4.4BSD/usr/src/contrib/gcc-2.3.3/gcc.info-9
This is Info file gcc.info, produced by Makeinfo-1.49 from the input
file gcc.texi.
This file documents the use and the internals of the GNU compiler.
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File: gcc.info, Node: Accessors, Next: Flags, Prev: RTL Objects, Up: RTL
Access to Operands
==================
For each expression type `rtl.def' specifies the number of contained
objects and their kinds, with four possibilities: `e' for expression
(actually a pointer to an expression), `i' for integer, `w' for wide
integer, `s' for string, and `E' for vector of expressions. The
sequence of letters for an expression code is called its "format".
Thus, the format of `subreg' is `ei'.
A few other format characters are used occasionally:
`u'
`u' is equivalent to `e' except that it is printed differently in
debugging dumps. It is used for pointers to insns.
`n'
`n' is equivalent to `i' except that it is printed differently in
debugging dumps. It is used for the line number or code number of
a `note' insn.
`S'
`S' indicates a string which is optional. In the RTL objects in
core, `S' is equivalent to `s', but when the object is read, from
an `md' file, the string value of this operand may be omitted. An
omitted string is taken to be the null string.
`V'
`V' indicates a vector which is optional. In the RTL objects in
core, `V' is equivalent to `E', but when the object is read from
an `md' file, the vector value of this operand may be omitted. An
omitted vector is effectively the same as a vector of no elements.
`0'
`0' means a slot whose contents do not fit any normal category.
`0' slots are not printed at all in dumps, and are often used in
special ways by small parts of the compiler.
There are macros to get the number of operands, the format, and the
class of an expression code:
`GET_RTX_LENGTH (CODE)'
Number of operands of an RTX of code CODE.
`GET_RTX_FORMAT (CODE)'
The format of an RTX of code CODE, as a C string.
`GET_RTX_CLASS (CODE)'
A single character representing the type of RTX operation that code
CODE performs.
The following classes are defined:
`o'
An RTX code that represents an actual object, such as `reg' or
`mem'. `subreg' is not in this class.
`<'
An RTX code for a comparison. The codes in this class are
`NE', `EQ', `LE', `LT', `GE', `GT', `LEU', `LTU', `GEU',
`GTU'.
`1'
An RTX code for a unary arithmetic operation, such as `neg'.
`c'
An RTX code for a commutative binary operation, other than
`NE' and `EQ' (which have class `<').
`2'
An RTX code for a noncommutative binary operation, such as
`MINUS'.
`b'
An RTX code for a bitfield operation (`ZERO_EXTRACT' and
`SIGN_EXTRACT').
`3'
An RTX code for other three input operations, such as
`IF_THEN_ELSE'.
`i'
An RTX code for a machine insn (`INSN', `JUMP_INSN', and
`CALL_INSN').
`m'
An RTX code for something that matches in insns, such as
`MATCH_DUP'.
`x'
All other RTX codes.
Operands of expressions are accessed using the macros `XEXP',
`XINT', `XWINT' and `XSTR'. Each of these macros takes two arguments:
an expression-pointer (RTX) and an operand number (counting from zero).
Thus,
XEXP (X, 2)
accesses operand 2 of expression X, as an expression.
XINT (X, 2)
accesses the same operand as an integer. `XSTR', used in the same
fashion, would access it as a string.
Any operand can be accessed as an integer, as an expression or as a
string. You must choose the correct method of access for the kind of
value actually stored in the operand. You would do this based on the
expression code of the containing expression. That is also how you
would know how many operands there are.
For example, if X is a `subreg' expression, you know that it has two
operands which can be correctly accessed as `XEXP (X, 0)' and `XINT (X,
1)'. If you did `XINT (X, 0)', you would get the address of the
expression operand but cast as an integer; that might occasionally be
useful, but it would be cleaner to write `(int) XEXP (X, 0)'. `XEXP
(X, 1)' would also compile without error, and would return the second,
integer operand cast as an expression pointer, which would probably
result in a crash when accessed. Nothing stops you from writing `XEXP
(X, 28)' either, but this will access memory past the end of the
expression with unpredictable results.
Access to operands which are vectors is more complicated. You can
use the macro `XVEC' to get the vector-pointer itself, or the macros
`XVECEXP' and `XVECLEN' to access the elements and length of a vector.
`XVEC (EXP, IDX)'
Access the vector-pointer which is operand number IDX in EXP.
`XVECLEN (EXP, IDX)'
Access the length (number of elements) in the vector which is in
operand number IDX in EXP. This value is an `int'.
`XVECEXP (EXP, IDX, ELTNUM)'
Access element number ELTNUM in the vector which is in operand
number IDX in EXP. This value is an RTX.
It is up to you to make sure that ELTNUM is not negative and is
less than `XVECLEN (EXP, IDX)'.
All the macros defined in this section expand into lvalues and
therefore can be used to assign the operands, lengths and vector
elements as well as to access them.
