Go to the previous, next section.
Most of the work of the compiler is done on an intermediate representation called register transfer language. In this language, the instructions to be output are described, pretty much one by one, in an algebraic form that describes what the instruction does.
RTL is inspired by Lisp lists. It has both an internal form, made up of structures that point at other structures, and a textual form that is used in the machine description and in printed debugging dumps. The textual form uses nested parentheses to indicate the pointers in the internal form.
RTL uses five kinds of objects: expressions, integers, wide integers,
strings and vectors. Expressions are the most important ones. An RTL
expression ("RTX", for short) is a C structure, but it is usually
referred to with a pointer; a type that is given the typedef name
rtx.
An integer is simply an int; their written form uses decimal digits.
A wide integer is an integral object whose type is HOST_WIDE_INT
(see section The Configuration File); their written form uses decimal digits.
A string is a sequence of characters. In core it is represented as a
char * in usual C fashion, and it is written in C syntax as well.
However, strings in RTL may never be null. If you write an empty string in
a machine description, it is represented in core as a null pointer rather
than as a pointer to a null character. In certain contexts, these null
pointers instead of strings are valid. Within RTL code, strings are most
commonly found inside symbol_ref expressions, but they appear in
other contexts in the RTL expressions that make up machine descriptions.
A vector contains an arbitrary number of pointers to expressions. The number of elements in the vector is explicitly present in the vector. The written form of a vector consists of square brackets (`[...]') surrounding the elements, in sequence and with whitespace separating them. Vectors of length zero are not created; null pointers are used instead.
Expressions are classified by expression codes (also called RTX
codes). The expression code is a name defined in `rtl.def', which is
also (in upper case) a C enumeration constant. The possible expression
codes and their meanings are machine-independent. The code of an RTX can
be extracted with the macro GET_CODE (x) and altered with
PUT_CODE (x, newcode).
The expression code determines how many operands the expression contains,
and what kinds of objects they are. In RTL, unlike Lisp, you cannot tell
by looking at an operand what kind of object it is. Instead, you must know
from its context--from the expression code of the containing expression.
For example, in an expression of code subreg, the first operand is
to be regarded as an expression and the second operand as an integer. In
an expression of code plus, there are two operands, both of which
are to be regarded as expressions. In a symbol_ref expression,
there is one operand, which is to be regarded as a string.
Expressions are written as parentheses containing the name of the expression type, its flags and machine mode if any, and then the operands of the expression (separated by spaces).
Expression code names in the `md' file are written in lower case,
but when they appear in C code they are written in upper case. In this
manual, they are shown as follows: const_int.
In a few contexts a null pointer is valid where an expression is normally
wanted. The written form of this is (nil).
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
n
note insn.
S
V
0
There are macros to get the number of operands, the format, and the class of an expression code:
GET_RTX_LENGTH (code)
GET_RTX_FORMAT (code)
GET_RTX_CLASS (code)
The following classes are defined:
o
reg or
mem. subreg is not in this class.
<
NE, EQ, LE, LT, GE, GT,
LEU, LTU, GEU, GTU.
1
neg.
c
NE
and EQ (which have class `<').
2
MINUS.
b
ZERO_EXTRACT or
SIGN_EXTRACT.
3
IF_THEN_ELSE.
i
INSN, JUMP_INSN, and
CALL_INSN).
m
MATCH_DUP.
x
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)
XVECLEN (exp, idx)
int.
XVECEXP (exp, idx, eltnum)
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.
RTL expressions contain several flags (one-bit bitfields) that are used in certain types of expression. Most often they are accessed with the following macros:
MEM_VOLATILE_P (x)
mem expressions, nonzero for volatile memory references.
Stored in the volatil field and printed as `/v'.
MEM_IN_STRUCT_P (x)
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
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)
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)
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
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 (see section 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
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)
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)
integrated field and printed as `/i'. This
may be deleted; nothing currently depends on it.
SYMBOL_REF_USED (x)
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)
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
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)
volatil field and printed as `/v'.
INSN_ANNULLED_BRANCH_P (insn)
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)
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)
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)
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)
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)
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
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
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
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 loop.
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
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 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.
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 (see section Storage Layout).
QImode
HImode
PSImode
SImode
PDImode
DImode
TImode
SFmode
DFmode
XFmode
TFmode
CCmode
cc0 (see see section Condition Code Status).
BLKmode
BLKmode will not appear in RTL.
VOIDmode
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
SFmode,
DFmode, XFmode, and TFmode, respectively.
CQImode, CHImode, CSImode, CDImode, CTImode, COImode
QImode, HImode,
SImode, DImode, TImode, and OImode,
respectively.
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
QImode, HImode,
SImode, DImode, and TImode.
MODE_PARTIAL_INT
PSImode and PDImode.
MODE_FLOAT
SFmode, DFmode,
XFmode and TFmode.
