rtl.texi

理解和实践操作系统的一本好书

for it.  In the C code, machine modes are represented by an enumeration
type, @code{enum machine_mode}, defined in @file{machmode.def}.  Each RTL
expression has room for a machine mode and so do certain kinds of tree
expressions (declarations and types, to be precise).

In debugging dumps and machine descriptions, the machine mode of an RTL
expression is written after the expression code with a colon to separate
them.  The letters @samp{mode} which appear at the end of each machine mode
name are omitted.  For example, @code{(reg:SI 38)} is a @code{reg}
expression with machine mode @code{SImode}.  If the mode is
@code{VOIDmode}, it is not written at all.

Here is a table of machine modes.  The term ``byte'' below refers to an
object of @code{BITS_PER_UNIT} bits (@pxref{Storage Layout}).

@table @code
@findex BImode
@item BImode
``Bit'' mode represents a single bit, for predicate registers.

@findex QImode
@item QImode
``Quarter-Integer'' mode represents a single byte treated as an integer.

@findex HImode
@item HImode
``Half-Integer'' mode represents a two-byte integer.

@findex PSImode
@item PSImode
``Partial Single Integer'' mode represents an integer which occupies
four bytes but which doesn't really use all four.  On some machines,
this is the right mode to use for pointers.

@findex SImode
@item SImode
``Single Integer'' mode represents a four-byte integer.

@findex PDImode
@item PDImode
``Partial Double Integer'' mode represents an integer which occupies
eight bytes but which doesn't really use all eight.  On some machines,
this is the right mode to use for certain pointers.

@findex DImode
@item DImode
``Double Integer'' mode represents an eight-byte integer.

@findex TImode
@item TImode
``Tetra Integer'' (?) mode represents a sixteen-byte integer.

@findex OImode
@item OImode
``Octa Integer'' (?) mode represents a thirty-two-byte integer.

@findex QFmode
@item QFmode
``Quarter-Floating'' mode represents a quarter-precision (single byte)
floating point number.

@findex HFmode
@item HFmode
``Half-Floating'' mode represents a half-precision (two byte) floating
point number.

@findex TQFmode
@item TQFmode
``Three-Quarter-Floating'' (?) mode represents a three-quarter-precision
(three byte) floating point number.

@findex SFmode
@item SFmode
``Single Floating'' mode represents a four byte floating point number.
In the common case, of a processor with IEEE arithmetic and 8-bit bytes,
this is a single-precision IEEE floating point number; it can also be
used for double-precision (on processors with 16-bit bytes) and
single-precision VAX and IBM types.

@findex DFmode
@item DFmode
``Double Floating'' mode represents an eight byte floating point number.
In the common case, of a processor with IEEE arithmetic and 8-bit bytes,
this is a double-precision IEEE floating point number.

@findex XFmode
@item XFmode
``Extended Floating'' mode represents an IEEE extended floating point
number.  This mode only has 80 meaningful bits (ten bytes).  Some
processors require such numbers to be padded to twelve bytes, others
to sixteen; this mode is used for either.

@findex SDmode
@item SDmode
``Single Decimal Floating'' mode represents a four byte decimal
floating point number (as distinct from conventional binary floating
point).

@findex DDmode
@item DDmode
``Double Decimal Floating'' mode represents an eight byte decimal
floating point number.

@findex TDmode
@item TDmode
``Tetra Decimal Floating'' mode represents a sixteen byte decimal
floating point number all 128 of whose bits are meaningful.

@findex TFmode
@item TFmode
``Tetra Floating'' mode represents a sixteen byte floating point number
all 128 of whose bits are meaningful.  One common use is the
IEEE quad-precision format.

@findex QQmode
@item QQmode
``Quarter-Fractional'' mode represents a single byte treated as a signed
fractional number.  The default format is ``s.7''.

@findex HQmode
@item HQmode
``Half-Fractional'' mode represents a two-byte signed fractional number.
The default format is ``s.15''.