File: gcc.info, Node: Flags, Next: Machine Modes, Prev: Accessors, Up: RTL
Flags in an RTL Expression
==========================
RTL expressions contain several flags (one-bit bit-fields) that are
used in certain types of expression. Most often they are accessed with
the following macros:
`MEM_VOLATILE_P (X)'
In `mem' expressions, nonzero for volatile memory references.
Stored in the `volatil' field and printed as `/v'.
`MEM_IN_STRUCT_P (X)'
In `mem' expressions, nonzero for reference to an entire
structure, union or array, or to a component of one. Zero for
references to a scalar variable or through a pointer to a scalar.
Stored in the `in_struct' field and printed as `/s'.
`REG_LOOP_TEST_P'
In `reg' expressions, nonzero if this register's entire life is
contained in the exit test code for some loop. Stored in the
`in_struct' field and printed as `/s'.
`REG_USERVAR_P (X)'
In a `reg', nonzero if it corresponds to a variable present in the
user's source code. Zero for temporaries generated internally by
the compiler. Stored in the `volatil' field and printed as `/v'.
`REG_FUNCTION_VALUE_P (X)'
Nonzero in a `reg' if it is the place in which this function's
value is going to be returned. (This happens only in a hard
register.) Stored in the `integrated' field and printed as `/i'.
The same hard register may be used also for collecting the values
of functions called by this one, but `REG_FUNCTION_VALUE_P' is zero
in this kind of use.
`SUBREG_PROMOTED_VAR_P'
Nonzero in a `subreg' if it was made when accessing an object that
was promoted to a wider mode in accord with the `PROMOTED_MODE'
machine description macro (*note Storage Layout::.). In this
case, the mode of the `subreg' is the declared mode of the object
and the mode of `SUBREG_REG' is the mode of the register that
holds the object. Promoted variables are always either sign- or
zero-extended to the wider mode on every assignment. Stored in
the `in_struct' field and printed as `/s'.
`SUBREG_PROMOTED_UNSIGNED_P'
Nonzero in a `subreg' that has `SUBREG_PROMOTED_VAR_P' nonzero if
the object being referenced is kept zero-extended and zero if it
is kept sign-extended. Stored in the `unchanging' field and
printed as `/u'.
`RTX_UNCHANGING_P (X)'
Nonzero in a `reg' or `mem' if the value is not changed. (This
flag is not set for memory references via pointers to constants.
Such pointers only guarantee that the object will not be changed
explicitly by the current function. The object might be changed by
other functions or by aliasing.) Stored in the `unchanging' field
and printed as `/u'.
`RTX_INTEGRATED_P (INSN)'
Nonzero in an insn if it resulted from an in-line function call.
Stored in the `integrated' field and printed as `/i'. This may be
deleted; nothing currently depends on it.
`SYMBOL_REF_USED (X)'
In a `symbol_ref', indicates that X has been used. This is
normally only used to ensure that X is only declared external
once. Stored in the `used' field.
`SYMBOL_REF_FLAG (X)'
In a `symbol_ref', this is used as a flag for machine-specific
purposes. Stored in the `volatil' field and printed as `/v'.
`LABEL_OUTSIDE_LOOP_P'
In `label_ref' expressions, nonzero if this is a reference to a
label that is outside the innermost loop containing the reference
to the label. Stored in the `in_struct' field and printed as `/s'.
`INSN_DELETED_P (INSN)'
In an insn, nonzero if the insn has been deleted. Stored in the
`volatil' field and printed as `/v'.
`INSN_ANNULLED_BRANCH_P (INSN)'
In an `insn' in the delay slot of a branch insn, indicates that an
annulling branch should be used. See the discussion under
`sequence' below. Stored in the `unchanging' field and printed as
`/u'.
`INSN_FROM_TARGET_P (INSN)'
In an `insn' in a delay slot of a branch, indicates that the insn
is from the target of the branch. If the branch insn has
`INSN_ANNULLED_BRANCH_P' set, this insn should only be executed if
the branch is taken. For annulled branches with this bit clear,
the insn should be executed only if the branch is not taken.
Stored in the `in_struct' field and printed as `/s'.
`CONSTANT_POOL_ADDRESS_P (X)'
Nonzero in a `symbol_ref' if it refers to part of the current
function's "constants pool". These are addresses close to the
beginning of the function, and GNU CC assumes they can be addressed
directly (perhaps with the help of base registers). Stored in the
`unchanging' field and printed as `/u'.
`CONST_CALL_P (X)'
In a `call_insn', indicates that the insn represents a call to a
const function. Stored in the `unchanging' field and printed as
`/u'.
`LABEL_PRESERVE_P (X)'
In a `code_label', indicates that the label can never be deleted.
Labels referenced by a non-local goto will have this bit set.
Stored in the `in_struct' field and printed as `/s'.
`SCHED_GROUP_P (INSN)'
During instruction scheduling, in an insn, indicates that the
previous insn must be scheduled together with this insn. This is
used to ensure that certain groups of instructions will not be
split up by the instruction scheduling pass, for example, `use'
insns before a `call_insn' may not be separated from the
`call_insn'. Stored in the `in_struct' field and printed as `/s'.
