rtl.texi

早期freebsd实现

@findex XCmode
@findex TCmode
@item SCmode, DCmode, XCmode, TCmode
These modes stand for a complex number represented as a pair of
floating point values.  The values are in @code{SFmode}, @code{DFmode},
@code{XFmode}, and @code{TFmode}, respectively.  Since C does not
support complex numbers, these machine modes are only partially
implemented.
@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{QImode}, @code{HImode},
@code{SImode}, @code{DImode}, and @code{TImode}.

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

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

@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{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 modes listed in the @code{EXTRA_CC_MODES} macro.  @xref{Jump Patterns},
also see @ref{Condition Code}.

@findex MODE_RANDOM
@item MODE_RANDOM
This is a catchall mode class for modes which don't fit into the above
classes.  Currently @code{VOIDmode} and @code{BLKmode} are in
@code{MODE_RANDOM}.
@end table

Here are some C macros that relate to machine modes:

@table @code
@findex GET_MODE
@item GET_MODE (@var{x})
Returns the machine mode of the RTX @var{x}.

@findex PUT_MODE
@item PUT_MODE (@var{x}, @var{newmode})
Alters the machine mode of the RTX @var{x} to be @var{newmode}.

@findex NUM_MACHINE_MODES
@item NUM_MACHINE_MODES
Stands for the number of machine modes available on the target
machine.  This is one greater than the largest numeric value of any
machine mode.

@findex GET_MODE_NAME
@item GET_MODE_NAME (@var{m})
Returns the name of mode @var{m} as a string.

@findex GET_MODE_CLASS
@item GET_MODE_CLASS (@var{m})
Returns the mode class of mode @var{m}.

@findex GET_MODE_WIDER_MODE
@item GET_MODE_WIDER_MODE (@var{m})
Returns the next wider natural mode.  E.g.,
@code{GET_WIDER_MODE(QImode)} returns @code{HImode}.

@findex GET_MODE_SIZE
@item GET_MODE_SIZE (@var{m})
Returns the size in bytes of a datum of mode @var{m}.

@findex GET_MODE_BITSIZE
@item GET_MODE_BITSIZE (@var{m})
Returns the size in bits of a datum of mode @var{m}.

@findex GET_MODE_MASK
@item GET_MODE_MASK (@var{m})
Returns a bitmask containing 1 for all bits in a word that fit within
mode @var{m}.  This macro can only be used for modes whose bitsize is
less than or equal to @code{HOST_BITS_PER_INT}.

@findex GET_MODE_ALIGNMENT
@item GET_MODE_ALIGNMENT (@var{m)})
Return the required alignment, in bits, for an object of mode @var{m}.

@findex GET_MODE_UNIT_SIZE
@item GET_MODE_UNIT_SIZE (@var{m})
Returns the size in bytes of the subunits of a datum of mode @var{m}.
This is the same as @code{GET_MODE_SIZE} except in the case of complex
modes.  For them, the unit size is the size of the real or imaginary
part.

@findex GET_MODE_NUNITS
@item GET_MODE_NUNITS (@var{m})
Returns the number of units contained in a mode, i.e.,
@code{GET_MODE_SIZE} divided by @code{GET_MODE_UNIT_SIZE}.

@findex GET_CLASS_NARROWEST_MODE
@item GET_CLASS_NARROWEST_MODE (@var{c})
Returns the narrowest mode in mode class @var{c}.
@end table

@findex byte_mode
@findex word_mode
The global variables @code{byte_mode} and @code{word_mode} contain
modes whose classes are @code{MODE_INT} and whose bitsizes are
@code{BITS_PER_UNIT} or @code{BITS_PER_WORD}, respectively.  On 32-bit
machines, these are @code{QImode} and @code{SImode}, respectively.

@node Constants, Regs and Memory, Machine Modes, RTL
@section Constant Expression Types
@cindex RTL constants
@cindex RTL constant expression types

The simplest RTL expressions are those that represent constant values.

@table @code
@findex const_int
@item (const_int @var{i})
This type of expression represents the integer value @var{i}.  @var{i}
is customarily accessed with the macro @code{INTVAL} as in
@code{INTVAL (@var{exp})}, which is equivalent to @code{XWINT (@var{exp}, 0)}.

@findex const0_rtx
@findex const1_rtx
@findex const2_rtx
@findex constm1_rtx
There is only one expression object for the integer value zero; it is
the value of the variable @code{const0_rtx}.  Likewise, the only
expression for integer value one is found in @code{const1_rtx}, the only
expression for integer value two is found in @code{const2_rtx}, and the
only expression for integer value negative one is found in
@code{constm1_rtx}.  Any attempt to create an expression of code
@code{const_int} and value zero, one, two or negative one will return
@code{const0_rtx}, @code{const1_rtx}, @code{const2_rtx} or
@code{constm1_rtx} as appropriate.@refill

@findex const_true_rtx
Similarly, there is only one object for the integer whose value is
@code{STORE_FLAG_VALUE}.  It is found in @code{const_true_rtx}.  If
@code{STORE_FLAG_VALUE} is one, @code{const_true_rtx} and
@code{const1_rtx} will point to the same object.  If
@code{STORE_FLAG_VALUE} is -1, @code{const_true_rtx} and
@code{constm1_rtx} will point to the same object.@refill

@findex const_double
@item (const_double:@var{m} @var{addr} @var{i0} @var{i1} @dots{})
Represents either a floating-point constant of mode @var{m} or an
integer constant that is too large to fit into @code{HOST_BITS_PER_WIDE_INT}
bits but small enough to fit within twice that number of bits (GNU CC
does not provide a mechanism to represent even larger constants).  In
the latter case, @var{m} will be @code{VOIDmode}.

