tm.texi

GUN开源阻止下的编译器GCC

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

@findex DEFAULT_SIGNED_CHAR
@item DEFAULT_SIGNED_CHAR
An expression whose value is 1 or 0, according to whether the type
@code{char} should be signed or unsigned by default.  The user can
always override this default with the options @samp{-fsigned-char}
and @samp{-funsigned-char}.

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

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

@findex SIZE_TYPE
@item SIZE_TYPE
A C expression for a string describing the name of the data type to use
for size values.  The typedef name @code{size_t} is defined using the
contents of the string.

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

If you don't define this macro, the default is @code{"long unsigned
int"}.

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

If you don't define this macro, the default is @code{"long int"}.

@findex WCHAR_TYPE
@item WCHAR_TYPE
A C expression for a string describing the name of the data type to use
for wide characters.  The typedef name @code{wchar_t} is defined using
the contents of the string.  See @code{SIZE_TYPE} above for more
information.

If you don't define this macro, the default is @code{"int"}.

@findex WCHAR_TYPE_SIZE
@item WCHAR_TYPE_SIZE
A C expression for the size in bits of the data type for wide
characters.  This is used in @code{cpp}, which cannot make use of
@code{WCHAR_TYPE}.

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

@findex OBJC_INT_SELECTORS
@item OBJC_INT_SELECTORS
Define this macro if the type of Objective C selectors should be
@code{int}.

If this macro is not defined, then selectors should have the type
@code{struct objc_selector *}.

@findex OBJC_SELECTORS_WITHOUT_LABELS
@item OBJC_SELECTORS_WITHOUT_LABELS
Define this macro if the compiler can group all the selectors together
into a vector and use just one label at the beginning of the vector.
Otherwise, the compiler must give each selector its own assembler
label.

On certain machines, it is important to have a separate label for each
selector because this enables the linker to eliminate duplicate selectors.

@findex TARGET_BELL
@item TARGET_BELL
A C constant expression for the integer value for escape sequence
@samp{\a}.

@findex TARGET_TAB
@findex TARGET_BS
@findex TARGET_NEWLINE
@item TARGET_BS
@itemx TARGET_TAB
@itemx TARGET_NEWLINE
C constant expressions for the integer values for escape sequences
@samp{\b}, @samp{\t} and @samp{\n}.

@findex TARGET_VT
@findex TARGET_FF
@findex TARGET_CR
@item TARGET_VT
@itemx TARGET_FF
@itemx TARGET_CR
C constant expressions for the integer values for escape sequences
@samp{\v}, @samp{\f} and @samp{\r}.
@end table

@node Registers
@section Register Usage
@cindex 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 @ref{Register Classes}.  For information
on using registers to access a stack frame, see @ref{Frame Registers}.
For passing values in registers, see @ref{Register Arguments}.
For returning values in registers, see @ref{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.
@end menu

@node Register Basics
@subsection Basic Characteristics of Registers

@c prevent bad page break with this line
Registers have various characteristics.

@table @code
@findex FIRST_PSEUDO_REGISTER
@item FIRST_PSEUDO_REGISTER
Number of hardware registers known to the compiler.  They receive
numbers 0 through @code{FIRST_PSEUDO_REGISTER-1}; thus, the first
pseudo register's number really is assigned the number
@code{FIRST_PSEUDO_REGISTER}.

@item FIXED_REGISTERS
@findex FIXED_REGISTERS
@cindex fixed register
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 @var{n}th number is 1 if
register @var{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 @code{CONDITIONAL_REGISTER_USAGE}, or by
the user with the command options @samp{-ffixed-@var{reg}},
@samp{-fcall-used-@var{reg}} and @samp{-fcall-saved-@var{reg}}.

@findex CALL_USED_REGISTERS
@item CALL_USED_REGISTERS
@cindex call-used register
@cindex call-clobbered register
@cindex call-saved register
Like @code{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 @code{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.

@findex CONDITIONAL_REGISTER_USAGE
@findex fixed_regs
@findex call_used_regs
@item CONDITIONAL_REGISTER_USAGE
Zero or more C statements that may conditionally modify two variables
@code{fixed_regs} and @code{call_used_regs} (both of type @code{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.

@cindex disabling certain registers
@cindex controlling register usage 
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
@code{fixed_regs} and @code{call_used_regs} to 1 for each of the
registers in the classes which should not be used by GCC.  Also define
the macro @code{REG_CLASS_FROM_LETTER} to return @code{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 @code{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.)

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

@findex INCOMING_REGNO
@item INCOMING_REGNO (@var{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 @var{out} as seen by the calling
function.  Return @var{out} if register number @var{out} is not an
outbound register.

@findex OUTGOING_REGNO
@item OUTGOING_REGNO (@var{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 @var{in} as seen by the called
function.  Return @var{in} if register number @var{in} is not an inbound
register.

@ignore
@findex PC_REGNUM
@item PC_REGNUM
If the program counter has a register number, define this as that
register number.  Otherwise, do not define it.
@end ignore
@end table

@node Allocation Order
@subsection Order of Allocation of Registers
@cindex order of register allocation
@cindex register allocation order

@c prevent bad page break with this line
Registers are allocated in order.

@table @code
@findex REG_ALLOC_ORDER
@item 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 @code{REG_ALLOC_ORDER} to be an initializer that lists
the highest numbered allocatable register first.

@findex ORDER_REGS_FOR_LOCAL_ALLOC
@item 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 @code{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
@code{reg_alloc_order} before execution of the macro.

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

@node Values in Registers
@subsection 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.

@table @code
@findex HARD_REGNO_NREGS
@item HARD_REGNO_NREGS (@var{regno}, @var{mode})
A C expression for the number of consecutive hard registers, starting
at register number @var{regno}, required to hold a value of mode
@var{mode}.

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

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

@findex HARD_REGNO_MODE_OK
@item HARD_REGNO_MODE_OK (@var{regno}, @var{mode})
A C expression that is nonzero if it is permissible to store a value
of mode @var{mode} in hard register number @var{regno} (or in several
registers starting with that one).  For a machine where all registers
are equivalent, a suitable definition is

@smallexample
#define HARD_REGNO_MODE_OK(REGNO, MODE) 1
@end smallexample

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.

@cindex register pairs
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.

@ignore
@c I think this is not true now
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
@code{HARD_REGNO_MODE_OK} must not allow this mode in those registers.
@end ignore