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

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File: gcc.info,  Node: 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'.

`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.

`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 a 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 non-zero 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 non-zero
     value on these machines.  When this macro has a non-zero value, the
     compiler will try to minimize the lifetime of hard registers.

     It is always safe to define this macro with a non-zero 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 non-zero 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_SIZE'
     If defined, a C expression for a class that contains registers
     which the compiler must always access in a mode that is the same
     size as the mode in which it loaded the register.

     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 this macro as
     `FLOAT_REGS'.

`TARGET_REG_CLASS_INIT'
     This macro can be used to change the contents of the register
     classes in a target specific way on a per function basis.  It is
     called at the start of the local alloc pass, just before the
     register class data structures are initialised.  If defined, the
     macro should contain C code to perform any target specific
     initialisation of the register classes.  This is used for example
     by the ARM port in order to change the REG_ALLOC_ORDER for fast
     interrupt handlers.

   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 (`Q', `R', `S', `T', `U') that can be used to
     segregate specific types of operands, usually memory references,
     for the target machine.  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: gcc.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.

* Menu:

* Frame Layout::
* Stack Checking::
* Frame Registers::
* Elimination::
* Stack Arguments::
* Register Arguments::
* Scalar Return::
* Aggregate Return::
* Caller Saves::
* Function Entry::
* Profiling::


File: gcc.info,  Node: Frame Layout,  Next: Stack Checking,  Up: Stack and Calling

Basic Stack Layout
------------------

   Here is the basic 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.

`ARGS_GROW_DOWNWARD'
     Define this macro if successive arguments to a function occupy
     decreasing addresses on the stack.

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

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

`STACK_POINTER_OFFSET'
     Offset from the stack pointer register to the first location at
     which outgoing arguments are placed.  If not specified, the
     default value of zero is used.  This is the proper value for most
     machines.

     If `ARGS_GROW_DOWNWARD', this is the offset to the location above
     the first location at which outgoing arguments are placed.

`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.

     If `ARGS_GROW_DOWNWARD', this is the offset to the location above
     the first argument's address.

`STACK_DYNAMIC_OFFSET (FUNDECL)'
     Offset from the stack pointer register to an item dynamically
     allocated on the stack, e.g., by `alloca'.

     The default value for this macro is `STACK_POINTER_OFFSET' plus the
     length of the outgoing arguments.  The default is correct for most
     machines.  See `function.c' for details.

`DYNAMIC_CHAIN_ADDRESS (FRAMEADDR)'
     A C expression whose value is RTL representing the address in a
     stack frame where the pointer to the caller's frame is stored.
     Assume that FRAMEADDR is an RTL expression for the address of the
     stack frame itself.

     If you don't define this macro, the default is to return the value
     of FRAMEADDR--that is, the stack frame address is also the address
     of the stack word that points to the previous frame.

`SETUP_FRAME_ADDRESSES'
     If defined, a C expression that produces the machine-specific code
     to setup the stack so that arbitrary frames can be accessed.  For
     example, on the Sparc, we must flush all of the register windows
     to the stack before we can access arbitrary stack frames.  You
     will seldom need to define this macro.

`BUILTIN_SETJMP_FRAME_VALUE'
     If defined, a C expression that contains an rtx that is used to
     store the address of the current frame into the built in `setjmp'
     buffer.  The default value, `virtual_stack_vars_rtx', is correct
     for most machines.  One reason you may need to define this macro
     is if `hard_frame_pointer_rtx' is the appropriate value on your
     machine.

`RETURN_ADDR_RTX (COUNT, FRAMEADDR)'
     A C expression whose value is RTL representing the value of the
     return address for the frame COUNT steps up from the current
     frame, after the prologue.  FRAMEADDR is the frame pointer of the
     COUNT frame, or the frame pointer of the COUNT - 1 frame if
     `RETURN_ADDR_IN_PREVIOUS_FRAME' is defined.

     The value of the expression must always be the correct address when
     COUNT is zero, but may be `NULL_RTX' if there is not way to
     determine the return address of other frames.

`RETURN_ADDR_IN_PREVIOUS_FRAME'
     Define this if the return address of a particular stack frame is
     accessed from the frame pointer of the previous stack frame.

`INCOMING_RETURN_ADDR_RTX'
     A C expression whose value is RTL representing the location of the
     incoming return address at the beginning of any function, before
     the prologue.  This RTL is either a `REG', indicating that the
     return value is saved in `REG', or a `MEM' representing a location
     in the stack.

     You only need to define this macro if you want to support call
     frame debugging information like that provided by DWARF 2.