@findex CONST_DOUBLE_MEM
@findex CONST_DOUBLE_CHAIN
@var{addr} is used to contain the @code{mem} expression that corresponds
to the location in memory that at which the constant can be found.  If
it has not been allocated a memory location, but is on the chain of all
@code{const_double} expressions in this compilation (maintained using an
undisplayed field), @var{addr} contains @code{const0_rtx}.  If it is not
on the chain, @var{addr} contains @code{cc0_rtx}.  @var{addr} is
customarily accessed with the macro @code{CONST_DOUBLE_MEM} and the
chain field via @code{CONST_DOUBLE_CHAIN}.@refill

@findex CONST_DOUBLE_LOW
If @var{m} is @code{VOIDmode}, the bits of the value are stored in
@var{i0} and @var{i1}.  @var{i0} is customarily accessed with the macro
@code{CONST_DOUBLE_LOW} and @var{i1} with @code{CONST_DOUBLE_HIGH}.

If the constant is floating point (either single or double precision),
then the number of integers used to store the value depends on the size
of @code{REAL_VALUE_TYPE} (@pxref{Cross-compilation}).  The integers
represent a @code{double}.  To convert them to a @code{double}, do

@example
union real_extract u;
bcopy (&CONST_DOUBLE_LOW (x), &u, sizeof u);
@end example

@noindent
and then refer to @code{u.d}.

@findex CONST0_RTX
@findex CONST1_RTX
@findex CONST2_RTX
The macro @code{CONST0_RTX (@var{mode})} refers to an expression with
value 0 in mode @var{mode}. If mode @var{mode} is of mode class
@code{MODE_INT}, it returns @code{const0_rtx}.  Otherwise, it returns a
@code{CONST_DOUBLE} expression in mode @var{mode}.  Similarly, the macro
@code{CONST1_RTX (@var{mode})} refers to an expression with value 1 in
mode @var{mode} and similarly for @code{CONST2_RTX}.

@findex const_string
@item (const_string @var{str})
Represents a constant string with value @var{str}.  Currently this is
used only for insn attributes (@pxref{Insn Attributes}) since constant
strings in C are placed in memory.

@findex symbol_ref
@item (symbol_ref:@var{mode} @var{symbol})
Represents the value of an assembler label for data.  @var{symbol} is
a string that describes the name of the assembler label.  If it starts
with a @samp{*}, the label is the rest of @var{symbol} not including
the @samp{*}.  Otherwise, the label is @var{symbol}, usually prefixed
with @samp{_}.

The @code{symbol_ref} contains a mode, which is usually @code{Pmode}.
Usually that is the only mode for which a symbol is directly valid.

@findex label_ref
@item (label_ref @var{label})
Represents the value of an assembler label for code.  It contains one
operand, an expression, which must be a @code{code_label} that appears
in the instruction sequence to identify the place where the label
should go.

The reason for using a distinct expression type for code label
references is so that jump optimization can distinguish them.

@item (const:@var{m} @var{exp})
Represents a constant that is the result of an assembly-time
arithmetic computation.  The operand, @var{exp}, is an expression that
contains only constants (@code{const_int}, @code{symbol_ref} and
@code{label_ref} expressions) combined with @code{plus} and
@code{minus}.  However, not all combinations are valid, since the
assembler cannot do arbitrary arithmetic on relocatable symbols.

@var{m} should be @code{Pmode}.

@findex high
@item (high:@var{m} @var{exp})
Represents the high-order bits of @var{exp}, usually a
@code{symbol_ref}.  The number of bits is machine-dependent and is
normally the number of bits specified in an instruction that initializes
the high order bits of a register.  It is used with @code{lo_sum} to
represent the typical two-instruction sequence used in RISC machines to
reference a global memory location.

@var{m} should be @code{Pmode}.
@end table

@node Regs and Memory, Arithmetic, Constants, RTL
@section Registers and Memory
@cindex RTL register expressions
@cindex RTL memory expressions

Here are the RTL expression types for describing access to machine
registers and to main memory.

@table @code
@findex reg
@cindex hard registers
@cindex pseudo registers
@item (reg:@var{m} @var{n})
For small values of the integer @var{n} (less than
@code{FIRST_PSEUDO_REGISTER}), this stands for a reference to machine
register number @var{n}: a @dfn{hard register}.  For larger values of
@var{n}, it stands for a temporary value or @dfn{pseudo register}.
The compiler's strategy is to generate code assuming an unlimited
number of such pseudo registers, and later convert them into hard
registers or into memory references.

@var{m} is the machine mode of the reference.  It is necessary because
machines can generally refer to each register in more than one mode.
For example, a register may contain a full word but there may be
instructions to refer to it as a half word or as a single byte, as
well as instructions to refer to it as a floating point number of
various precisions.

Even for a register that the machine can access in only one mode,
the mode must always be specified.

The symbol @code{FIRST_PSEUDO_REGISTER} is defined by the machine
description, since the number of hard registers on the machine is an
invariant characteristic of the machine.  Note, however, that not
all of the machine registers must be general registers.  All the
machine registers that can be used for storage of data are given
hard register numbers, even those that can be used only in certain
instructions or can hold only certain types of data.

A hard register may be accessed in various modes throughout one
function, but each pseudo register is given a natural mode
and is accessed only in that mode.  When it is necessary to describe
an access to a pseudo register using a nonnatural mode, a @code{subreg}
expression is used.

A @code{reg} expression with a machine mode that specifies more than
one word of data may actually stand for several consecutive registers.
If in addition the register number specifies a hardware register, then
it actually represents several consecutive hardware registers starting
with the specified one.

Each pseudo register number used in a function's RTL code is
represented by a unique @code{reg} expression.

@findex FIRST_VIRTUAL_REGISTER
@findex LAST_VIRTUAL_REGISTER