These are the fields which the above macros refer to:
`used'
Normally, this flag is used only momentarily, at the end of RTL
generation for a function, to count the number of times an
expression appears in insns. Expressions that appear more than
once are copied, according to the rules for shared structure
(*note Sharing::.).
In a `symbol_ref', it indicates that an external declaration for
the symbol has already been written.
In a `reg', it is used by the leaf register renumbering code to
ensure that each register is only renumbered once.
`volatil'
This flag is used in `mem', `symbol_ref' and `reg' expressions and
in insns. In RTL dump files, it is printed as `/v'.
In a `mem' expression, it is 1 if the memory reference is volatile.
Volatile memory references may not be deleted, reordered or
combined.
In a `symbol_ref' expression, it is used for machine-specific
purposes.
In a `reg' expression, it is 1 if the value is a user-level
variable. 0 indicates an internal compiler temporary.
In an insn, 1 means the insn has been deleted.
`in_struct'
In `mem' expressions, it is 1 if the memory datum referred to is
all or part of a structure or array; 0 if it is (or might be) a
scalar variable. A reference through a C pointer has 0 because
the pointer might point to a scalar variable. This information
allows the compiler to determine something about possible cases of
aliasing.
In an insn in the delay slot of a branch, 1 means that this insn
is from the target of the branch.
During instruction scheduling, in an insn, 1 means that this insn
must be scheduled as part of a group together with the previous
insn.
In `reg' expressions, it is 1 if the register has its entire life
contained within the test expression of some loopl.
In `subreg' expressions, 1 means that the `subreg' is accessing an
object that has had its mode promoted from a wider mode.
In `label_ref' expressions, 1 means that the referenced label is
outside the innermost loop containing the insn in which the
`label_ref' was found.
In `code_label' expressions, it is 1 if the label may never be
deleted. This is used for labels which are the target of non-local
gotos.
In an RTL dump, this flag is represented as `/s'.
`unchanging'
In `reg' and `mem' expressions, 1 means that the value of the
expression never changes.
In `subreg' expressions, it is 1 if the `subreg' references an
unsigned object whose mode has been promoted to a wider mode.
In an insn, 1 means that this is an annulling branch.
In a `symbol_ref' expression, 1 means that this symbol addresses
something in the per-function constants pool.
In a `call_insn', 1 means that this instruction is a call to a
const function.
In an RTL dump, this flag is represented as `/u'.
`integrated'
In some kinds of expressions, including insns, this flag means the
rtl was produced by procedure integration.
In a `reg' expression, this flag indicates the register containing
the value to be returned by the current function. On machines
that pass parameters in registers, the same register number may be
used for parameters as well, but this flag is not set on such uses.
File: gcc.info, Node: Machine Modes, Next: Constants, Prev: Flags, Up: RTL
Machine Modes
=============
A machine mode describes a size of data object and the
representation used for it. In the C code, machine modes are
represented by an enumeration type, `enum machine_mode', defined in
`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 `mode' which appear at the end of each
machine mode name are omitted. For example, `(reg:SI 38)' is a `reg'
expression with machine mode `SImode'. If the mode is `VOIDmode', it
is not written at all.
Here is a table of machine modes. The term "byte" below refers to an
object of `BITS_PER_UNIT' bits (*note Storage Layout::.).
`QImode'
"Quarter-Integer" mode represents a single byte treated as an
integer.
`HImode'
"Half-Integer" mode represents a two-byte integer.
`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.
`SImode'
"Single Integer" mode represents a four-byte integer.
`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.
`DImode'
"Double Integer" mode represents an eight-byte integer.
`TImode'
"Tetra Integer" (?) mode represents a sixteen-byte integer.
`SFmode'
"Single Floating" mode represents a single-precision (four byte)
floating point number.
`DFmode'
"Double Floating" mode represents a double-precision (eight byte)
floating point number.
`XFmode'
"Extended Floating" mode represents a triple-precision (twelve
byte) floating point number. This mode is used for IEEE extended
floating point.
`TFmode'
"Tetra Floating" mode represents a quadruple-precision (sixteen
byte) floating point number.
`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 `cc0' (see *note Condition Code::.).
`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,
`BLKmode' will not appear in RTL.
`VOIDmode'
Void mode means the absence of a mode or an unspecified mode. For
example, RTL expressions of code `const_int' have mode `VOIDmode'
because they can be taken to have whatever mode the context
requires. In debugging dumps of RTL, `VOIDmode' is expressed by
the absence of any mode.
`SCmode, DCmode, XCmode, TCmode'
These modes stand for a complex number represented as a pair of
floating point values. The values are in `SFmode', `DFmode',
`XFmode', and `TFmode', respectively. Since C does not support
complex numbers, these machine modes are only partially
implemented.
The machine description defines `Pmode' as a C macro which expands
into the machine mode used for addresses. Normally this is the mode
whose size is `BITS_PER_WORD', `SImode' on 32-bit machines.
