This is Info file gcc.info, produced by Makeinfo-1.47 from the input
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   This file documents the use and the internals of the GNU compiler.

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File: gcc.info,  Node: Registers,  Next: Register Classes,  Prev: Storage Layout,  Up: Machine Macros

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

`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.

`DEFAULT_CALLER_SAVES'
     Define this macro if function calls on the target machine do not
     preserve any registers; in other words, if `CALL_USED_REGISTERS'
     has 1 for all registers.  This macro enables `-fcaller-saves' by
     default. Eventually that option will be enabled by default on all
     machines and both the option and this macro will be eliminated.

`CONDITIONAL_REGISTER_USAGE'
     Zero or more C statements that may conditionally modify two
     variables `fixed_regs' and `call_used_regs' (both of type `char
     []') after they have been initialized from the two preceding
     macros.

     This is necessary in case the fixed or call-clobbered registers
     depend on target flags.

     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.)

`OVERLAPPING_REGNO_P (REGNO)'
     If defined, this is a C expression whose value is nonzero if hard
     register number REGNO is an overlapping register.  This means a
     hard register which overlaps a hard register with a different
     number. (Such overlap is undesirable, but occasionally it allows a
     machine to be supported which otherwise could not be.)  This macro
     must return nonzero for *all* the registers which overlap each
     other.  GNU CC can use an overlapping register only in certain
     limited ways.  It can be used for allocation within a basic block,
     and may be spilled for reloading; that is all.

     If this macro is not defined, it means that none of the hard
     registers overlap each other.  This is the usual situation.

`INSN_CLOBBERS_REGNO_P (INSN, REGNO)'
     If defined, this is a C expression whose value should be nonzero if
     the insn INSN has the effect of mysteriously clobbering the
     contents of hard register number REGNO.  By "mysterious" we mean
     that the insn's RTL expression doesn't describe such an effect.

     If this macro is not defined, it means that no insn clobbers
     registers mysteriously.  This is the usual situation; all else
     being equal, it is best for the RTL expression to show all the
     activity.

`PRESERVE_DEATH_INFO_REGNO_P (REGNO)'
     If defined, this is a C expression whose value is nonzero if
     accurate `REG_DEAD' notes are needed for hard register number REGNO
     at the time of outputting the assembler code.  When this is so, a
     few optimizations that take place after register allocation and
     could invalidate the death notes are not done when this register is
     involved.

     You would arrange to preserve death info for a register when some
     of the code in the machine description which is executed to write
     the assembler code looks at the death notes.  This is necessary
     only when the actual hardware feature which GNU CC thinks of as a
     register is not actually a register of the usual sort.  (It might,
     for example, be a hardware stack.)

     If this macro is not defined, it means that no death notes need to
     be preserved.  This is the usual situation.

`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

     It is not necessary for this macro 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.  The way to implement that is to define this macro
     to reject odd register numbers for such modes.

     GNU CC assumes that it can always move values between registers and
     (suitably addressed) memory locations.  If it is impossible to
     move a value of a certain mode between memory and certain
     registers, then `HARD_REGNO_MODE_OK' must not allow this mode in
     those registers.

     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.

     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 and this macro should 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 insn's
     constraint asks for one.

`MODES_TIEABLE_P (MODE1, MODE2)'
     A C expression that is nonzero if it is desirable to choose
     register allocation so as to avoid move instructions between a
     value of mode MODE1 and a value of mode MODE2.

     If `HARD_REGNO_MODE_OK (R, MODE1)' and `HARD_REGNO_MODE_OK (R,
     MODE2)' are ever different for any R, then `MODES_TIEABLE_P (MODE1,
     MODE2)' must be zero.

`PC_REGNUM'
     If the program counter has a register number, define this as that
     register number.  Otherwise, do not define it.

`STACK_POINTER_REGNUM'
     The register number of the stack pointer register, which must also
     be a fixed register according to `FIXED_REGISTERS'.  On many
     machines, the hardware determines which register this is.