@findex SQmode
@item SQmode
``Single Fractional'' mode represents a four-byte signed fractional number.
The default format is ``s.31''.

@findex DQmode
@item DQmode
``Double Fractional'' mode represents an eight-byte signed fractional number.
The default format is ``s.63''.

@findex TQmode
@item TQmode
``Tetra Fractional'' mode represents a sixteen-byte signed fractional number.
The default format is ``s.127''.

@findex UQQmode
@item UQQmode
``Unsigned Quarter-Fractional'' mode represents a single byte treated as an
unsigned fractional number.  The default format is ``.8''.

@findex UHQmode
@item UHQmode
``Unsigned Half-Fractional'' mode represents a two-byte unsigned fractional
number.  The default format is ``.16''.

@findex USQmode
@item USQmode
``Unsigned Single Fractional'' mode represents a four-byte unsigned fractional
number.  The default format is ``.32''.

@findex UDQmode
@item UDQmode
``Unsigned Double Fractional'' mode represents an eight-byte unsigned
fractional number.  The default format is ``.64''.

@findex UTQmode
@item UTQmode
``Unsigned Tetra Fractional'' mode represents a sixteen-byte unsigned
fractional number.  The default format is ``.128''.

@findex HAmode
@item HAmode
``Half-Accumulator'' mode represents a two-byte signed accumulator.
The default format is ``s8.7''.

@findex SAmode
@item SAmode
``Single Accumulator'' mode represents a four-byte signed accumulator.
The default format is ``s16.15''.

@findex DAmode
@item DAmode
``Double Accumulator'' mode represents an eight-byte signed accumulator.
The default format is ``s32.31''.

@findex TAmode
@item TAmode
``Tetra Accumulator'' mode represents a sixteen-byte signed accumulator.
The default format is ``s64.63''.

@findex UHAmode
@item UHAmode
``Unsigned Half-Accumulator'' mode represents a two-byte unsigned accumulator.
The default format is ``8.8''.

@findex USAmode
@item USAmode
``Unsigned Single Accumulator'' mode represents a four-byte unsigned
accumulator.  The default format is ``16.16''.

@findex UDAmode
@item UDAmode
``Unsigned Double Accumulator'' mode represents an eight-byte unsigned
accumulator.  The default format is ``32.32''.

@findex UTAmode
@item UTAmode
``Unsigned Tetra Accumulator'' mode represents a sixteen-byte unsigned
accumulator.  The default format is ``64.64''.

@findex CCmode
@item CCmode
``Condition Code'' mode represents the value of a condition code, which
is a machine-specific set of bits used to represent the result of a
comparison operation.  Other machine-specific modes may also be used for
the condition code.  These modes are not used on machines that use
@code{cc0} (see @pxref{Condition Code}).

@findex BLKmode
@item BLKmode
``Block'' mode represents values that are aggregates to which none of
the other modes apply.  In RTL, only memory references can have this mode,
and only if they appear in string-move or vector instructions.  On machines
which have no such instructions, @code{BLKmode} will not appear in RTL@.

@findex VOIDmode
@item VOIDmode
Void mode means the absence of a mode or an unspecified mode.
For example, RTL expressions of code @code{const_int} have mode
@code{VOIDmode} because they can be taken to have whatever mode the context
requires.  In debugging dumps of RTL, @code{VOIDmode} is expressed by
the absence of any mode.

@findex QCmode
@findex HCmode
@findex SCmode
@findex DCmode
@findex XCmode
@findex TCmode
@item QCmode, HCmode, SCmode, DCmode, XCmode, TCmode
These modes stand for a complex number represented as a pair of floating
point values.  The floating point values are in @code{QFmode},
@code{HFmode}, @code{SFmode}, @code{DFmode}, @code{XFmode}, and
@code{TFmode}, respectively.