The only modes which a machine description must support are
`QImode', and the modes corresponding to `BITS_PER_WORD',
`FLOAT_TYPE_SIZE' and `DOUBLE_TYPE_SIZE'. The compiler will attempt to
use `DImode' for 8-byte structures and unions, but this can be
prevented by overriding the definition of `MAX_FIXED_MODE_SIZE'.
Alternatively, you can have the compiler use `TImode' for 16-byte
structures and unions. Likewise, you can arrange for the C type `short
int' to avoid using `HImode'.
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 `enum mode_class' defined in `machmode.h'. The
possible mode classes are:
`MODE_INT'
Integer modes. By default these are `QImode', `HImode', `SImode',
`DImode', and `TImode'.
`MODE_PARTIAL_INT'
The "partial integer" modes, `PSImode' and `PDImode'.
`MODE_FLOAT'
floating point modes. By default these are `SFmode', `DFmode',
`XFmode' and `TFmode'.
`MODE_COMPLEX_INT'
Complex integer modes. (These are not currently implemented).
`MODE_COMPLEX_FLOAT'
Complex floating point modes. By default these are `SCmode',
`DCmode', `XCmode', and `TCmode'.
`MODE_FUNCTION'
Algol or Pascal function variables including a static chain.
(These are not currently implemented).
`MODE_CC'
Modes representing condition code values. These are `CCmode' plus
any modes listed in the `EXTRA_CC_MODES' macro. *Note Jump
Patterns::, also see *Note Condition Code::.
`MODE_RANDOM'
This is a catchall mode class for modes which don't fit into the
above classes. Currently `VOIDmode' and `BLKmode' are in
`MODE_RANDOM'.
Here are some C macros that relate to machine modes:
`GET_MODE (X)'
Returns the machine mode of the RTX X.
`PUT_MODE (X, NEWMODE)'
Alters the machine mode of the RTX X to be NEWMODE.
`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.
`GET_MODE_NAME (M)'
Returns the name of mode M as a string.
`GET_MODE_CLASS (M)'
Returns the mode class of mode M.
`GET_MODE_WIDER_MODE (M)'
Returns the next wider natural mode. E.g.,
`GET_WIDER_MODE(QImode)' returns `HImode'.
`GET_MODE_SIZE (M)'
Returns the size in bytes of a datum of mode M.
`GET_MODE_BITSIZE (M)'
Returns the size in bits of a datum of mode M.
`GET_MODE_MASK (M)'
Returns a bitmask containing 1 for all bits in a word that fit
within mode M. This macro can only be used for modes whose
bitsize is less than or equal to `HOST_BITS_PER_INT'.
`GET_MODE_ALIGNMENT (M))'
Return the required alignment, in bits, for an object of mode M.
`GET_MODE_UNIT_SIZE (M)'
Returns the size in bytes of the subunits of a datum of mode M.
This is the same as `GET_MODE_SIZE' except in the case of complex
modes. For them, the unit size is the size of the real or
imaginary part.
`GET_MODE_NUNITS (M)'
Returns the number of units contained in a mode, i.e.,
`GET_MODE_SIZE' divided by `GET_MODE_UNIT_SIZE'.
`GET_CLASS_NARROWEST_MODE (C)'
Returns the narrowest mode in mode class C.
The global variables `byte_mode' and `word_mode' contain modes whose
classes are `MODE_INT' and whose bitsizes are `BITS_PER_UNIT' or
`BITS_PER_WORD', respectively. On 32-bit machines, these are `QImode'
and `SImode', respectively.
File: gcc.info, Node: Constants, Next: Regs and Memory, Prev: Machine Modes, Up: RTL
Constant Expression Types
=========================
The simplest RTL expressions are those that represent constant
values.
`(const_int I)'
This type of expression represents the integer value I. I is
customarily accessed with the macro `INTVAL' as in `INTVAL (EXP)',
which is equivalent to `XWINT (EXP, 0)'.
There is only one expression object for the integer value zero; it
is the value of the variable `const0_rtx'. Likewise, the only
expression for integer value one is found in `const1_rtx', the only
expression for integer value two is found in `const2_rtx', and the
only expression for integer value negative one is found in
`constm1_rtx'. Any attempt to create an expression of code
`const_int' and value zero, one, two or negative one will return
`const0_rtx', `const1_rtx', `const2_rtx' or `constm1_rtx' as
appropriate.
Similarly, there is only one object for the integer whose value is
`STORE_FLAG_VALUE'. It is found in `const_true_rtx'. If
`STORE_FLAG_VALUE' is one, `const_true_rtx' and `const1_rtx' will
point to the same object. If `STORE_FLAG_VALUE' is -1,
`const_true_rtx' and `constm1_rtx' will point to the same object.
`(const_double:M ADDR I0 I1 ...)'
Represents either a floating-point constant of mode M or an
integer constant that is too large to fit into
`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, M will be
`VOIDmode'.