`FRAME_POINTER_REGNUM'
     The register number of the frame pointer register, which is used to
     access automatic variables in the stack frame.  On some machines,
     the hardware determines which register this is.  On other
     machines, you can choose any register you wish for this purpose.

`FRAME_POINTER_REQUIRED'
     A C expression which is nonzero if a function must have and use a
     frame pointer.  This expression is evaluated twice: at the
     beginning of generating RTL, and in the reload pass.  If its value
     is nonzero at either time, then the function will have a frame
     pointer.

     The expression can in principle examine the current function and
     decide according to the facts, but on most machines the constant 0
     or the constant 1 suffices.  Use 0 when the machine allows code to
     be generated with no frame pointer, and doing so saves some time
     or space.  Use 1 when there is no possible advantage to avoiding a
     frame pointer.

     In certain cases, the compiler does not know how to produce valid
     code without a frame pointer.  The compiler recognizes those cases
     and automatically gives the function a frame pointer regardless of
     what `FRAME_POINTER_REQUIRED' says.  You don't need to worry about
     them.

     In a function that does not require a frame pointer, the frame
     pointer register can be allocated for ordinary usage, unless you
     mark it as a fixed register.  See `FIXED_REGISTERS' for more
     information.

`ARG_POINTER_REGNUM'
     The register number of the arg pointer register, which is used to
     access the function's argument list.  On some machines, this is the
     same as the frame pointer register.  On some machines, the hardware
     determines which register this is.  On other machines, you can
     choose any register you wish for this purpose.  If this is not the
     same register as the frame pointer register, then you must mark it
     as a fixed register according to `FIXED_REGISTERS'.

`STATIC_CHAIN_REGNUM'
     The register number used for passing a function's static chain
     pointer.  This is needed for languages such as Pascal and Algol
     where functions defined within other functions can access the local
     variables of the outer functions; it is not currently used because
     C does not provide this feature, but you must define the macro.

     The static chain register need not be a fixed register.

`STRUCT_VALUE_REGNUM'
     When a function's value's mode is `BLKmode', the value is not
     returned according to `FUNCTION_VALUE'.  Instead, the caller
     passes the address of a block of memory in which the value should
     be stored.

     If this value is passed in a register, then `STRUCT_VALUE_REGNUM'
     should be the number of that register.

`STRUCT_VALUE'
     If the structure value address is not passed in a register, define
     `STRUCT_VALUE' as an expression returning an RTX for the place
     where the address is passed.  If it returns a `mem' RTX, the
     address is passed as an "invisible" first argument.

`STRUCT_VALUE_INCOMING_REGNUM'
     On some architectures the place where the structure value address
     is found by the called function is not the same place that the
     caller put it.  This can be due to register windows, or it could
     be because the function prologue moves it to a different place.

     If the incoming location of the structure value address is in a
     register, define this macro as the register number.

`STRUCT_VALUE_INCOMING'
     If the incoming location is not a register, define
     `STRUCT_VALUE_INCOMING' as an expression for an RTX for where the
     called function should find the value.  If it should find the
     value on the stack, define this to create a `mem' which refers to
     the frame pointer.  If the value is a `mem', the compiler assumes
     it is for an invisible first argument, and leaves space for it when
     finding the first real argument.

`REG_ALLOC_ORDER'
     If defined, an initializer for a vector of integers, containing the
     numbers of hard registers in the order in which the GNU CC 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 the 360, where the highest numbered
     registers must always be saved and the save-multiple-registers
     instruction supports only sequences of consecutive registers.  This
     macro is defined to cause the highest numbered allocatable
     registers to be used first.


File: gcc.info,  Node: Register Classes,  Next: Stack Layout,  Prev: Registers,  Up: Machine 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'.

   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'.

`REGNO_REG_CLASS (REGNO)'
     A C expression whose value is a register class containing hard
     register REGNO.  In general there is more that 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.

