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Initial commit of GCC 2.8.1
2020-09-21 03:06:00 +02:00
This is Info file gcc.info, produced by Makeinfo version 1.67 from the
input file gcc.texi.
This file documents the use and the internals of the GNU compiler.
Published by the Free Software Foundation 59 Temple Place - Suite 330
Boston, MA 02111-1307 USA
Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998
Free Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.
Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided also
that the sections entitled "GNU General Public License," "Funding for
Free Software," and "Protect Your Freedom--Fight `Look And Feel'" are
included exactly as in the original, and provided that the entire
resulting derived work is distributed under the terms of a permission
notice identical to this one.
Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that the sections entitled "GNU General Public
License," "Funding for Free Software," and "Protect Your Freedom--Fight
`Look And Feel'", and this permission notice, may be included in
translations approved by the Free Software Foundation instead of in the
original English.

File: gcc.info, Node: Registers, Next: Register Classes, Prev: Type Layout, Up: Target Macros
Register Usage
==============
This section explains how to describe what registers the target
machine has, and how (in general) they can be used.
The description of which registers a specific instruction can use is
done with register classes; see *Note Register Classes::. For
information on using registers to access a stack frame, see *Note Frame
Registers::. For passing values in registers, see *Note Register
Arguments::. For returning values in registers, see *Note Scalar
Return::.
* Menu:
* Register Basics:: Number and kinds of registers.
* Allocation Order:: Order in which registers are allocated.
* Values in Registers:: What kinds of values each reg can hold.
* Leaf Functions:: Renumbering registers for leaf functions.
* Stack Registers:: Handling a register stack such as 80387.
* Obsolete Register Macros:: Macros formerly used for the 80387.

File: gcc.info, Node: Register Basics, Next: Allocation Order, Up: Registers
Basic Characteristics of Registers
----------------------------------
Registers have various characteristics.
`FIRST_PSEUDO_REGISTER'
Number of hardware registers known to the compiler. They receive
numbers 0 through `FIRST_PSEUDO_REGISTER-1'; thus, the first
pseudo register's number really is assigned the number
`FIRST_PSEUDO_REGISTER'.
`FIXED_REGISTERS'
An initializer that says which registers are used for fixed
purposes all throughout the compiled code and are therefore not
available for general allocation. These would include the stack
pointer, the frame pointer (except on machines where that can be
used as a general register when no frame pointer is needed), the
program counter on machines where that is considered one of the
addressable registers, and any other numbered register with a
standard use.
This information is expressed as a sequence of numbers, separated
by commas and surrounded by braces. The Nth number is 1 if
register N is fixed, 0 otherwise.
The table initialized from this macro, and the table initialized by
the following one, may be overridden at run time either
automatically, by the actions of the macro
`CONDITIONAL_REGISTER_USAGE', or by the user with the command
options `-ffixed-REG', `-fcall-used-REG' and `-fcall-saved-REG'.
`CALL_USED_REGISTERS'
Like `FIXED_REGISTERS' but has 1 for each register that is
clobbered (in general) by function calls as well as for fixed
registers. This macro therefore identifies the registers that are
not available for general allocation of values that must live
across function calls.
If a register has 0 in `CALL_USED_REGISTERS', the compiler
automatically saves it on function entry and restores it on
function exit, if the register is used within the function.
`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.)
`NON_SAVING_SETJMP'
If this macro is defined and has a nonzero value, it means that
`setjmp' and related functions fail to save the registers, or that
`longjmp' fails to restore them. To compensate, the compiler
avoids putting variables in registers in functions that use
`setjmp'.
`INCOMING_REGNO (OUT)'
Define this macro if the target machine has register windows.
This C expression returns the register number as seen by the
called function corresponding to the register number OUT as seen
by the calling function. Return OUT if register number OUT is not
an outbound register.
`OUTGOING_REGNO (IN)'
Define this macro if the target machine has register windows.
This C expression returns the register number as seen by the
calling function corresponding to the register number IN as seen
by the called function. Return IN if register number IN is not an
inbound register.

File: gcc.info, Node: Allocation Order, Next: Values in Registers, Prev: Register Basics, Up: Registers
Order of Allocation of Registers
--------------------------------
Registers are allocated in order.
`REG_ALLOC_ORDER'
If defined, an initializer for a vector of integers, containing the
numbers of hard registers in the order in which 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 machines where the highest numbered
registers must always be saved and the save-multiple-registers
instruction supports only sequences of consecutive registers. On
such machines, define `REG_ALLOC_ORDER' to be an initializer that
lists the highest numbered allocable register first.
`ORDER_REGS_FOR_LOCAL_ALLOC'
A C statement (sans semicolon) to choose the order in which to
allocate hard registers for pseudo-registers local to a basic
block.
Store the desired register order in the array `reg_alloc_order'.
Element 0 should be the register to allocate first; element 1, the
next register; and so on.
The macro body should not assume anything about the contents of
`reg_alloc_order' before execution of the macro.
On most machines, it is not necessary to define this macro.

