     register available for reallocation, you should not change the
     definition of this macro since the only effect of such a
     definition would be to slow down register allocation.

`CLASS_MAX_NREGS (CLASS, MODE)'
     A C expression for the maximum number of consecutive registers of
     class CLASS needed to hold a value of mode MODE.

     This is closely related to the macro `HARD_REGNO_NREGS'.  In fact,
     the value of the macro `CLASS_MAX_NREGS (CLASS, MODE)' should be
     the maximum value of `HARD_REGNO_NREGS (REGNO, MODE)' for all
     REGNO values in the class CLASS.

     This macro helps control the handling of multiple-word values in
     the reload pass.

`CLASS_CANNOT_CHANGE_MODE'
     If defined, a C expression for a class that contains registers for
     which the compiler may not change modes arbitrarily.

`CLASS_CANNOT_CHANGE_MODE_P(FROM, TO)'
     A C expression that is true if, for a register in
     `CLASS_CANNOT_CHANGE_MODE', the requested mode punning is invalid.

     For the example, loading 32-bit integer or floating-point objects
     into floating-point registers on the Alpha extends them to 64 bits.
     Therefore loading a 64-bit object and then storing it as a 32-bit
     object does not store the low-order 32 bits, as would be the case
     for a normal register.  Therefore, `alpha.h' defines
     `CLASS_CANNOT_CHANGE_MODE' as `FLOAT_REGS' and
     `CLASS_CANNOT_CHANGE_MODE_P' restricts mode changes to same-size
     modes.

     Compare this to IA-64, which extends floating-point values to 82
     bits, and stores 64-bit integers in a different format than 64-bit
     doubles.  Therefore `CLASS_CANNOT_CHANGE_MODE_P' is always true.

   Three other special macros describe which operands fit which
constraint letters.

`CONST_OK_FOR_LETTER_P (VALUE, C)'
     A C expression that defines the machine-dependent operand
     constraint letters (`I', `J', `K', ... `P') that specify
     particular ranges of integer values.  If C is one of those
     letters, the expression should check that VALUE, an integer, is in
     the appropriate range and return 1 if so, 0 otherwise.  If C is
     not one of those letters, the value should be 0 regardless of
     VALUE.

`CONST_DOUBLE_OK_FOR_LETTER_P (VALUE, C)'
     A C expression that defines the machine-dependent operand
     constraint letters that specify particular ranges of
     `const_double' values (`G' or `H').

     If C is one of those letters, the expression should check that
     VALUE, an RTX of code `const_double', is in the appropriate range
     and return 1 if so, 0 otherwise.  If C is not one of those
     letters, the value should be 0 regardless of VALUE.

     `const_double' is used for all floating-point constants and for
     `DImode' fixed-point constants.  A given letter can accept either
     or both kinds of values.  It can use `GET_MODE' to distinguish
     between these kinds.

`EXTRA_CONSTRAINT (VALUE, C)'
     A C expression that defines the optional machine-dependent
     constraint letters that can be used to segregate specific types of
     operands, usually memory references, for the target machine.  Any
     letter that is not elsewhere defined and not matched by
     `REG_CLASS_FROM_LETTER' may be used.  Normally this macro will not
     be defined.

     If it is required for a particular target machine, it should
     return 1 if VALUE corresponds to the operand type represented by
     the constraint letter C.  If C is not defined as an extra
     constraint, the value returned should be 0 regardless of VALUE.

     For example, on the ROMP, load instructions cannot have their
     output in r0 if the memory reference contains a symbolic address.
     Constraint letter `Q' is defined as representing a memory address
     that does _not_ contain a symbolic address.  An alternative is
     specified with a `Q' constraint on the input and `r' on the
     output.  The next alternative specifies `m' on the input and a
     register class that does not include r0 on the output.


File: gccint.info,  Node: Stack and Calling,  Next: Varargs,  Prev: Register Classes,  Up: Target Macros

Stack Layout and Calling Conventions
====================================

   This describes the stack layout and calling conventions.

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