`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'.

`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_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 definition

          #define PREFERRED_RELOAD_CLASS(X,CLASS) CLASS

     is safe.

     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.

     In a shift instruction or a bitwise-and instruction, the mode of X,
     the value being reloaded, may not be the same as the mode of the
     instruction's operand.  (They will both be fixed-point modes,
     however.)  In such a case, CLASS may not be a safe value to
     return.  CLASS is certainly valid for the instruction, but it may
     not be valid for reloading X.  This problem can occur on machines
     such as the 68000 and 80386 where some registers can handle
     full-word values but cannot handle single-byte values.

     On such machines, this macro must examine the mode of X and return
     a subclass of CLASS which can handle loads and stores of that
     mode.  On the 68000, where address registers cannot handle
     `QImode', if X has `QImode' then you must return `DATA_REGS'.  If
     CLASS is `ADDR_REGS', then there is no correct value to return;
     but the shift and bitwise-and instructions don't use `ADDR_REGS',
     so this fatal case never arises.

`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.

   Two other special macros describe which constants fit which
constraint letters.

`CONST_OK_FOR_LETTER_P (VALUE, C)'
     A C expression that defines the machine-dependent operand
     constraint letters 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 floating
     values.  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.


File: gcc.info,  Node: Stack Layout,  Next: Library Calls,  Prev: Register Classes,  Up: Machine Macros

Describing Stack Layout
=======================

`STACK_GROWS_DOWNWARD'
     Define this macro if pushing a word onto the stack moves the stack
     pointer to a smaller address.

     When we say, "define this macro if ...," it means that the
     compiler checks this macro only with `#ifdef' so the precise
     definition used does not matter.

`FRAME_GROWS_DOWNWARD'
     Define this macro if the addresses of local variable slots are at
     negative offsets from the frame pointer.

`STARTING_FRAME_OFFSET'
     Offset from the frame pointer to the first local variable slot to
     be allocated.

     If `FRAME_GROWS_DOWNWARD', the next slot's offset is found by
     subtracting the length of the first slot from
     `STARTING_FRAME_OFFSET'. Otherwise, it is found by adding the
     length of the first slot to the value `STARTING_FRAME_OFFSET'.

`PUSH_ROUNDING (NPUSHED)'
     A C expression that is the number of bytes actually pushed onto the
     stack when an instruction attempts to push NPUSHED bytes.

     If the target machine does not have a push instruction, do not
     define this macro.  That directs GNU CC to use an alternate
     strategy: to allocate the entire argument block and then store the
     arguments into it.

     On some machines, the definition

          #define PUSH_ROUNDING(BYTES) (BYTES)

     will suffice.  But on other machines, instructions that appear to
     push one byte actually push two bytes in an attempt to maintain
     alignment.  Then the definition should be

          #define PUSH_ROUNDING(BYTES) (((BYTES) + 1) & ~1)

`FIRST_PARM_OFFSET (FUNDECL)'
     Offset from the argument pointer register to the first argument's
     address.  On some machines it may depend on the data type of the
     function.  (In the next version of GNU CC, the argument will be
     changed to the function data type rather than its declaration.)

`FIRST_PARM_CALLER_OFFSET (FUNDECL)'
     Define this macro on machines where register parameters have shadow
     locations on the stack, at addresses below the nominal parameter.
     This matters because certain arguments cannot be passed on the
     stack. On these machines, such arguments must be stored into the
     shadow locations.

     This macro should expand into a C expression whose value is the
     offset of the first parameter's shadow location from the nominal
     stack pointer value.  (That value is itself computed by adding the
     value of `STACK_POINTER_OFFSET' to the stack pointer register.)

`REG_PARM_STACK_SPACE'
     Define this macro if functions should assume that stack space has
     been allocated for arguments even when their values are passed in
     registers.

