tm.texi

理解和实践操作系统的一本好书

@end defmac

@deftypefn {Target Hook} bool TARGET_ALIGN_ANON_BITFIELDS (void)
When @code{PCC_BITFIELD_TYPE_MATTERS} is true this hook will determine
whether unnamed bitfields affect the alignment of the containing
structure.  The hook should return true if the structure should inherit
the alignment requirements of an unnamed bitfield's type.
@end deftypefn

@deftypefn {Target Hook} bool TARGET_NARROW_VOLATILE_BITFIELDS (void)
This target hook should return @code{true} if accesses to volatile bitfields
should use the narrowest mode possible.  It should return @code{false} if
these accesses should use the bitfield container type.

The default is @code{!TARGET_STRICT_ALIGN}.
@end deftypefn

@defmac MEMBER_TYPE_FORCES_BLK (@var{field}, @var{mode})
Return 1 if a structure or array containing @var{field} should be accessed using
@code{BLKMODE}.

If @var{field} is the only field in the structure, @var{mode} is its
mode, otherwise @var{mode} is VOIDmode.  @var{mode} is provided in the
case where structures of one field would require the structure's mode to
retain the field's mode.

Normally, this is not needed.
@end defmac

@defmac ROUND_TYPE_ALIGN (@var{type}, @var{computed}, @var{specified})
Define this macro as an expression for the alignment of a type (given
by @var{type} 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}
@end defmac

@defmac 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.
@end defmac

@defmac STACK_SAVEAREA_MODE (@var{save_level})
If defined, an expression of type @code{enum machine_mode} that
specifies the mode of the save area operand of a
@code{save_stack_@var{level}} named pattern (@pxref{Standard Names}).
@var{save_level} is one of @code{SAVE_BLOCK}, @code{SAVE_FUNCTION}, or
@code{SAVE_NONLOCAL} and selects which of the three named patterns is
having its mode specified.

You need not define this macro if it always returns @code{Pmode}.  You
would most commonly define this macro if the
@code{save_stack_@var{level}} patterns need to support both a 32- and a
64-bit mode.
@end defmac

@defmac STACK_SIZE_MODE
If defined, an expression of type @code{enum machine_mode} that
specifies the mode of the size increment operand of an
@code{allocate_stack} named pattern (@pxref{Standard Names}).

You need not define this macro if it always returns @code{word_mode}.
You would most commonly define this macro if the @code{allocate_stack}
pattern needs to support both a 32- and a 64-bit mode.
@end defmac

@deftypefn {Target Hook} {enum machine_mode} TARGET_LIBGCC_CMP_RETURN_MODE ()
This target hook should return the mode to be used for the return value
of compare instructions expanded to libgcc calls.  If not defined
@code{word_mode} is returned which is the right choice for a majority of
targets.
@end deftypefn

@deftypefn {Target Hook} {enum machine_mode} TARGET_LIBGCC_SHIFT_COUNT_MODE ()
This target hook should return the mode to be used for the shift count operand
of shift instructions expanded to libgcc calls.  If not defined
@code{word_mode} is returned which is the right choice for a majority of
targets.
@end deftypefn

@defmac TARGET_FLOAT_FORMAT
A code distinguishing the floating point format of the target machine.
There are two defined values:

@ftable @code
@item IEEE_FLOAT_FORMAT
This code indicates IEEE floating point.  It is the default; there is no
need to define @code{TARGET_FLOAT_FORMAT} when the format is IEEE@.

@item VAX_FLOAT_FORMAT
This code indicates the ``F float'' (for @code{float}) and ``D float''
or ``G float'' formats (for @code{double}) used on the VAX and PDP-11@.
@end ftable

If your target uses a floating point format other than these, you must
define a new @var{name}_FLOAT_FORMAT code for it, and add support for
it to @file{real.c}.

The ordering of the component words of floating point values stored in
memory is controlled by @code{FLOAT_WORDS_BIG_ENDIAN}.
@end defmac

@defmac MODE_HAS_NANS (@var{mode})
When defined, this macro should be true if @var{mode} has a NaN
representation.  The compiler assumes that NaNs are not equal to
anything (including themselves) and that addition, subtraction,
multiplication and division all return NaNs when one operand is
NaN@.

By default, this macro is true if @var{mode} is a floating-point
mode and the target floating-point format is IEEE@.
@end defmac

@defmac MODE_HAS_INFINITIES (@var{mode})
This macro should be true if @var{mode} can represent infinity.  At
present, the compiler uses this macro to decide whether @samp{x - x}
is always defined.  By default, the macro is true when @var{mode}
is a floating-point mode and the target format is IEEE@.
@end defmac

@defmac MODE_HAS_SIGNED_ZEROS (@var{mode})
True if @var{mode} distinguishes between positive and negative zero.
The rules are expected to follow the IEEE standard:

@itemize @bullet
@item
@samp{x + x} has the same sign as @samp{x}.

@item
If the sum of two values with opposite sign is zero, the result is
positive for all rounding modes expect towards @minus{}infinity, for
which it is negative.

@item
The sign of a product or quotient is negative when exactly one
of the operands is negative.
@end itemize

The default definition is true if @var{mode} is a floating-point
mode and the target format is IEEE@.
@end defmac

@defmac MODE_HAS_SIGN_DEPENDENT_ROUNDING (@var{mode})
If defined, this macro should be true for @var{mode} if it has at
least one rounding mode in which @samp{x} and @samp{-x} can be
rounded to numbers of different magnitude.  Two such modes are
towards @minus{}infinity and towards +infinity.

The default definition of this macro is true if @var{mode} is
a floating-point mode and the target format is IEEE@.
@end defmac

@defmac ROUND_TOWARDS_ZERO
If defined, this macro should be true if the prevailing rounding
mode is towards zero.  A true value has the following effects:

@itemize @bullet
@item
@code{MODE_HAS_SIGN_DEPENDENT_ROUNDING} will be false for all modes.