`INCOMING_FRAME_SP_OFFSET'
     A C expression whose value is an integer giving the offset, in
     bytes, from the value of the stack pointer register to the top of
     the stack frame at the beginning of any function, before the
     prologue.  The top of the frame is defined to be the value of the
     stack pointer in the previous frame, just before the call
     instruction.

     You only need to define this macro if you want to support call
     frame debugging information like that provided by DWARF 2.

`ARG_POINTER_CFA_OFFSET'
     A C expression whose value is an integer giving the offset, in
     bytes, from the argument pointer to the canonical frame address
     (cfa).  The final value should coincide with that calculated by
     `INCOMING_FRAME_SP_OFFSET'.  Which is unfortunately not usable
     during virtual register instantiation.

     You only need to define this macro if you want to support call
     frame debugging information like that provided by DWARF 2.


File: gcc.info,  Node: Stack Checking,  Next: Frame Registers,  Prev: Frame Layout,  Up: Stack and Calling

Specifying How Stack Checking is Done
-------------------------------------

   GCC will check that stack references are within the boundaries of
the stack, if the `-fstack-check' is specified, in one of three ways:

  1. If the value of the `STACK_CHECK_BUILTIN' macro is nonzero, GCC
     will assume that you have arranged for stack checking to be done at
     appropriate places in the configuration files, e.g., in
     `FUNCTION_PROLOGUE'.  GCC will do not other special processing.

  2. If `STACK_CHECK_BUILTIN' is zero and you defined a named pattern
     called `check_stack' in your `md' file, GCC will call that pattern
     with one argument which is the address to compare the stack value
     against.  You must arrange for this pattern to report an error if
     the stack pointer is out of range.

  3. If neither of the above are true, GCC will generate code to
     periodically "probe" the stack pointer using the values of the
     macros defined below.

   Normally, you will use the default values of these macros, so GCC
will use the third approach.

`STACK_CHECK_BUILTIN'
     A nonzero value if stack checking is done by the configuration
     files in a machine-dependent manner.  You should define this macro
     if stack checking is require by the ABI of your machine or if you
     would like to have to stack checking in some more efficient way
     than GCC's portable approach.  The default value of this macro is
     zero.

`STACK_CHECK_PROBE_INTERVAL'
     An integer representing the interval at which GCC must generate
     stack probe instructions.  You will normally define this macro to
     be no larger than the size of the "guard pages" at the end of a
     stack area.  The default value of 4096 is suitable for most
     systems.

`STACK_CHECK_PROBE_LOAD'
     A integer which is nonzero if GCC should perform the stack probe
     as a load instruction and zero if GCC should use a store
     instruction.  The default is zero, which is the most efficient
     choice on most systems.

`STACK_CHECK_PROTECT'
     The number of bytes of stack needed to recover from a stack
     overflow, for languages where such a recovery is supported.  The
     default value of 75 words should be adequate for most machines.

`STACK_CHECK_MAX_FRAME_SIZE'
     The maximum size of a stack frame, in bytes.  GCC will generate
     probe instructions in non-leaf functions to ensure at least this
     many bytes of stack are available.  If a stack frame is larger
     than this size, stack checking will not be reliable and GCC will
     issue a warning.  The default is chosen so that GCC only generates
     one instruction on most systems.  You should normally not change
     the default value of this macro.

`STACK_CHECK_FIXED_FRAME_SIZE'
     GCC uses this value to generate the above warning message.  It
     represents the amount of fixed frame used by a function, not
     including space for any callee-saved registers, temporaries and
     user variables.  You need only specify an upper bound for this
     amount and will normally use the default of four words.

`STACK_CHECK_MAX_VAR_SIZE'
     The maximum size, in bytes, of an object that GCC will place in the
     fixed area of the stack frame when the user specifies
     `-fstack-check'.  GCC computed the default from the values of the
     above macros and you will normally not need to override that
     default.


File: gcc.info,  Node: Frame Registers,  Next: Elimination,  Prev: Stack Checking,  Up: Stack and Calling

Registers That Address the Stack Frame
--------------------------------------

   This discusses registers that address the stack frame.

`STACK_POINTER_REGNUM'
     The register number of the stack pointer register, which must also
     be a fixed register according to `FIXED_REGISTERS'.  On most
     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.

`HARD_FRAME_POINTER_REGNUM'
     On some machines the offset between the frame pointer and starting
     offset of the automatic variables is not known until after register
     allocation has been done (for example, because the saved registers
     are between these two locations).  On those machines, define
     `FRAME_POINTER_REGNUM' the number of a special, fixed register to
     be used internally until the offset is known, and define
     `HARD_FRAME_POINTER_REGNUM' to be the actual hard register number
     used for the frame pointer.