MODE_COMPLEX_INT
MODE_COMPLEX_FLOAT
SCmode,
DCmode, XCmode, and TCmode.
MODE_FUNCTION
MODE_CC
CCmode plus
any modes listed in the EXTRA_CC_MODES macro. See section Defining Jump Instruction Patterns,
also see section Condition Code Status.
MODE_RANDOM
VOIDmode and BLKmode are in
MODE_RANDOM.
Here are some C macros that relate to machine modes:
GET_MODE (x)
PUT_MODE (x, newmode)
NUM_MACHINE_MODES
GET_MODE_NAME (m)
GET_MODE_CLASS (m)
GET_MODE_WIDER_MODE (m)
GET_MODE_WIDER_MODE (QImode) returns HImode.
GET_MODE_SIZE (m)
GET_MODE_BITSIZE (m)
GET_MODE_MASK (m)
HOST_BITS_PER_INT.
GET_MODE_ALIGNMENT (m))
GET_MODE_UNIT_SIZE (m)
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)
GET_MODE_SIZE divided by GET_MODE_UNIT_SIZE.
GET_CLASS_NARROWEST_MODE (c)
The global variables byte_mode and word_mode contain modes
whose classes are MODE_INT and whose bitsizes are either
BITS_PER_UNIT or BITS_PER_WORD, respectively. On 32-bit
machines, these are QImode and SImode, respectively.
The simplest RTL expressions are those that represent constant values.
(const_int i)
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 ...)
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 (regardless of its precision), then
the number of integers used to store the value depends on the size of
REAL_VALUE_TYPE (see section Cross Compilation and Floating Point). The integers
represent a floating point number, but not precisely in the target
machine's or host machine's floating point format. To convert them to
the precise bit pattern used by the target machine, use the macro
REAL_VALUE_TO_TARGET_DOUBLE and friends (see section Output of Data).
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)
(symbol_ref:mode symbol)
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)
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)
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.
(high:m exp)
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.
Here are the RTL expression types for describing access to machine registers and to main memory.
(reg:m n)
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
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
FRAME_GROWS_DOWNWARD is defined, this points to immediately
above the first variable on the stack. Otherwise, it points to the
first variable on the stack.
VIRTUAL_STACK_VARS_REGNUM is replaced with the sum of the
register given by FRAME_POINTER_REGNUM and the value
STARTING_FRAME_OFFSET.
VIRTUAL_STACK_DYNAMIC_REGNUM
This virtual register is replaced by the sum of the register given by
STACK_POINTER_REGNUM and the value STACK_DYNAMIC_OFFSET.
VIRTUAL_OUTGOING_ARGS_REGNUM
STACK_POINTER_REGNUM).
This virtual register is replaced by the sum of the register given by
STACK_POINTER_REGNUM and the value STACK_POINTER_OFFSET.
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.
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.
Storing in a non-paradoxical subreg has undefined results for
bits belonging to the same word as the subreg. This laxity makes
it easier to generate efficient code for such instructions. To
represent an instruction that preserves all the bits outside of those in
the subreg, use strict_low_part around the subreg.
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.
reg by either the local register allocator or
the reload pass.
scratch is usually present inside a clobber operation
(see section Side Effect Expressions).
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).
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). See section Condition Code Status. 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) 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.
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)
(lo_sum:m x y)
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
(see section Constant Expression Types).
(minus:m x y)
plus but represents subtraction.
(compare:m x y)
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. See section Condition Code Status.
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)
(mult:m x y)
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)
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)
div but represents unsigned division.
(mod:m x y)
(umod:m x y)
div and udiv but represent the remainder instead of
the quotient.
(smin:m x y)
(smax:m x y)
smin) or larger (for smax) of
x and y, interpreted as signed integers in mode m.
(umin:m x y)
(umax:m x y)
smin and smax, but the values are interpreted as unsigned
integers.
(not:m x)
(and:m x y)
(ior:m x y)
(xor:m x y)
(ashift:m x c)
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.
(lshiftrt:m x c)
(ashiftrt:m x c)
ashift but for right shift. Unlike the case for left shift,
these two operations are distinct.
(rotate:m x c)
(rotatert:m x c)
rotate.
(abs:m x)
(sqrt:m x)
(ffs:m x)
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 (see section Miscellaneous Parameters)
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)
(ne:m x y)
(gt:m x y)
(gtu:m x y)
gt but does unsigned comparison, on fixed-point numbers only.
(lt:m x y)
(ltu:m x y)
gt and gtu but test for "less than".
(ge:m x y)
(geu:m x y)
gt and gtu but test for "greater than or equal".
(le:m x y)
(leu:m x y)
gt and gtu but test for "less than or equal".
(if_then_else cond then else)
On most machines, if_then_else expressions are valid only
to express conditional jumps.