File: gcc.info, Node: Values in Registers, Next: Leaf Functions, Prev: Allocation Order, Up: Registers
How Values Fit in Registers
---------------------------
This section discusses the macros that describe which kinds of values
(specifically, which machine modes) each register can hold, and how many
consecutive registers are needed for a given mode.
`HARD_REGNO_NREGS (REGNO, MODE)'
A C expression for the number of consecutive hard registers,
starting at register number REGNO, required to hold a value of mode
MODE.
On a machine where all registers are exactly one word, a suitable
definition of this macro is
#define HARD_REGNO_NREGS(REGNO, MODE) \
((GET_MODE_SIZE (MODE) + UNITS_PER_WORD - 1) \
/ UNITS_PER_WORD))
`HARD_REGNO_MODE_OK (REGNO, MODE)'
A C expression that is nonzero if it is permissible to store a
value of mode MODE in hard register number REGNO (or in several
registers starting with that one). For a machine where all
registers are equivalent, a suitable definition is
#define HARD_REGNO_MODE_OK(REGNO, MODE) 1
You need not include code to check for the numbers of fixed
registers, because the allocation mechanism considers them to be
always occupied.
On some machines, double-precision values must be kept in even/odd
register pairs. You can implement that by defining this macro to
reject odd register numbers for such modes.
The minimum requirement for a mode to be OK in a register is that
the `movMODE' instruction pattern support moves between the
register and other hard register in the same class and that moving
a value into the register and back out not alter it.
Since the same instruction used to move `word_mode' will work for
all narrower integer modes, it is not necessary on any machine for
`HARD_REGNO_MODE_OK' to distinguish between these modes, provided
you define patterns `movhi', etc., to take advantage of this. This
is useful because of the interaction between `HARD_REGNO_MODE_OK'
and `MODES_TIEABLE_P'; it is very desirable for all integer modes
to be tieable.
Many machines have special registers for floating point arithmetic.
Often people assume that floating point machine modes are allowed
only in floating point registers. This is not true. Any
registers that can hold integers can safely *hold* a floating
point machine mode, whether or not floating arithmetic can be done
on it in those registers. Integer move instructions can be used
to move the values.
On some machines, though, the converse is true: fixed-point machine
modes may not go in floating registers. This is true if the
floating registers normalize any value stored in them, because
storing a non-floating value there would garble it. In this case,
`HARD_REGNO_MODE_OK' should reject fixed-point machine modes in
floating registers. But if the floating registers do not
automatically normalize, if you can store any bit pattern in one
and retrieve it unchanged without a trap, then any machine mode
may go in a floating register, so you can define this macro to say
so.
The primary significance of special floating registers is rather
that they are the registers acceptable in floating point arithmetic
instructions. However, this is of no concern to
`HARD_REGNO_MODE_OK'. You handle it by writing the proper
constraints for those instructions.
On some machines, the floating registers are especially slow to
access, so that it is better to store a value in a stack frame
than in such a register if floating point arithmetic is not being
done. As long as the floating registers are not in class
`GENERAL_REGS', they will not be used unless some pattern's
constraint asks for one.
`MODES_TIEABLE_P (MODE1, MODE2)'
A C expression that is nonzero if a value of mode MODE1 is
accessible in mode MODE2 without copying.
If `HARD_REGNO_MODE_OK (R, MODE1)' and `HARD_REGNO_MODE_OK (R,
MODE2)' are always the same for any R, then `MODES_TIEABLE_P
(MODE1, MODE2)' should be nonzero. If they differ for any R, you
should define this macro to return zero unless some other
mechanism ensures the accessibility of the value in a narrower
mode.
You should define this macro to return nonzero in as many cases as
possible since doing so will allow GNU CC to perform better
register allocation.

