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早期freebsd实现

@item PCC_BITFIELD_TYPE_MATTERS
Define this if you wish to imitate the way many other C compilers handle
alignment of bitfields and the structures that contain them.

The behavior is that the type written for a bitfield (@code{int},
@code{short}, or other integer type) imposes an alignment for the
entire structure, as if the structure really did contain an ordinary
field of that type.  In addition, the bitfield is placed within the
structure so that it would fit within such a field, not crossing a
boundary for it.

Thus, on most machines, a bitfield whose type is written as @code{int}
would not cross a four-byte boundary, and would force four-byte
alignment for the whole structure.  (The alignment used may not be four
bytes; it is controlled by the other alignment parameters.)

If the macro is defined, its definition should be a C expression;
a nonzero value for the expression enables this behavior.

Note that if this macro is not defined, or its value is zero, some
bitfields may cross more than one alignment boundary.  The compiler can
support such references if there are @samp{insv}, @samp{extv}, and
@samp{extzv} insns that can directly reference memory.

The other known way of making bitfields work is to define
@code{STRUCTURE_SIZE_BOUNDARY} as large as @code{BIGGEST_ALIGNMENT}.
Then every structure can be accessed with fullwords.

Unless the machine has bitfield instructions or you define
@code{STRUCTURE_SIZE_BOUNDARY} that way, you must define
@code{PCC_BITFIELD_TYPE_MATTERS} to have a nonzero value.

If your aim is to make GNU CC use the same conventions for laying out
bitfields as are used by another compiler, here is how to investigate
what the other compiler does.  Compile and run this program:

@example
struct foo1
@{
  char x;
  char :0;
  char y;
@};

struct foo2
@{
  char x;
  int :0;
  char y;
@};

main ()
@{
  printf ("Size of foo1 is %d\n", sizeof (struct foo1));
  printf ("Size of foo2 is %d\n", sizeof (struct foo2));
  exit (0);
@}
@end example

If this prints 2 and 5, then the compiler's behavior is what you would
get from @code{PCC_BITFIELD_TYPE_MATTERS}.

@findex BITFIELD_NBYTES_LIMITED
@item BITFIELD_NBYTES_LIMITED
Like PCC_BITFIELD_TYPE_MATTERS except that its effect is limited to
aligning a bitfield within the structure.

@findex ROUND_TYPE_SIZE
@item ROUND_TYPE_SIZE (@var{struct}, @var{size}, @var{align})
Define this macro as an expression for the overall size of a structure 
(given by @var{struct} as a tree node) when the size computed from the
fields is @var{size} and the alignment is @var{align}.

The default is to round @var{size} up to a multiple of @var{align}.

@findex ROUND_TYPE_ALIGN
@item ROUND_TYPE_ALIGN (@var{struct}, @var{computed}, @var{specified})
Define this macro as an expression for the alignment of a structure 
(given by @var{struct} as a tree node) if the alignment computed in the
usual way is @var{computed} and the alignment explicitly specified was
@var{specified}.

The default is to use @var{specified} if it is larger; otherwise, use
the smaller of @var{computed} and @code{BIGGEST_ALIGNMENT}

@findex MAX_FIXED_MODE_SIZE
@item MAX_FIXED_MODE_SIZE
An integer expression for the size in bits of the largest integer
machine mode that should actually be used.  All integer machine modes of
this size or smaller can be used for structures and unions with the
appropriate sizes.  If this macro is undefined, @code{GET_MODE_BITSIZE
(DImode)} is assumed.

@findex CHECK_FLOAT_VALUE
@item CHECK_FLOAT_VALUE (@var{mode}, @var{value})
A C statement to validate the value @var{value} (of type
@code{double}) for mode @var{mode}.  This means that you check whether
@var{value} fits within the possible range of values for mode
@var{mode} on this target machine.  The mode @var{mode} is always
@code{SFmode} or @code{DFmode}.

@findex error
If @var{value} is not valid, you should call @code{error} to print an
error message and then assign some valid value to @var{value}.
Allowing an invalid value to go through the compiler can produce
incorrect assembler code which may even cause Unix assemblers to
crash.

This macro need not be defined if there is no work for it to do.

@findex TARGET_FLOAT_FORMAT
@item TARGET_FLOAT_FORMAT
A code distinguishing the floating point format of the target machine.
There are three defined values:

@table @code
@findex IEEE_FLOAT_FORMAT
@item IEEE_FLOAT_FORMAT
This code indicates IEEE floating point.  It is the default; there is no
need to define this macro when the format is IEEE.

@findex VAX_FLOAT_FORMAT
@item VAX_FLOAT_FORMAT
This code indicates the peculiar format used on the Vax.

@findex UNKNOWN_FLOAT_FORMAT
@item UNKNOWN_FLOAT_FORMAT
This code indicates any other format.
@end table

The value of this macro is compared with @code{HOST_FLOAT_FORMAT}
(@pxref{Config}) to determine whether the target machine has the same
format as the host machine.  If any other formats are actually in use on
supported machines, new codes should be defined for them.
@end table

@node Type Layout
@section Layout of Source Language Data Types

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

@table @code
@findex INT_TYPE_SIZE
@item INT_TYPE_SIZE
A C expression for the size in bits of the type @code{int} on the
target machine.  If you don't define this, the default is one word.

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

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

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

@findex CHAR_TYPE_SIZE
@item CHAR_TYPE_SIZE
A C expression for the size in bits of the type @code{char} on the
target machine.  If you don't define this, the default is one quarter
of a word.  (If this would be less than one storage unit, it is rounded up
to one unit.)

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

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

@findex LONG_DOUBLE_TYPE_SIZE
@item LONG_DOUBLE_TYPE_SIZE
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 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

@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