(cond [test1 value1 test2 value2 ...] default)
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. See section Instruction Attributes.
Special expression codes exist to represent bitfield 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)
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
(see section Standard Pattern Names For Generation) 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)
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.
All conversions between machine modes must be represented by
explicit conversion operations. For example, an expression
which is the sum of a byte and a full word cannot be written as
(plus:SI (reg:QI 34) (reg:SI 80)) because the plus
operation requires two operands of the same machine mode.
Therefore, the byte-sized operand is enclosed in a conversion
operation, as in
(plus:SI (sign_extend:SI (reg:QI 34)) (reg:SI 80))
The conversion operation is not a mere placeholder, because there may be more than one way of converting from a given starting mode to the desired final mode. The conversion operation code says how to do it.
For all conversion operations, x must not be VOIDmode
because the mode in which to do the conversion would not be known.
The conversion must either be done at compile-time or x
must be placed into a register.
(sign_extend:m x)
(zero_extend:m x)
(float_extend:m x)
(truncate:m x)
(float_truncate:m x)
(float:m x)
(unsigned_float:m x)
(fix:m x)
(unsigned_fix:m x)
(fix:m x)
Declaration expression codes do not represent arithmetic operations but rather state assertions about their operands.
(strict_low_part (subreg:m (reg:n r) 0))
set expression. In addition, the operand of this expression
must be a non-paradoxical subreg expression.
The presence of strict_low_part says that the part of the
register which is meaningful in mode n, but is not part of
mode m, is not to be altered. Normally, an assignment to such
a subreg is allowed to have undefined effects on the rest of the
register when m is less than a word.
The expression codes described so far represent values, not actions. But machine instructions never produce values; they are meaningful only for their side effects on the state of the machine. Special expression codes are used to represent side effects.
The body of an instruction is always one of these side effect codes; the codes described above, which represent values, appear only as the operands of these.
(set lval x)
reg (or
subreg or strict_low_part), mem, pc or
cc0.
If lval is a reg, subreg or mem, it has a
machine mode; then x must be valid for that mode.
If lval is a reg whose machine mode is less than the full
width of the register, then it means that the part of the register
specified by the machine mode is given the specified value and the
rest of the register receives an undefined value. Likewise, if
lval is a subreg whose machine mode is narrower than
the mode of the register, the rest of the register can be changed in
an undefined way.
If lval is a strict_low_part of a subreg, then the
part of the register specified by the machine mode of the
subreg is given the value x and the rest of the register
is not changed.
If lval is (cc0), it has no machine mode, and x may
be either a compare expression or a value that may have any mode.
The latter case represents a "test" instruction. The expression
(set (cc0) (reg:m n)) is equivalent to
(set (cc0) (compare (reg:m n) (const_int 0))).
Use the former expression to save space during the compilation.
If lval is (pc), we have a jump instruction, and the
possibilities for x are very limited. It may be a
label_ref expression (unconditional jump). It may be an
if_then_else (conditional jump), in which case either the
second or the third operand must be (pc) (for the case which
does not jump) and the other of the two must be a label_ref
(for the case which does jump). x may also be a mem or
(plus:SI (pc) y), where y may be a reg or a
mem; these unusual patterns are used to represent jumps through
branch tables.
If lval is neither (cc0) nor (pc), the mode of
lval must not be VOIDmode and the mode of x must be
valid for the mode of lval.
lval is customarily accessed with the SET_DEST macro and
x with the SET_SRC macro.
(return)
return expression code is never used.
Inside an if_then_else expression, represents the value to be
placed in pc to return to the caller.
Note that an insn pattern of (return) is logically equivalent to
(set (pc) (return)), but the latter form is never used.
(call function nargs)
mem expression
whose address is the address of the function to be called.
nargs is an expression which can be used for two purposes: on
some machines it represents the number of bytes of stack argument; on
others, it represents the number of argument registers.
Each machine has a standard machine mode which function must
have. The machine description defines macro FUNCTION_MODE to
expand into the requisite mode name. The purpose of this mode is to
specify what kind of addressing is allowed, on machines where the
allowed kinds of addressing depend on the machine mode being
addressed.
(clobber x)
reg,
scratch or mem expression.
One place this is used is in string instructions that store standard values into particular hard registers. It may not be worth the trouble to describe the values that are stored, but it is essential to inform the compiler that the registers will be altered, lest it attempt to keep data in them across the string instruction.
If x is (mem:BLK (const_int 0)), it means that all memory
locations must be presumed clobbered.
Note that the machine description classifies certain hard registers as
"call-clobbered". All function call instructions are assumed by
default to clobber these registers, so there is no need to use
clobber expressions to indicate this fact. Also, each function
call is assumed to have the potential to alter any memory location,
unless the function is declared const.