     You should define this macro only in the very rare circumstances
     when it is not possible to calculate the offset between the frame
     pointer and the automatic variables until after register
     allocation has been completed.  When this macro is defined, you
     must also indicate in your definition of `ELIMINABLE_REGS' how to
     eliminate `FRAME_POINTER_REGNUM' into either
     `HARD_FRAME_POINTER_REGNUM' or `STACK_POINTER_REGNUM'.

     Do not define this macro if it would be the same as
     `FRAME_POINTER_REGNUM'.

`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', or
     arrange to be able to eliminate it (*note Elimination::.).

`RETURN_ADDRESS_POINTER_REGNUM'
     The register number of the return address pointer register, which
     is used to access the current function's return address from the
     stack.  On some machines, the return address is not at a fixed
     offset from the frame pointer or stack pointer or argument
     pointer.  This register can be defined to point to the return
     address on the stack, and then be converted by `ELIMINABLE_REGS'
     into either the frame pointer or stack pointer.

     Do not define this macro unless there is no other way to get the
     return address from the stack.

`STATIC_CHAIN_REGNUM'
`STATIC_CHAIN_INCOMING_REGNUM'
     Register numbers used for passing a function's static chain
     pointer.  If register windows are used, the register number as
     seen by the called function is `STATIC_CHAIN_INCOMING_REGNUM',
     while the register number as seen by the calling function is
     `STATIC_CHAIN_REGNUM'.  If these registers are the same,
     `STATIC_CHAIN_INCOMING_REGNUM' need not be defined.

     The static chain register need not be a fixed register.

     If the static chain is passed in memory, these macros should not be
     defined; instead, the next two macros should be defined.

`STATIC_CHAIN'
`STATIC_CHAIN_INCOMING'
     If the static chain is passed in memory, these macros provide rtx
     giving `mem' expressions that denote where they are stored.
     `STATIC_CHAIN' and `STATIC_CHAIN_INCOMING' give the locations as
     seen by the calling and called functions, respectively.  Often the
     former will be at an offset from the stack pointer and the latter
     at an offset from the frame pointer.

     The variables `stack_pointer_rtx', `frame_pointer_rtx', and
     `arg_pointer_rtx' will have been initialized prior to the use of
     these macros and should be used to refer to those items.

     If the static chain is passed in a register, the two previous
     macros should be defined instead.


File: gcc.info,  Node: Elimination,  Next: Stack Arguments,  Prev: Frame Registers,  Up: Stack and Calling

Eliminating Frame Pointer and Arg Pointer
-----------------------------------------

   This is about eliminating the frame pointer and arg pointer.

`FRAME_POINTER_REQUIRED'
     A C expression which is nonzero if a function must have and use a
     frame pointer.  This expression is evaluated  in the reload pass.
     If its value is nonzero 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.

`INITIAL_FRAME_POINTER_OFFSET (DEPTH-VAR)'
     A C statement to store in the variable DEPTH-VAR the difference
     between the frame pointer and the stack pointer values immediately
     after the function prologue.  The value would be computed from
     information such as the result of `get_frame_size ()' and the
     tables of registers `regs_ever_live' and `call_used_regs'.

     If `ELIMINABLE_REGS' is defined, this macro will be not be used and
     need not be defined.  Otherwise, it must be defined even if
     `FRAME_POINTER_REQUIRED' is defined to always be true; in that
     case, you may set DEPTH-VAR to anything.

`ELIMINABLE_REGS'
     If defined, this macro specifies a table of register pairs used to
     eliminate unneeded registers that point into the stack frame.  If
     it is not defined, the only elimination attempted by the compiler
     is to replace references to the frame pointer with references to
     the stack pointer.

     The definition of this macro is a list of structure
     initializations, each of which specifies an original and
     replacement register.

     On some machines, the position of the argument pointer is not
     known until the compilation is completed.  In such a case, a
     separate hard register must be used for the argument pointer.
     This register can be eliminated by replacing it with either the
     frame pointer or the argument pointer, depending on whether or not
     the frame pointer has been eliminated.

     In this case, you might specify:
          #define ELIMINABLE_REGS  \
          {{ARG_POINTER_REGNUM, STACK_POINTER_REGNUM}, \
           {ARG_POINTER_REGNUM, FRAME_POINTER_REGNUM}, \
           {FRAME_POINTER_REGNUM, STACK_POINTER_REGNUM}}

     Note that the elimination of the argument pointer with the stack
     pointer is specified first since that is the preferred elimination.