ADDR is used to contain the `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 `const_double' expressions in this compilation (maintained
using an undisplayed field), ADDR contains `const0_rtx'. If it is
not on the chain, ADDR contains `cc0_rtx'. ADDR is customarily
accessed with the macro `CONST_DOUBLE_MEM' and the chain field via
`CONST_DOUBLE_CHAIN'.
If M is `VOIDmode', the bits of the value are stored in I0 and I1.
I0 is customarily accessed with the macro `CONST_DOUBLE_LOW' and
I1 with `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 `REAL_VALUE_TYPE' (*note
Cross-compilation::.). The integers represent a `double'. To
convert them to a `double', do
union real_extract u;
bcopy (&CONST_DOUBLE_LOW (x), &u, sizeof u);
and then refer to `u.d'.
The macro `CONST0_RTX (MODE)' refers to an expression with value 0
in mode MODE. If mode MODE is of mode class `MODE_INT', it returns
`const0_rtx'. Otherwise, it returns a `CONST_DOUBLE' expression
in mode MODE. Similarly, the macro `CONST1_RTX (MODE)' refers to
an expression with value 1 in mode MODE and similarly for
`CONST2_RTX'.
`(const_string STR)'
Represents a constant string with value STR. Currently this is
used only for insn attributes (*note Insn Attributes::.) since
constant strings in C are placed in memory.
`(symbol_ref:MODE SYMBOL)'
Represents the value of an assembler label for data. SYMBOL is a
string that describes the name of the assembler label. If it
starts with a `*', the label is the rest of SYMBOL not including
the `*'. Otherwise, the label is SYMBOL, usually prefixed with
`_'.
The `symbol_ref' contains a mode, which is usually `Pmode'.
Usually that is the only mode for which a symbol is directly valid.
`(label_ref LABEL)'
Represents the value of an assembler label for code. It contains
one operand, an expression, which must be a `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.
`(const:M EXP)'
Represents a constant that is the result of an assembly-time
arithmetic computation. The operand, EXP, is an expression that
contains only constants (`const_int', `symbol_ref' and `label_ref'
expressions) combined with `plus' and `minus'. However, not all
combinations are valid, since the assembler cannot do arbitrary
arithmetic on relocatable symbols.
M should be `Pmode'.
`(high:M EXP)'
Represents the high-order bits of EXP, usually a `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 `lo_sum' to represent
the typical two-instruction sequence used in RISC machines to
reference a global memory location.
M should be `Pmode'.
File: gcc.info, Node: Regs and Memory, Next: Arithmetic, Prev: Constants, Up: RTL
Registers and Memory
====================
Here are the RTL expression types for describing access to machine
registers and to main memory.
`(reg:M N)'
For small values of the integer N (less than
`FIRST_PSEUDO_REGISTER'), this stands for a reference to machine
register number N: a "hard register". For larger values of N, it
stands for a temporary value or "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.
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 `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 `subreg'
expression is used.
A `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 `reg' expression.
Some pseudo register numbers, those within the range of
`FIRST_VIRTUAL_REGISTER' to `LAST_VIRTUAL_REGISTER' only appear
during the RTL generation phase and are eliminated before the
optimization phases. These represent locations in the stack frame
that cannot be determined until RTL generation for the function
has been completed. The following virtual register numbers are
defined:
`VIRTUAL_INCOMING_ARGS_REGNUM'
This points to the first word of the incoming arguments
passed on the stack. Normally these arguments are placed
there by the caller, but the callee may have pushed some
arguments that were previously passed in registers.
When RTL generation is complete, this virtual register is
replaced by the sum of the register given by
`ARG_POINTER_REGNUM' and the value of `FIRST_PARM_OFFSET'.
`VIRTUAL_STACK_VARS_REGNUM'
If `FRAME_GROWS_DOWNWARDS' is defined, this points to
immediately above the first variable on the stack.
Otherwise, it points to the first variable on the stack.
It is replaced with the sum of the register given by
`FRAME_POINTER_REGNUM' and the value `STARTING_FRAME_OFFSET'.
`VIRTUAL_STACK_DYNAMIC_REGNUM'
This points to the location of dynamically allocated memory
on the stack immediately after the stack pointer has been
adjusted by the amount of memory desired.
It is replaced by the sum of the register given by
`STACK_POINTER_REGNUM' and the value `STACK_DYNAMIC_OFFSET'.
`VIRTUAL_OUTGOING_ARGS_REGNUM'
This points to the location in the stack at which outgoing
arguments should be written when the stack is pre-pushed
(arguments pushed using push insns should always use
`STACK_POINTER_REGNUM').
It is replaced by the sum of the register given by
`STACK_POINTER_REGNUM' and the value `STACK_POINTER_OFFSET'.
`(subreg:M REG WORDNUM)'
`subreg' expressions are used to refer to a register in a machine
mode other than its natural one, or to refer to one register of a
multi-word `reg' that actually refers to several registers.