If the last group of expressions in a parallel are each a
clobber expression whose arguments are reg or
match_scratch (see section RTL Template) expressions, the combiner
phase can add the appropriate clobber expressions to an insn it
has constructed when doing so will cause a pattern to be matched.
This feature can be used, for example, on a machine that whose multiply and add instructions don't use an MQ register but which has an add-accumulate instruction that does clobber the MQ register. Similarly, a combined instruction might require a temporary register while the constituent instructions might not.
When a clobber expression for a register appears inside a
parallel with other side effects, the register allocator
guarantees that the register is unoccupied both before and after that
insn. However, the reload phase may allocate a register used for one of
the inputs unless the `&' constraint is specified for the selected
alternative (see section Constraint Modifier Characters). You can clobber either a specific hard
register, a pseudo register, or a scratch expression; in the
latter two cases, GNU CC will allocate a hard register that is available
there for use as a temporary.
For instructions that require a temporary register, you should use
scratch instead of a pseudo-register because this will allow the
combiner phase to add the clobber when required. You do this by
coding (clobber (match_scratch ...)). If you do
clobber a pseudo register, use one which appears nowhere else--generate
a new one each time. Otherwise, you may confuse CSE.
There is one other known use for clobbering a pseudo register in a
parallel: when one of the input operands of the insn is also
clobbered by the insn. In this case, using the same pseudo register in
the clobber and elsewhere in the insn produces the expected results.
(use x)
reg expression.
During the delayed branch scheduling phase, x may be an insn.
This indicates that x previously was located at this place in the
code and its data dependencies need to be taken into account. These
use insns will be deleted before the delayed branch scheduling
phase exits.
(parallel [x0 x1 ...])
parallel is a
vector of expressions. x0, x1 and so on are individual
side effect expressions--expressions of code set, call,
return, clobber or use."In parallel" means that first all the values used in the individual side-effects are computed, and second all the actual side-effects are performed. For example,
(parallel [(set (reg:SI 1) (mem:SI (reg:SI 1)))
(set (mem:SI (reg:SI 1)) (reg:SI 1))])
says unambiguously that the values of hard register 1 and the memory
location addressed by it are interchanged. In both places where
(reg:SI 1) appears as a memory address it refers to the value
in register 1 before the execution of the insn.
It follows that it is incorrect to use parallel and
expect the result of one set to be available for the next one.
For example, people sometimes attempt to represent a jump-if-zero
instruction this way:
(parallel [(set (cc0) (reg:SI 34))
(set (pc) (if_then_else
(eq (cc0) (const_int 0))
(label_ref ...)
(pc)))])
But this is incorrect, because it says that the jump condition depends on the condition code value before this instruction, not on the new value that is set by this instruction.
Peephole optimization, which takes place together with final assembly
code output, can produce insns whose patterns consist of a parallel
whose elements are the operands needed to output the resulting
assembler code--often reg, mem or constant expressions.
This would not be well-formed RTL at any other stage in compilation,
but it is ok then because no further optimization remains to be done.
However, the definition of the macro NOTICE_UPDATE_CC, if
any, must deal with such insns if you define any peephole optimizations.
(sequence [insns ...])
insn, jump_insn, call_insn,
code_label, barrier or note.
A sequence RTX is never placed in an actual insn during RTL
generation. It represents the sequence of insns that result from a
define_expand before those insns are passed to
emit_insn to insert them in the chain of insns. When actually
inserted, the individual sub-insns are separated out and the
sequence is forgotten.
After delay-slot scheduling is completed, an insn and all the insns that
reside in its delay slots are grouped together into a sequence.
The insn requiring the delay slot is the first insn in the vector;
subsequent insns are to be placed in the delay slot.
INSN_ANNULLED_BRANCH_P is set on an insn in a delay slot to
indicate that a branch insn should be used that will conditionally annul
the effect of the insns in the delay slots. In such a case,
INSN_FROM_TARGET_P indicates that the insn is from the target of
the branch and should be executed only if the branch is taken; otherwise
the insn should be executed only if the branch is not taken.
See section Delay Slot Scheduling.
These expression codes appear in place of a side effect, as the body of an insn, though strictly speaking they do not always describe side effects as such:
(asm_input s)
(unspec [operands ...] index)
(unspec_volatile [operands ...] index)
unspec_volatile is used for volatile operations and operations
that may trap; unspec is used for other operations.
These codes may appear inside a pattern of an
insn, inside a parallel, or inside an expression.
(addr_vec:m [lr0 lr1 ...])
label_ref expressions. The mode m specifies
how much space is given to each address; normally m would be
Pmode.
(addr_diff_vec:m base [lr0 lr1 ...])
label_ref
expressions and so is base. The mode m specifies how much
space is given to each address-difference.Four special side-effect expression codes appear as memory addresses.