`CAN_ELIMINATE (FROM-REG, TO-REG)'
     A C expression that returns non-zero if the compiler is allowed to
     try to replace register number FROM-REG with register number
     TO-REG.  This macro need only be defined if `ELIMINABLE_REGS' is
     defined, and will usually be the constant 1, since most of the
     cases preventing register elimination are things that the compiler
     already knows about.

`INITIAL_ELIMINATION_OFFSET (FROM-REG, TO-REG, OFFSET-VAR)'
     This macro is similar to `INITIAL_FRAME_POINTER_OFFSET'.  It
     specifies the initial difference between the specified pair of
     registers.  This macro must be defined if `ELIMINABLE_REGS' is
     defined.

`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: Stack Arguments,  Next: Register Arguments,  Prev: Elimination,  Up: Stack and Calling

Passing Function Arguments on the Stack
---------------------------------------

   The macros in this section control how arguments are passed on the
stack.  See the following section for other macros that control passing
certain arguments in registers.

`PROMOTE_PROTOTYPES'
     A C expression whose value is nonzero if an argument declared in a
     prototype as an integral type smaller than `int' should actually
     be passed as an `int'.  In addition to avoiding errors in certain
     cases of mismatch, it also makes for better code on certain
     machines.  If the macro is not defined in target header files, it
     defaults to 0.

`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 GCC 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)

`ACCUMULATE_OUTGOING_ARGS'
     If defined, the maximum amount of space required for outgoing
     arguments will be computed and placed into the variable
     `current_function_outgoing_args_size'.  No space will be pushed
     onto the stack for each call; instead, the function prologue should
     increase the stack frame size by this amount.

     Defining both `PUSH_ROUNDING' and `ACCUMULATE_OUTGOING_ARGS' is
     not proper.

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

     The value of this macro is the size, in bytes, of the area
     reserved for arguments passed in registers for the function
     represented by FNDECL, which can be zero if GCC is calling a
     library function.

     This space can be allocated by the caller, or be a part of the
     machine-dependent stack frame: `OUTGOING_REG_PARM_STACK_SPACE' says
     which.

`MAYBE_REG_PARM_STACK_SPACE'
`FINAL_REG_PARM_STACK_SPACE (CONST_SIZE, VAR_SIZE)'
     Define these macros in addition to the one above if functions might
     allocate stack space for arguments even when their values are
     passed in registers.  These should be used when the stack space
     allocated for arguments in registers is not a simple constant
     independent of the function declaration.

     The value of the first macro is the size, in bytes, of the area
     that we should initially assume would be reserved for arguments
     passed in registers.

     The value of the second macro is the actual size, in bytes, of the
     area that will be reserved for arguments passed in registers.
     This takes two arguments: an integer representing the number of
     bytes of fixed sized arguments on the stack, and a tree
     representing the number of bytes of variable sized arguments on
     the stack.

     When these macros are defined, `REG_PARM_STACK_SPACE' will only be
     called for libcall functions, the current function, or for a
     function being called when it is known that such stack space must
     be allocated.  In each case this value can be easily computed.

     When deciding whether a called function needs such stack space,
     and how much space to reserve, GCC uses these two macros instead of
     `REG_PARM_STACK_SPACE'.

`OUTGOING_REG_PARM_STACK_SPACE'
     Define this if it is the responsibility of the caller to allocate
     the area reserved for arguments passed in registers.

     If `ACCUMULATE_OUTGOING_ARGS' is defined, this macro controls
     whether the space for these arguments counts in the value of
     `current_function_outgoing_args_size'.

`STACK_PARMS_IN_REG_PARM_AREA'
     Define this macro if `REG_PARM_STACK_SPACE' is defined, but the
     stack parameters don't skip the area specified by it.

     Normally, when a parameter is not passed in registers, it is
     placed on the stack beyond the `REG_PARM_STACK_SPACE' area.
     Defining this macro suppresses this behavior and causes the
     parameter to be passed on the stack in its natural location.

`RETURN_POPS_ARGS (FUNDECL, FUNTYPE, STACK-SIZE)'
     A C expression that should indicate the number of bytes of its own
     arguments that a function pops on returning, or 0 if the function
     pops no arguments and the caller must therefore pop them all after
     the function returns.

     FUNDECL is a C variable whose value is a tree node that describes
     the function in question.  Normally it is a node of type
     `FUNCTION_DECL' that describes the declaration of the function.
     From this you can obtain the DECL_MACHINE_ATTRIBUTES of the
     function.

     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, FUNDECL
     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.

     STACK-SIZE is the number of bytes of arguments passed on the
     stack.  If a variable number of bytes is passed, it is zero, and
     argument popping will always be the responsibility of the calling
     function.

     On the Vax, all functions always pop their arguments, so the
     definition of this macro is STACK-SIZE.  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.