Each pseudo-register has a natural mode. If it is necessary to
operate on it in a different mode--for example, to perform a
fullword move instruction on a pseudo-register that contains a
single byte--the pseudo-register must be enclosed in a `subreg'.
In such a case, WORDNUM is zero.
Usually M is at least as narrow as the mode of REG, in which case
it is restricting consideration to only the bits of REG that are
in M. However, sometimes M is wider than the mode of REG. These
`subreg' expressions are often called "paradoxical". They are
used in cases where we want to refer to an object in a wider mode
but do not care what value the additional bits have. The reload
pass ensures that paradoxical references are only made to hard
registers.
The other use of `subreg' is to extract the individual registers of
a multi-register value. Machine modes such as `DImode' and
`TImode' can indicate values longer than a word, values which
usually require two or more consecutive registers. To access one
of the registers, use a `subreg' with mode `SImode' and a WORDNUM
that says which register.
The compilation parameter `WORDS_BIG_ENDIAN', if set to 1, says
that word number zero is the most significant part; otherwise, it
is the least significant part.
Between the combiner pass and the reload pass, it is possible to
have a paradoxical `subreg' which contains a `mem' instead of a
`reg' as its first operand. After the reload pass, it is also
possible to have a non-paradoxical `subreg' which contains a
`mem'; this usually occurs when the `mem' is a stack slot which
replaced a pseudo register.
Note that it is not valid to access a `DFmode' value in `SFmode'
using a `subreg'. On some machines the most significant part of a
`DFmode' value does not have the same format as a single-precision
floating value.
It is also not valid to access a single word of a multi-word value
in a hard register when less registers can hold the value than
would be expected from its size. For example, some 32-bit
machines have floating-point registers that can hold an entire
`DFmode' value. If register 10 were such a register `(subreg:SI
(reg:DF 10) 1)' would be invalid because there is no way to
convert that reference to a single machine register. The reload
pass prevents `subreg' expressions such as these from being formed.
The first operand of a `subreg' expression is customarily accessed
with the `SUBREG_REG' macro and the second operand is customarily
accessed with the `SUBREG_WORD' macro.
`(scratch:M)'
This represents a scratch register that will be required for the
execution of a single instruction and not used subsequently. It is
converted into a `reg' by either the local register allocator or
the reload pass.
`scratch' is usually present inside a `clobber' operation (*note
Side Effects::.).
`(cc0)'
This refers to the machine's condition code register. It has no
operands and may not have a machine mode. There are two ways to
use it:
* To stand for a complete set of condition code flags. This is
best on most machines, where each comparison sets the entire
series of flags.
With this technique, `(cc0)' may be validly used in only two
contexts: as the destination of an assignment (in test and
compare instructions) and in comparison operators comparing
against zero (`const_int' with value zero; that is to say,
`const0_rtx').
* To stand for a single flag that is the result of a single
condition. This is useful on machines that have only a single
flag bit, and in which comparison instructions must specify
the condition to test.
With this technique, `(cc0)' may be validly used in only two
contexts: as the destination of an assignment (in test and
compare instructions) where the source is a comparison
operator, and as the first operand of `if_then_else' (in a
conditional branch).
There is only one expression object of code `cc0'; it is the value
of the variable `cc0_rtx'. Any attempt to create an expression of
code `cc0' will return `cc0_rtx'.
Instructions can set the condition code implicitly. On many
machines, nearly all instructions set the condition code based on
the value that they compute or store. It is not necessary to
record these actions explicitly in the RTL because the machine
description includes a prescription for recognizing the
instructions that do so (by means of the macro
`NOTICE_UPDATE_CC'). *Note Condition Code::. Only instructions
whose sole purpose is to set the condition code, and instructions
that use the condition code, need mention `(cc0)'.
On some machines, the condition code register is given a register
number and a `reg' is used instead of `(cc0)'. This is usually the
preferable approach if only a small subset of instructions modify
the condition code. Other machines store condition codes in
general registers; in such cases a pseudo register should be used.
Some machines, such as the Sparc and RS/6000, have two sets of
arithmetic instructions, one that sets and one that does not set
the condition code. This is best handled by normally generating
the instruction that does not set the condition code, and making a
pattern that both performs the arithmetic and sets the condition
code register (which would not be `(cc0)' in this case). For
examples, search for `addcc' and `andcc' in `sparc.md'.
`(pc)'
This represents the machine's program counter. It has no operands
and may not have a machine mode. `(pc)' may be validly used only
in certain specific contexts in jump instructions.
There is only one expression object of code `pc'; it is the value
of the variable `pc_rtx'. Any attempt to create an expression of
code `pc' will return `pc_rtx'.
All instructions that do not jump alter the program counter
implicitly by incrementing it, but there is no need to mention
this in the RTL.
`(mem:M ADDR)'
This RTX represents a reference to main memory at an address
represented by the expression ADDR. M specifies how large a unit
of memory is accessed.