(pre_dec:m x)
reg or mem, but most
machines allow only a reg. m must be the machine mode
for pointers on the machine in use. The amount x is decremented
by is the length in bytes of the machine mode of the containing memory
reference of which this expression serves as the address. Here is an
example of its use:
(mem:DF (pre_dec:SI (reg:SI 39)))
This says to decrement pseudo register 39 by the length of a DFmode
value and use the result to address a DFmode value.
(pre_inc:m x)
(post_dec:m x)
pre_dec but a different
value. The value represented here is the value x has before
being decremented.
(post_inc:m x)
These embedded side effect expressions must be used with care. Instruction patterns may not use them. Until the `flow' pass of the compiler, they may occur only to represent pushes onto the stack. The `flow' pass finds cases where registers are incremented or decremented in one instruction and used as an address shortly before or after; these cases are then transformed to use pre- or post-increment or -decrement.
If a register used as the operand of these expressions is used in another address in an insn, the original value of the register is used. Uses of the register outside of an address are not permitted within the same insn as a use in an embedded side effect expression because such insns behave differently on different machines and hence must be treated as ambiguous and disallowed.
An instruction that can be represented with an embedded side effect
could also be represented using parallel containing an additional
set to describe how the address register is altered. This is not
done because machines that allow these operations at all typically
allow them wherever a memory address is called for. Describing them as
additional parallel stores would require doubling the number of entries
in the machine description.
The RTX code asm_operands represents a value produced by a
user-specified assembler instruction. It is used to represent
an asm statement with arguments. An asm statement with
a single output operand, like this:
asm ("foo %1,%2,%0" : "=a" (outputvar) : "g" (x + y), "di" (*z));
is represented using a single asm_operands RTX which represents
the value that is stored in outputvar:
(set rtx-for-outputvar
(asm_operands "foo %1,%2,%0" "a" 0
[rtx-for-addition-result rtx-for-*z]
[(asm_input:m1 "g")
(asm_input:m2 "di")]))
Here the operands of the asm_operands RTX are the assembler
template string, the output-operand's constraint, the index-number of the
output operand among the output operands specified, a vector of input
operand RTX's, and a vector of input-operand modes and constraints. The
mode m1 is the mode of the sum x+y; m2 is that of
*z.
When an asm statement has multiple output values, its insn has
several such set RTX's inside of a parallel. Each set
contains a asm_operands; all of these share the same assembler
template and vectors, but each contains the constraint for the respective
output operand. They are also distinguished by the output-operand index
number, which is 0, 1, ... for successive output operands.
The RTL representation of the code for a function is a doubly-linked
chain of objects called insns. Insns are expressions with
special codes that are used for no other purpose. Some insns are
actual instructions; others represent dispatch tables for switch
statements; others represent labels to jump to or various sorts of
declarative information.
In addition to its own specific data, each insn must have a unique
id-number that distinguishes it from all other insns in the current
function (after delayed branch scheduling, copies of an insn with the
same id-number may be present in multiple places in a function, but
these copies will always be identical and will only appear inside a
sequence), and chain pointers to the preceding and following
insns. These three fields occupy the same position in every insn,
independent of the expression code of the insn. They could be accessed
with XEXP and XINT, but instead three special macros are
always used:
INSN_UID (i)
PREV_INSN (i)
NEXT_INSN (i)
The first insn in the chain is obtained by calling get_insns; the
last insn is the result of calling get_last_insn. Within the
chain delimited by these insns, the NEXT_INSN and
PREV_INSN pointers must always correspond: if insn is not
the first insn,
NEXT_INSN (PREV_INSN (insn)) == insn
is always true and if insn is not the last insn,
PREV_INSN (NEXT_INSN (insn)) == insn
is always true.
After delay slot scheduling, some of the insns in the chain might be
sequence expressions, which contain a vector of insns. The value
of NEXT_INSN in all but the last of these insns is the next insn
in the vector; the value of NEXT_INSN of the last insn in the vector
is the same as the value of NEXT_INSN for the sequence in
which it is contained. Similar rules apply for PREV_INSN.
This means that the above invariants are not necessarily true for insns
inside sequence expressions. Specifically, if insn is the
first insn in a sequence, NEXT_INSN (PREV_INSN (insn))
is the insn containing the sequence expression, as is the value
of PREV_INSN (NEXT_INSN (insn)) is insn is the last
insn in the sequence expression. You can use these expressions
to find the containing sequence expression.
Every insn has one of the following six expression codes:
insn
insn is used for instructions that do not jump
and do not do function calls. sequence expressions are always
contained in insns with code insn even if one of those insns
should jump or do function calls.
Insns with code insn have four additional fields beyond the three
mandatory ones listed above. These four are described in a table below.
jump_insn
jump_insn is used for instructions that may
jump (or, more generally, may contain label_ref expressions). If
there is an instruction to return from the current function, it is
recorded as a jump_insn.
jump_insn insns have the same extra fields as insn insns,
accessed in the same way and in addition contain a field
JUMP_LABEL which is defined once jump optimization has completed.