@item
@file{libgcc.a}'s floating-point emulator will round towards zero
rather than towards nearest.

@item
The compiler's floating-point emulator will round towards zero after
doing arithmetic, and when converting from the internal float format to
the target format.
@end itemize

The macro does not affect the parsing of string literals.  When the
primary rounding mode is towards zero, library functions like
@code{strtod} might still round towards nearest, and the compiler's
parser should behave like the target's @code{strtod} where possible.

Not defining this macro is equivalent to returning zero.
@end defmac

@defmac LARGEST_EXPONENT_IS_NORMAL (@var{size})
This macro should return true if floats with @var{size}
bits do not have a NaN or infinity representation, but use the largest
exponent for normal numbers instead.

Defining this macro to true for @var{size} causes @code{MODE_HAS_NANS}
and @code{MODE_HAS_INFINITIES} to be false for @var{size}-bit modes.
It also affects the way @file{libgcc.a} and @file{real.c} emulate
floating-point arithmetic.

The default definition of this macro returns false for all sizes.
@end defmac

@deftypefn {Target Hook} bool TARGET_VECTOR_OPAQUE_P (tree @var{type})
This target hook should return @code{true} a vector is opaque.  That
is, if no cast is needed when copying a vector value of type
@var{type} into another vector lvalue of the same size.  Vector opaque
types cannot be initialized.  The default is that there are no such
types.
@end deftypefn

@deftypefn {Target Hook} bool TARGET_MS_BITFIELD_LAYOUT_P (tree @var{record_type})
This target hook returns @code{true} if bit-fields in the given
@var{record_type} are to be laid out following the rules of Microsoft
Visual C/C++, namely: (i) a bit-field won't share the same storage
unit with the previous bit-field if their underlying types have
different sizes, and the bit-field will be aligned to the highest
alignment of the underlying types of itself and of the previous
bit-field; (ii) a zero-sized bit-field will affect the alignment of
the whole enclosing structure, even if it is unnamed; except that
(iii) a zero-sized bit-field will be disregarded unless it follows
another bit-field of nonzero size.  If this hook returns @code{true},
other macros that control bit-field layout are ignored.

When a bit-field is inserted into a packed record, the whole size
of the underlying type is used by one or more same-size adjacent
bit-fields (that is, if its long:3, 32 bits is used in the record,
and any additional adjacent long bit-fields are packed into the same
chunk of 32 bits.  However, if the size changes, a new field of that
size is allocated).  In an unpacked record, this is the same as using
alignment, but not equivalent when packing.

If both MS bit-fields and @samp{__attribute__((packed))} are used,
the latter will take precedence.  If @samp{__attribute__((packed))} is
used on a single field when MS bit-fields are in use, it will take
precedence for that field, but the alignment of the rest of the structure
may affect its placement.
@end deftypefn

@deftypefn {Target Hook} {bool} TARGET_DECIMAL_FLOAT_SUPPORTED_P (void)
Returns true if the target supports decimal floating point.
@end deftypefn

@deftypefn {Target Hook} {bool} TARGET_FIXED_POINT_SUPPORTED_P (void)
Returns true if the target supports fixed-point arithmetic.
@end deftypefn

@deftypefn {Target Hook} void TARGET_EXPAND_TO_RTL_HOOK (void)
This hook is called just before expansion into rtl, allowing the target
to perform additional initializations or analysis before the expansion.
For example, the rs6000 port uses it to allocate a scratch stack slot
for use in copying SDmode values between memory and floating point
registers whenever the function being expanded has any SDmode
usage.
@end deftypefn

@deftypefn {Target Hook} void TARGET_INSTANTIATE_DECLS (void)
This hook allows the backend to perform additional instantiations on rtl
that are not actually in any insns yet, but will be later.
@end deftypefn

@deftypefn {Target Hook} {const char *} TARGET_MANGLE_TYPE (tree @var{type})
If your target defines any fundamental types, or any types your target
uses should be mangled differently from the default, define this hook
to return the appropriate encoding for these types as part of a C++
mangled name.  The @var{type} argument is the tree structure representing
the type to be mangled.  The hook may be applied to trees which are
not target-specific fundamental types; it should return @code{NULL}
for all such types, as well as arguments it does not recognize.  If the
return value is not @code{NULL}, it must point to a statically-allocated
string constant.

Target-specific fundamental types might be new fundamental types or
qualified versions of ordinary fundamental types.  Encode new
fundamental types as @samp{@w{u @var{n} @var{name}}}, where @var{name}
is the name used for the type in source code, and @var{n} is the
length of @var{name} in decimal.  Encode qualified versions of
ordinary types as @samp{@w{U @var{n} @var{name} @var{code}}}, where
@var{name} is the name used for the type qualifier in source code,
@var{n} is the length of @var{name} as above, and @var{code} is the
code used to represent the unqualified version of this type.  (See
@code{write_builtin_type} in @file{cp/mangle.c} for the list of
codes.)  In both cases the spaces are for clarity; do not include any
spaces in your string.

This hook is applied to types prior to typedef resolution.  If the mangled
name for a particular type depends only on that type's main variant, you
can perform typedef resolution yourself using @code{TYPE_MAIN_VARIANT}
before mangling.

The default version of this hook always returns @code{NULL}, which is
appropriate for a target that does not define any new fundamental
types.
@end deftypefn

@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.

@defmac 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.
@end defmac

@defmac 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.)
@end defmac

@defmac 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.
@end defmac

@defmac ADA_LONG_TYPE_SIZE
On some machines, the size used for the Ada equivalent of the ty