File: gcc.info, Node: Arithmetic, Next: Comparisons, Prev: Regs and Memory, Up: RTL
RTL Expressions for Arithmetic
==============================
Unless otherwise specified, all the operands of arithmetic
expressions must be valid for mode M. An operand is valid for mode M
if it has mode M, or if it is a `const_int' or `const_double' and M is
a mode of class `MODE_INT'.
For commutative binary operations, constants should be placed in the
second operand.
`(plus:M X Y)'
Represents the sum of the values represented by X and Y carried
out in machine mode M.
`(lo_sum:M X Y)'
Like `plus', except that it represents that sum of X and the
low-order bits of Y. The number of low order bits is
machine-dependent but is normally the number of bits in a `Pmode'
item minus the number of bits set by the `high' code (*note
Constants::.).
M should be `Pmode'.
`(minus:M X Y)'
Like `plus' but represents subtraction.
`(compare:M X Y)'
Represents the result of subtracting Y from X for purposes of
comparison. The result is computed without overflow, as if with
infinite precision.
Of course, machines can't really subtract with infinite precision.
However, they can pretend to do so when only the sign of the
result will be used, which is the case when the result is stored
in the condition code. And that is the only way this kind of
expression may validly be used: as a value to be stored in the
condition codes.
The mode M is not related to the modes of X and Y, but instead is
the mode of the condition code value. If `(cc0)' is used, it is
`VOIDmode'. Otherwise it is some mode in class `MODE_CC', often
`CCmode'. *Note Condition Code::.
Normally, X and Y must have the same mode. Otherwise, `compare'
is valid only if the mode of X is in class `MODE_INT' and Y is a
`const_int' or `const_double' with mode `VOIDmode'. The mode of X
determines what mode the comparison is to be done in; thus it must
not be `VOIDmode'.
If one of the operands is a constant, it should be placed in the
second operand and the comparison code adjusted as appropriate.
A `compare' specifying two `VOIDmode' constants is not valid since
there is no way to know in what mode the comparison is to be
performed; the comparison must either be folded during the
compilation or the first operand must be loaded into a register
while its mode is still known.
`(neg:M X)'
Represents the negation (subtraction from zero) of the value
represented by X, carried out in mode M.
`(mult:M X Y)'
Represents the signed product of the values represented by X and Y
carried out in machine mode M.
Some machines support a multiplication that generates a product
wider than the operands. Write the pattern for this as
(mult:M (sign_extend:M X) (sign_extend:M Y))
where M is wider than the modes of X and Y, which need not be the
same.
Write patterns for unsigned widening multiplication similarly using
`zero_extend'.
`(div:M X Y)'
Represents the quotient in signed division of X by Y, carried out
in machine mode M. If M is a floating point mode, it represents
the exact quotient; otherwise, the integerized quotient.
Some machines have division instructions in which the operands and
quotient widths are not all the same; you should represent such
instructions using `truncate' and `sign_extend' as in,
(truncate:M1 (div:M2 X (sign_extend:M2 Y)))
`(udiv:M X Y)'
Like `div' but represents unsigned division.
`(mod:M X Y)'
`(umod:M X Y)'
Like `div' and `udiv' but represent the remainder instead of the
quotient.
`(smin:M X Y)'
`(smax:M X Y)'
Represents the smaller (for `smin') or larger (for `smax') of X
and Y, interpreted as signed integers in mode M.
`(umin:M X Y)'
`(umax:M X Y)'
Like `smin' and `smax', but the values are interpreted as unsigned
integers.
`(not:M X)'
Represents the bitwise complement of the value represented by X,
carried out in mode M, which must be a fixed-point machine mode.
`(and:M X Y)'
Represents the bitwise logical-and of the values represented by X
and Y, carried out in machine mode M, which must be a fixed-point
machine mode.
`(ior:M X Y)'
Represents the bitwise inclusive-or of the values represented by X
and Y, carried out in machine mode M, which must be a fixed-point
mode.
`(xor:M X Y)'
Represents the bitwise exclusive-or of the values represented by X
and Y, carried out in machine mode M, which must be a fixed-point
mode.
`(ashift:M X C)'
Represents the result of arithmetically shifting X left by C
places. X have mode M, a fixed-point machine mode. C be a
fixed-point mode or be a constant with mode `VOIDmode'; which mode
is determined by the mode called for in the machine description
entry for the left-shift instruction. For example, on the Vax,
the mode of C is `QImode' regardless of M.
`(lshift:M X C)'
Like `ashift' but for logical left shift. `ashift' and `lshift'
are identical operations; we customarily use `ashift' for both.
`(lshiftrt:M X C)'
`(ashiftrt:M X C)'
Like `lshift' and `ashift' but for right shift. Unlike the case
for left shift, these two operations are distinct.
`(rotate:M X C)'
`(rotatert:M X C)'
Similar but represent left and right rotate. If C is a constant,
use `rotate'.
`(abs:M X)'
Represents the absolute value of X, computed in mode M.
`(sqrt:M X)'
Represents the square root of X, computed in mode M. Most often M
will be a floating point mode.