For simple conditional and unconditional jumps, this field contains the
code_label to which this insn will (possibly conditionally)
branch. In a more complex jump, JUMP_LABEL records one of the
labels that the insn refers to; the only way to find the others
is to scan the entire body of the insn.
Return insns count as jumps, but since they do not refer to any labels,
they have zero in the JUMP_LABEL field.
call_insn
call_insn is used for instructions that may do
function calls. It is important to distinguish these instructions because
they imply that certain registers and memory locations may be altered
unpredictably.
call_insn insns have the same extra fields as insn insns,
accessed in the same way and in addition contain a field
CALL_INSN_FUNCTION_USAGE, which contains a list (chain of
expr_list expressions) containing use and clobber
expressions that denote hard registers used or clobbered by the called
function. A register specified in a clobber in this list is
modified after the execution of the call_insn, while a
register in a clobber in the body of the call_insn is
clobbered before the insn completes execution. clobber
expressions in this list augment registers specified in
CALL_USED_REGISTERS (see section Basic Characteristics of Registers).
code_label
code_label insn represents a label that a jump insn can jump
to. It contains two special fields of data in addition to the three
standard ones. CODE_LABEL_NUMBER is used to hold the label
number, a number that identifies this label uniquely among all the
labels in the compilation (not just in the current function).
Ultimately, the label is represented in the assembler output as an
assembler label, usually of the form `Ln' where n is
the label number.
When a code_label appears in an RTL expression, it normally
appears within a label_ref which represents the address of
the label, as a number.
The field LABEL_NUSES is only defined once the jump optimization
phase is completed and contains the number of times this label is
referenced in the current function.
barrier
volatile functions, which do not return (e.g., exit).
They contain no information beyond the three standard fields.
note
note insns are used to represent additional debugging and
declarative information. They contain two nonstandard fields, an
integer which is accessed with the macro NOTE_LINE_NUMBER and a
string accessed with NOTE_SOURCE_FILE.
If NOTE_LINE_NUMBER is positive, the note represents the
position of a source line and NOTE_SOURCE_FILE is the source file name
that the line came from. These notes control generation of line
number data in the assembler output.
Otherwise, NOTE_LINE_NUMBER is not really a line number but a
code with one of the following values (and NOTE_SOURCE_FILE
must contain a null pointer):
NOTE_INSN_DELETED
NOTE_INSN_BLOCK_BEG
NOTE_INSN_BLOCK_END
NOTE_INSN_LOOP_BEG
NOTE_INSN_LOOP_END
while or for loop. They enable the loop optimizer
to find loops quickly.
NOTE_INSN_LOOP_CONT
continue statements jump to.
NOTE_INSN_LOOP_VTOP
NOTE_INSN_FUNCTION_END
return statements jump to (on machine where a single instruction
does not suffice for returning). This note may be deleted by jump
optimization.
NOTE_INSN_SETJMP
setjmp or a related function.
These codes are printed symbolically when they appear in debugging dumps.
The machine mode of an insn is normally VOIDmode, but some
phases use the mode for various purposes; for example, the reload pass
sets it to HImode if the insn needs reloading but not register
elimination and QImode if both are required. The common
subexpression elimination pass sets the mode of an insn to QImode
when it is the first insn in a block that has already been processed.
Here is a table of the extra fields of insn, jump_insn
and call_insn insns:
PATTERN (i)
set, call, use,
clobber, return, asm_input, asm_output,
addr_vec, addr_diff_vec, trap_if, unspec,
unspec_volatile, parallel, or sequence. If it is a parallel,
each element of the parallel must be one these codes, except that
parallel expressions cannot be nested and addr_vec and
addr_diff_vec are not permitted inside a parallel expression.
INSN_CODE (i)
Such matching is never attempted and this field remains -1 on an insn
whose pattern consists of a single use, clobber,
asm_input, addr_vec or addr_diff_vec expression.
Matching is also never attempted on insns that result from an asm
statement. These contain at least one asm_operands expression.
The function asm_noperands returns a non-negative value for
such insns.
In the debugging output, this field is printed as a number followed by a symbolic representation that locates the pattern in the `md' file as some small positive or negative offset from a named pattern.
LOG_LINKS (i)
insn_list expressions) giving information about
dependencies between instructions within a basic block. Neither a jump
nor a label may come between the related insns.
REG_NOTES (i)
expr_list and insn_list expressions)
giving miscellaneous information about the insn. It is often information
pertaining to the registers used in this insn.
The LOG_LINKS field of an insn is a chain of insn_list
expressions. Each of these has two operands: the first is an insn,
and the second is another insn_list expression (the next one in
the chain). The last insn_list in the chain has a null pointer
as second operand. The significant thing about the chain is which
insns appear in it (as first operands of insn_list
expressions). Their order is not significant.