     The actual allocation of such space would be done either by the
     call instruction or by the function prologue, or by defining
     `FIRST_PARM_CALLER_OFFSET'.

`STACK_ARGS_ADJUST (SIZE)'
     Define this macro if the machine requires padding on the stack for
     certain function calls.  This is padding on a per-function-call
     basis, not padding for individual arguments.

     The argument SIZE will be a C variable of type `struct arg_data'
     which contains two fields, an integer named `constant' and an RTX
     named `var'.  These together represent a size measured in bytes
     which is the sum of the integer and the RTX.  Most of the time
     `var' is 0, which means that the size is simply the integer.

     The definition should be a C statement or compound statement which
     alters the variable supplied in whatever way you wish.

     Note that the value you leave in the variable `size' will
     ultimately be rounded up to a multiple of `STACK_BOUNDARY' bits.

     This macro is not fully implemented for machines which have push
     instructions (i.e., on which `PUSH_ROUNDING' is defined).

`RETURN_POPS_ARGS (FUNTYPE)'
     A C expression that should be 1 if a function pops its own
     arguments on returning, or 0 if the function pops no arguments and
     the caller must therefore pop them all after the function returns.

     FUNTYPE is a C variable whose value is a tree node that describes
     the function in question.  Normally it is a node of type
     `FUNCTION_TYPE' that describes the data type of the function. From
     this it is possible to obtain the data types of the value and
     arguments (if known).

     When a call to a library function is being considered, FUNTYPE
     will contain an identifier node for the library function.  Thus, if
     you need to distinguish among various library functions, you can
     do so by their names.  Note that "library function" in this
     context means a function used to perform arithmetic, whose name is
     known specially in the compiler and was not mentioned in the C
     code being compiled.

     On the Vax, all functions always pop their arguments, so the
     definition of this macro is 1.  On the 68000, using the standard
     calling convention, no functions pop their arguments, so the value
     of the macro is always 0 in this case.  But an alternative calling
     convention is available in which functions that take a fixed
     number of arguments pop them but other functions (such as
     `printf') pop nothing (the caller pops all).  When this convention
     is in use, FUNTYPE is examined to determine whether a function
     takes a fixed number of arguments.

     When this macro returns nonzero, the macro `FRAME_POINTER_REQUIRED'
     must also return nonzero for proper operation.

`FUNCTION_VALUE (VALTYPE, FUNC)'
     A C expression to create an RTX representing the place where a
     function returns a value of data type VALTYPE.  VALTYPE is a tree
     node representing a data type.  Write `TYPE_MODE (VALTYPE)' to get
     the machine mode used to represent that type. On many machines,
     only the mode is relevant.  (Actually, on most machines, scalar
     values are returned in the same place regardless of mode).

     If the precise function being called is known, FUNC is a tree node
     (`FUNCTION_DECL') for it; otherwise, FUNC is a null pointer.  This
     makes it possible to use a different value-returning convention
     for specific functions when all their calls are known.

`FUNCTION_OUTGOING_VALUE (VALTYPE, FUNC)'
     Define this macro if the target machine has "register windows" so
     that the register in which a function returns its value is not the
     same as the one in which the caller sees the value.

     For such machines, `FUNCTION_VALUE' computes the register in which
     the caller will see the value, and `FUNCTION_OUTGOING_VALUE'
     should be defined in a similar fashion to tell the function where
     to put the value.

     If `FUNCTION_OUTGOING_VALUE' is not defined, `FUNCTION_VALUE'
     serves both purposes.

`RETURN_IN_MEMORY (TYPE)'
     A C expression which can inhibit the returning of certain function
     values in registers, based on the type of value.  A nonzero value
     says to return the function value in memory, just as large
     structures are always returned.  Here TYPE will be a C expression
     of type `tree', representing the data type of the value.

     Note that values of mode `BLKmode' are returned in memory
     regardless of this macro.  Also, the option `-fpcc-struct-return'
     takes effect regardless of this macro.  On most systems, it is
     possible to leave the macro undefined; this causes a default
     definition to be used, whose value is the constant 0.