File: gcc.info, Node: Leaf Functions, Next: Stack Registers, Prev: Values in Registers, Up: Registers
Handling Leaf Functions
-----------------------
On some machines, a leaf function (i.e., one which makes no calls)
can run more efficiently if it does not make its own register window.
Often this means it is required to receive its arguments in the
registers where they are passed by the caller, instead of the registers
where they would normally arrive.
The special treatment for leaf functions generally applies only when
other conditions are met; for example, often they may use only those
registers for its own variables and temporaries. We use the term "leaf
function" to mean a function that is suitable for this special
handling, so that functions with no calls are not necessarily "leaf
functions".
GNU CC assigns register numbers before it knows whether the function
is suitable for leaf function treatment. So it needs to renumber the
registers in order to output a leaf function. The following macros
accomplish this.
`LEAF_REGISTERS'
A C initializer for a vector, indexed by hard register number,
which contains 1 for a register that is allowable in a candidate
for leaf function treatment.
If leaf function treatment involves renumbering the registers,
then the registers marked here should be the ones before
renumbering--those that GNU CC would ordinarily allocate. The
registers which will actually be used in the assembler code, after
renumbering, should not be marked with 1 in this vector.
Define this macro only if the target machine offers a way to
optimize the treatment of leaf functions.
`LEAF_REG_REMAP (REGNO)'
A C expression whose value is the register number to which REGNO
should be renumbered, when a function is treated as a leaf
function.
If REGNO is a register number which should not appear in a leaf
function before renumbering, then the expression should yield -1,
which will cause the compiler to abort.
Define this macro only if the target machine offers a way to
optimize the treatment of leaf functions, and registers need to be
renumbered to do this.
Normally, `FUNCTION_PROLOGUE' and `FUNCTION_EPILOGUE' must treat
leaf functions specially. It can test the C variable `leaf_function'
which is nonzero for leaf functions. (The variable `leaf_function' is
defined only if `LEAF_REGISTERS' is defined.)

File: gcc.info, Node: Stack Registers, Next: Obsolete Register Macros, Prev: Leaf Functions, Up: Registers
Registers That Form a Stack
---------------------------
There are special features to handle computers where some of the
"registers" form a stack, as in the 80387 coprocessor for the 80386.
Stack registers are normally written by pushing onto the stack, and are
numbered relative to the top of the stack.
Currently, GNU CC can only handle one group of stack-like registers,
and they must be consecutively numbered.
`STACK_REGS'
Define this if the machine has any stack-like registers.
`FIRST_STACK_REG'
The number of the first stack-like register. This one is the top
of the stack.
`LAST_STACK_REG'
The number of the last stack-like register. This one is the
bottom of the stack.

File: gcc.info, Node: Obsolete Register Macros, Prev: Stack Registers, Up: Registers
Obsolete Macros for Controlling Register Usage
----------------------------------------------
These features do not work very well. They exist because they used
to be required to generate correct code for the 80387 coprocessor of the
80386. They are no longer used by that machine description and may be
removed in a later version of the compiler. Don't use 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
correct `REG_DEAD' notes are needed for hard register number REGNO
after reload.
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.)
It is also useful for peepholes and linker relaxation.
If this macro is not defined, it means that no death notes need to
be preserved, and some may even be incorrect. This is the usual
situation.

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'
Normally the compiler avoids choosing registers that have been
explicitly mentioned in the rtl as spill registers (these
registers are normally those used to pass parameters and return
values). However, some machines have so few registers of certain
classes that there would not be enough registers to use as spill
registers if this were done.
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 allows registers explicitly used in the rtl to be used as
spill registers but avoids extending the lifetime of these
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 pseudo 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'.
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. This
macro will seldom need to be defined.
`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.

File: gcc.info, Node: Stack Checking, Next: Frame Registers, Prev: Frame Layout, Up: Stack and Calling
Specifying How Stack Checking is Done
-------------------------------------
GNU CC 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, GNU CC
will assume that you have arranged for stack checking to be done at
appropriate places in the configuration files, e.g., in
`FUNCTION_PROLOGUE'. GNU CC 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, GNU CC 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, GNU CC 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 GNU CC
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 GNU CC's portable approach. The default value of this macro
is zero.
`STACK_CHECK_PROBE_INTERVAL'
An integer representing the interval at which GNU CC 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 GNU CC should perform the stack probe
as a load instruction and zero if GNU CC 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. GNU CC 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 GNU CC
will issue a warning. The default is chosen so that GNU CC only
generates one instruction on most systems. You should normally
not change the default value of this macro.
`STACK_CHECK_FIXED_FRAME_SIZE'
GNU CC 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 GNU CC will place in
the fixed area of the stack frame when the user specifies
`-fstack-check'. GNU CC computed the default from the values of
the above macros and you will normally not need to override that
default.
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