This list is originally set up by the flow analysis pass; it is a null
pointer until then. Flow only adds links for those data dependencies
which can be used for instruction combination. For each insn, the flow
analysis pass adds a link to insns which store into registers values
that are used for the first time in this insn. The instruction
scheduling pass adds extra links so that every dependence will be
represented. Links represent data dependencies, antidependencies and
output dependencies; the machine mode of the link distinguishes these
three types: antidependencies have mode REG_DEP_ANTI, output
dependencies have mode REG_DEP_OUTPUT, and data dependencies have
mode VOIDmode.
The REG_NOTES field of an insn is a chain similar to the
LOG_LINKS field but it includes expr_list expressions in
addition to insn_list expressions. There are several kinds
of register notes, which are distinguished by the machine mode, which
in a register note is really understood as being an enum reg_note.
The first operand op of the note is data whose meaning depends on
the kind of note.
The macro REG_NOTE_KIND (x) returns the kind of
register note. Its counterpart, the macro PUT_REG_NOTE_KIND
(x, newkind) sets the register note type of x to be
newkind.
Register notes are of three classes: They may say something about an
input to an insn, they may say something about an output of an insn, or
they may create a linkage between two insns. There are also a set
of values that are only used in LOG_LINKS.
These register notes annotate inputs to an insn:
REG_DEAD
This does not necessarily mean that the register op has no useful
value after this insn since it may also be an output of the insn. In
such a case, however, a REG_DEAD note would be redundant and is
usually not present until after the reload pass, but no code relies on
this fact.
REG_INC
post_inc, pre_inc,
post_dec or pre_dec expression.
REG_NONNEG
The REG_NONNEG note is added to insns only if the machine
description has a `decrement_and_branch_until_zero' pattern.
REG_NO_CONFLICT
Insns with this note are usually part of a block that begins with a
clobber insn specifying a multi-word pseudo register (which will
be the output of the block), a group of insns that each set one word of
the value and have the REG_NO_CONFLICT note attached, and a final
insn that copies the output to itself with an attached REG_EQUAL
note giving the expression being computed. This block is encapsulated
with REG_LIBCALL and REG_RETVAL notes on the first and
last insns, respectively.
REG_LABEL
code_label, but is not a
jump_insn. The presence of this note allows jump optimization to
be aware that op is, in fact, being used.
The following notes describe attributes of outputs of an insn:
REG_EQUIV
REG_EQUAL
set is a strict_low_part expression,
the note refers to the register that is contained in SUBREG_REG
of the subreg expression.
For REG_EQUIV, the register is equivalent to op throughout
the entire function, and could validly be replaced in all its
occurrences by op. ("Validly" here refers to the data flow of
the program; simple replacement may make some insns invalid.) For
example, when a constant is loaded into a register that is never
assigned any other value, this kind of note is used.
When a parameter is copied into a pseudo-register at entry to a function, a note of this kind records that the register is equivalent to the stack slot where the parameter was passed. Although in this case the register may be set by other insns, it is still valid to replace the register by the stack slot throughout the function.
In the case of REG_EQUAL, the register that is set by this insn
will be equal to op at run time at the end of this insn but not
necessarily elsewhere in the function. In this case, op
is typically an arithmetic expression. For example, when a sequence of
insns such as a library call is used to perform an arithmetic operation,
this kind of note is attached to the insn that produces or copies the
final value.
These two notes are used in different ways by the compiler passes.
REG_EQUAL is used by passes prior to register allocation (such as
common subexpression elimination and loop optimization) to tell them how
to think of that value. REG_EQUIV notes are used by register
allocation to indicate that there is an available substitute expression
(either a constant or a mem expression for the location of a
parameter on the stack) that may be used in place of a register if
insufficient registers are available.
Except for stack homes for parameters, which are indicated by a
REG_EQUIV note and are not useful to the early optimization
passes and pseudo registers that are equivalent to a memory location
throughout there entire life, which is not detected until later in
the compilation, all equivalences are initially indicated by an attached
REG_EQUAL note. In the early stages of register allocation, a
REG_EQUAL note is changed into a REG_EQUIV note if
op is a constant and the insn represents the only set of its
destination register.
Thus, compiler passes prior to register allocation need only check for
REG_EQUAL notes and passes subsequent to register allocation
need only check for REG_EQUIV notes.
REG_UNUSED
REG_DEAD note, which
indicates that the value in an input will not be used subsequently.
These two notes are independent; both may be present for the same
register.
REG_WAS_0
note;
its absence implies nothing.
These notes describe linkages between insns. They occur in pairs: one insn has one of a pair of notes that points to a second insn, which has the inverse note pointing back to the first insn.