`LIBCALL_VALUE (MODE)'
     A C expression to create an RTX representing the place where a
     library function returns a value of mode MODE.  If the precise
     function being called is known, FUNC is a tree node
     (`FUNCTION_DECL') for it; otherwise, FUNC is a null pointer.  This
     makes it possible to use a different value-returning convention
     for specific functions when all their calls are known.

     Note that "library function" in this context means a compiler
     support routine, used to perform arithmetic, whose name is known
     specially by the compiler and was not mentioned in the C code being
     compiled.

`FUNCTION_VALUE_REGNO_P (REGNO)'
     A C expression that is nonzero if REGNO is the number of a hard
     register in which the values of called function may come back.

     A register whose use for returning values is limited to serving as
     the second of a pair (for a value of type `double', say) need not
     be recognized by this macro.  So for most machines, this definition
     suffices:

          #define FUNCTION_VALUE_REGNO_P(N) ((N) == 0)

     If the machine has register windows, so that the caller and the
     called function use different registers for the return value, this
     macro should recognize only the caller's register numbers.

`FUNCTION_ARG (CUM, MODE, TYPE, NAMED)'
     A C expression that controls whether a function argument is passed
     in a register, and which register.

     The arguments are CUM, which summarizes all the previous
     arguments; MODE, the machine mode of the argument; TYPE, the data
     type of the argument as a tree node or 0 if that is not known
     (which happens for C support library functions); and NAMED, which
     is 1 for an ordinary argument and 0 for nameless arguments that
     correspond to `...' in the called function's prototype.

     The value of the expression should either be a `reg' RTX for the
     hard register in which to pass the argument, or zero to pass the
     argument on the stack.

     For the Vax and 68000, where normally all arguments are pushed,
     zero suffices as a definition.

     The usual way to make the ANSI library `stdarg.h' work on a machine
     where some arguments are usually passed in registers, is to cause
     nameless arguments to be passed on the stack instead.  This is done
     by making `FUNCTION_ARG' return 0 whenever NAMED is 0.

`FUNCTION_INCOMING_ARG (CUM, MODE, TYPE, NAMED)'
     Define this macro if the target machine has "register windows", so
     that the register in which a function sees an arguments is not
     necessarily the same as the one in which the caller passed the
     argument.

     For such machines, `FUNCTION_ARG' computes the register in which
     the caller passes the value, and `FUNCTION_INCOMING_ARG' should be
     defined in a similar fashion to tell the function being called
     where the arguments will arrive.

     If `FUNCTION_INCOMING_ARG' is not defined, `FUNCTION_ARG' serves
     both purposes.

`FUNCTION_ARG_PARTIAL_NREGS (CUM, MODE, TYPE, NAMED)'
     A C expression for the number of words, at the beginning of an
     argument, must be put in registers.  The value must be zero for
     arguments that are passed entirely in registers or that are
     entirely pushed on the stack.

     On some machines, certain arguments must be passed partially in
     registers and partially in memory.  On these machines, typically
     the first N words of arguments are passed in registers, and the
     rest on the stack.  If a multi-word argument (a `double' or a
     structure) crosses that boundary, its first few words must be
     passed in registers and the rest must be pushed.  This macro tells
     the compiler when this occurs, and how many of the words should go
     in registers.

     `FUNCTION_ARG' for these arguments should return the first
     register to be used by the caller for this argument; likewise
     `FUNCTION_INCOMING_ARG', for the called function.

`CUMULATIVE_ARGS'
     A C type for declaring a variable that is used as the first
     argument of `FUNCTION_ARG' and other related values.  For some
     target machines, the type `int' suffices and can hold the number of
     bytes of argument so far.