`(ffs:M X)'
Represents one plus the index of the least significant 1-bit in X,
represented as an integer of mode M. (The value is zero if X is
zero.) The mode of X need not be M; depending on the target
machine, various mode combinations may be valid.
File: gcc.info, Node: Comparisons, Next: Bit Fields, Prev: Arithmetic, Up: RTL
Comparison Operations
=====================
Comparison operators test a relation on two operands and are
considered to represent a machine-dependent nonzero value described by,
but not necessarily equal to, `STORE_FLAG_VALUE' (*note Misc::.) if the
relation holds, or zero if it does not. The mode of the comparison
operation is independent of the mode of the data being compared. If
the comparison operation is being tested (e.g., the first operand of an
`if_then_else'), the mode must be `VOIDmode'. If the comparison
operation is producing data to be stored in some variable, the mode
must be in class `MODE_INT'. All comparison operations producing data
must use the same mode, which is machine-specific.
There are two ways that comparison operations may be used. The
comparison operators may be used to compare the condition codes `(cc0)'
against zero, as in `(eq (cc0) (const_int 0))'. Such a construct
actually refers to the result of the preceding instruction in which the
condition codes were set. The instructing setting the condition code
must be adjacent to the instruction using the condition code; only
`note' insns may separate them.
Alternatively, a comparison operation may directly compare two data
objects. The mode of the comparison is determined by the operands; they
must both be valid for a common machine mode. A comparison with both
operands constant would be invalid as the machine mode could not be
deduced from it, but such a comparison should never exist in RTL due to
constant folding.
In the example above, if `(cc0)' were last set to `(compare X Y)',
the comparison operation is identical to `(eq X Y)'. Usually only one
style of comparisons is supported on a particular machine, but the
combine pass will try to merge the operations to produce the `eq' shown
in case it exists in the context of the particular insn involved.
Inequality comparisons come in two flavors, signed and unsigned.
Thus, there are distinct expression codes `gt' and `gtu' for signed and
unsigned greater-than. These can produce different results for the same
pair of integer values: for example, 1 is signed greater-than -1 but not
unsigned greater-than, because -1 when regarded as unsigned is actually
`0xffffffff' which is greater than 1.
The signed comparisons are also used for floating point values.
Floating point comparisons are distinguished by the machine modes of
the operands.
`(eq:M X Y)'
1 if the values represented by X and Y are equal, otherwise 0.
`(ne:M X Y)'
1 if the values represented by X and Y are not equal, otherwise 0.
`(gt:M X Y)'
1 if the X is greater than Y. If they are fixed-point, the
comparison is done in a signed sense.
`(gtu:M X Y)'
Like `gt' but does unsigned comparison, on fixed-point numbers
only.
`(lt:M X Y)'
`(ltu:M X Y)'
Like `gt' and `gtu' but test for "less than".
`(ge:M X Y)'
`(geu:M X Y)'
Like `gt' and `gtu' but test for "greater than or equal".
`(le:M X Y)'
`(leu:M X Y)'
Like `gt' and `gtu' but test for "less than or equal".
`(if_then_else COND THEN ELSE)'
This is not a comparison operation but is listed here because it is
always used in conjunction with a comparison operation. To be
precise, COND is a comparison expression. This expression
represents a choice, according to COND, between the value
represented by THEN and the one represented by ELSE.
On most machines, `if_then_else' expressions are valid only to
express conditional jumps.
`(cond [TEST1 VALUE1 TEST2 VALUE2 ...] DEFAULT)'
Similar to `if_then_else', but more general. Each of TEST1,
TEST2, ... is performed in turn. The result of this expression is
the VALUE corresponding to the first non-zero test, or DEFAULT if
none of the tests are non-zero expressions.
This is currently not valid for instruction patterns and is
supported only for insn attributes. *Note Insn Attributes::.
File: gcc.info, Node: Bit Fields, Next: Conversions, Prev: Comparisons, Up: RTL
Bit Fields
==========
Special expression codes exist to represent bit-field instructions.
These types of expressions are lvalues in RTL; they may appear on the
left side of an assignment, indicating insertion of a value into the
specified bit field.
`(sign_extract:M LOC SIZE POS)'
This represents a reference to a sign-extended bit field contained
or starting in LOC (a memory or register reference). The bit field
is SIZE bits wide and starts at bit POS. The compilation option
`BITS_BIG_ENDIAN' says which end of the memory unit POS counts
from.
If LOC is in memory, its mode must be a single-byte integer mode.
If LOC is in a register, the mode to use is specified by the
operand of the `insv' or `extv' pattern (*note Standard Names::.)
and is usually a full-word integer mode.
The mode of POS is machine-specific and is also specified in the
`insv' or `extv' pattern.
The mode M is the same as the mode that would be used for LOC if
it were a register.
`(zero_extract:M LOC SIZE POS)'
Like `sign_extract' but refers to an unsigned or zero-extended bit
field. The same sequence of bits are extracted, but they are
filled to an entire word with zeros instead of by sign-extension.