@findex CQImode
@findex CHImode
@findex CSImode
@findex CDImode
@findex CTImode
@findex COImode
@item CQImode, CHImode, CSImode, CDImode, CTImode, COImode
These modes stand for a complex number represented as a pair of integer
values.  The integer values are in @code{QImode}, @code{HImode},
@code{SImode}, @code{DImode}, @code{TImode}, and @code{OImode},
respectively.
@end table

The machine description defines @code{Pmode} as a C macro which expands
into the machine mode used for addresses.  Normally this is the mode
whose size is @code{BITS_PER_WORD}, @code{SImode} on 32-bit machines.

The only modes which a machine description @i{must} support are
@code{QImode}, and the modes corresponding to @code{BITS_PER_WORD},
@code{FLOAT_TYPE_SIZE} and @code{DOUBLE_TYPE_SIZE}.
The compiler will attempt to use @code{DImode} for 8-byte structures and
unions, but this can be prevented by overriding the definition of
@code{MAX_FIXED_MODE_SIZE}.  Alternatively, you can have the compiler
use @code{TImode} for 16-byte structures and unions.  Likewise, you can
arrange for the C type @code{short int} to avoid using @code{HImode}.

@cindex mode classes
Very few explicit references to machine modes remain in the compiler and
these few references will soon be removed.  Instead, the machine modes
are divided into mode classes.  These are represented by the enumeration
type @code{enum mode_class} defined in @file{machmode.h}.  The possible
mode classes are:

@table @code
@findex MODE_INT
@item MODE_INT
Integer modes.  By default these are @code{BImode}, @code{QImode},
@code{HImode}, @code{SImode}, @code{DImode}, @code{TImode}, and
@code{OImode}.

@findex MODE_PARTIAL_INT
@item MODE_PARTIAL_INT
The ``partial integer'' modes, @code{PQImode}, @code{PHImode},
@code{PSImode} and @code{PDImode}.

@findex MODE_FLOAT
@item MODE_FLOAT
Floating point modes.  By default these are @code{QFmode},
@code{HFmode}, @code{TQFmode}, @code{SFmode}, @code{DFmode},
@code{XFmode} and @code{TFmode}.

@findex MODE_DECIMAL_FLOAT
@item MODE_DECIMAL_FLOAT
Decimal floating point modes.  By default these are @code{SDmode},
@code{DDmode} and @code{TDmode}.

@findex MODE_FRACT
@item MODE_FRACT
Signed fractional modes.  By default these are @code{QQmode}, @code{HQmode},
@code{SQmode}, @code{DQmode} and @code{TQmode}.

@findex MODE_UFRACT
@item MODE_UFRACT
Unsigned fractional modes.  By default these are @code{UQQmode}, @code{UHQmode},
@code{USQmode}, @code{UDQmode} and @code{UTQmode}.

@findex MODE_ACCUM
@item MODE_ACCUM
Signed accumulator modes.  By default these are @code{HAmode},
@code{SAmode}, @code{DAmode} and @code{TAmode}.

@findex MODE_UACCUM
@item MODE_UACCUM
Unsigned accumulator modes.  By default these are @code{UHAmode},
@code{USAmode}, @code{UDAmode} and @code{UTAmode}.

@findex MODE_COMPLEX_INT
@item MODE_COMPLEX_INT
Complex integer modes.  (These are not currently implemented).

@findex MODE_COMPLEX_FLOAT
@item MODE_COMPLEX_FLOAT
Complex floating point modes.  By default these are @code{QCmode},
@code{HCmode}, @code{SCmode}, @code{DCmode}, @code{XCmode}, and
@code{TCmode}.

@findex MODE_FUNCTION
@item MODE_FUNCTION
Algol or Pascal function variables including a static chain.
(These are not currently implemented).

@findex MODE_CC
@item MODE_CC
Modes representing condition code values.  These are @code{CCmode} plus
any @code{CC_MODE} modes listed in the @file{@var{machine}-modes.def}.  
@xref{Jump Patterns},
also see @ref{Condition Code}.

@findex MODE_RANDOM
@item MODE_RANDOM