`INIT_CUMULATIVE_ARGS (CUM, FNTYPE)'
     A C statement (sans semicolon) for initializing the variable CUM
     for the state at the beginning of the argument list.  The variable
     has type `CUMULATIVE_ARGS'.  The value of FNTYPE is the tree node
     for the data type of the function which will receive the args, or 0
     if the args are to a compiler support library function.

`FUNCTION_ARG_ADVANCE (CUM, MODE, TYPE, NAMED)'
     A C statement (sans semicolon) to update the summarizer variable
     CUM to advance past an argument in the argument list.  The values
     MODE, TYPE and NAMED describe that argument. Once this is done,
     the variable CUM is suitable for analyzing the *following*
     argument with `FUNCTION_ARG', etc.

`FUNCTION_ARG_REGNO_P (REGNO)'
     A C expression that is nonzero if REGNO is the number of a hard
     register in which function arguments are sometimes passed.  This
     does *not* include implicit arguments such as the static chain and
     the structure-value address.  On many machines, no registers can be
     used for this purpose since all function arguments are pushed on
     the stack.

`FUNCTION_ARG_PADDING (MODE, SIZE)'
     If defined, a C expression which determines whether, and in which
     direction, to pad out an argument with extra space.  The value
     should be of type `enum direction': either `upward' to pad above
     the argument, `downward' to pad below, or `none' to inhibit
     padding.

     The argument SIZE is an RTX which describes the size of the
     argument, in bytes.  It should be used only if MODE is `BLKmode'. 
     Otherwise, SIZE is 0.

     This macro does not control the *amount* of padding; that is
     always just enough to reach the next multiple of `PARM_BOUNDARY'.

     This macro has a default definition which is right for most
     systems. For little-endian machines, the default is to pad upward.
      For big-endian machines, the default is to pad downward for an
     argument of constant size shorter than an `int', and upward
     otherwise.

`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, but not if it is one of the
     call-used registers.

     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.

     On machines where an argument may be passed partly in registers and
     partly in memory, this macro must examine the variable
     `current_function_pretend_args_size', and allocate that many bytes
     of uninitialized space on the stack just underneath the first
     argument arriving on the stack.  (This may not be at the very end
     of the stack, 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.)

`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.

`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.

`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 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.

     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 nonzero if the function should pop its own arguments.  If so,
     use the variable `current_function_args_size' as the number of
     bytes to pop.

`FIX_FRAME_POINTER_ADDRESS (ADDR, DEPTH)'
     A C compound statement to alter a memory address that uses the
     frame pointer register so that it uses the stack pointer register
     instead. This must be done in the instructions that load parameter
     values into registers, when the reload pass determines that a
     frame pointer is not necessary for the function.  ADDR will be a C
     variable name, and the updated address should be stored in that
     variable.  DEPTH will be the current depth of stack temporaries
     (number of bytes of arguments currently pushed).  The change in
     offset between a frame-pointer-relative address and a
     stack-pointer-relative address must include DEPTH.

     Even if your machine description specifies there will always be a
     frame pointer in the frame pointer register, you must still define
     `FIX_FRAME_POINTER_ADDRESS', but the definition will never be
     executed at run time, so it may be empty.

`LONGJMP_RESTORE_FROM_STACK'
     Define this macro if the `longjmp' function restores registers
     from the stack frames, rather than from those saved specifically by
     `setjmp'.  Certain quantities must not be kept in registers across
     a call to `setjmp' on such machines.


File: gcc.info,  Node: Library Calls,  Next: Addressing Modes,  Prev: Stack Layout,  Up: Machine Macros

Implicit Use of 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 `gnulib'.

`UMULSI3_LIBCALL'
     A C string constant giving the name of the function to call for
     multiplication of one unsigned full-word by another.  If you do not
     define this macro, the default name is used, which is `__umulsi3',
     a function defined in `gnulib'.

`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 `gnulib'.

`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 `gnulib'.

`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 `gnulib'.

`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 `gnulib'.

`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'.

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