REG_RETVAL
Loop optimization uses this note to treat such a sequence as a single operation for code motion purposes and flow analysis uses this note to delete such sequences whose results are dead.
A REG_EQUAL note will also usually be attached to this insn to
provide the expression being computed by the sequence.
REG_LIBCALL
REG_RETVAL: it is placed on the first
insn of a multi-insn sequence, and it points to the last one.
REG_CC_SETTER
REG_CC_USER
cc0, the insns which set and use cc0
set and use cc0 are adjacent. However, when branch delay slot
filling is done, this may no longer be true. In this case a
REG_CC_USER note will be placed on the insn setting cc0 to
point to the insn using cc0 and a REG_CC_SETTER note will
be placed on the insn using cc0 to point to the insn setting
cc0.
These values are only used in the LOG_LINKS field, and indicate
the type of dependency that each link represents. Links which indicate
a data dependence (a read after write dependence) do not use any code,
they simply have mode VOIDmode, and are printed without any
descriptive text.
REG_DEP_ANTI
REG_DEP_OUTPUT
For convenience, the machine mode in an insn_list or
expr_list is printed using these symbolic codes in debugging dumps.
The only difference between the expression codes insn_list and
expr_list is that the first operand of an insn_list is
assumed to be an insn and is printed in debugging dumps as the insn's
unique id; the first operand of an expr_list is printed in the
ordinary way as an expression.
Insns that call subroutines have the RTL expression code call_insn.
These insns must satisfy special rules, and their bodies must use a special
RTL expression code, call.
A call expression has two operands, as follows:
(call (mem:fm addr) nbytes)
Here nbytes is an operand that represents the number of bytes of
argument data being passed to the subroutine, fm is a machine mode
(which must equal as the definition of the FUNCTION_MODE macro in
the machine description) and addr represents the address of the
subroutine.
For a subroutine that returns no value, the call expression as
shown above is the entire body of the insn, except that the insn might
also contain use or clobber expressions.
For a subroutine that returns a value whose mode is not BLKmode,
the value is returned in a hard register. If this register's number is
r, then the body of the call insn looks like this:
(set (reg:m r)
(call (mem:fm addr) nbytes))
This RTL expression makes it clear (to the optimizer passes) that the appropriate register receives a useful value in this insn.
When a subroutine returns a BLKmode value, it is handled by
passing to the subroutine the address of a place to store the value.
So the call insn itself does not "return" any value, and it has the
same RTL form as a call that returns nothing.
On some machines, the call instruction itself clobbers some register,
for example to contain the return address. call_insn insns
on these machines should have a body which is a parallel
that contains both the call expression and clobber
expressions that indicate which registers are destroyed. Similarly,
if the call instruction requires some register other than the stack
pointer that is not explicitly mentioned it its RTL, a use
subexpression should mention that register.
Functions that are called are assumed to modify all registers listed in
the configuration macro CALL_USED_REGISTERS (see section Basic Characteristics of Registers) and, with the exception of const functions and library
calls, to modify all of memory.
Insns containing just use expressions directly precede the
call_insn insn to indicate which registers contain inputs to the
function. Similarly, if registers other than those in
CALL_USED_REGISTERS are clobbered by the called function, insns
containing a single clobber follow immediately after the call to
indicate which registers.
The compiler assumes that certain kinds of RTL expressions are unique; there do not exist two distinct objects representing the same value. In other cases, it makes an opposite assumption: that no RTL expression object of a certain kind appears in more than one place in the containing structure.
These assumptions refer to a single function; except for the RTL objects that describe global variables and external functions, and a few standard objects such as small integer constants, no RTL objects are common to two functions.
reg object to represent it,
and therefore only a single machine mode.
symbol_ref object
referring to it.
const_int expression with value 0, only
one with value 1, and only one with value -1.
Some other integer values are also stored uniquely.
pc expression.
cc0 expression.
const_double expression with value 0 for
each floating point mode. Likewise for values 1 and 2.
label_ref or scratch appears in more than one place in
the RTL structure; in other words, it is safe to do a tree-walk of all
the insns in the function and assume that each time a label_ref
or scratch is seen it is distinct from all others that are seen.
mem object is normally created for each static
variable or stack slot, so these objects are frequently shared in all
the places they appear. However, separate but equal objects for these
variables are occasionally made.
asm statement has multiple output operands, a
distinct asm_operands expression is made for each output operand.
However, these all share the vector which contains the sequence of input
operands. This sharing is used later on to test whether two
asm_operands expressions come from the same statement, so all
optimizations must carefully preserve the sharing if they copy the
vector at all.
unshare_all_rtl in `emit-rtl.c',
after which the above rules are guaranteed to be followed.
copy_rtx_if_shared, which is a subroutine of
unshare_all_rtl.
To read an RTL object from a file, call read_rtx. It takes one
argument, a stdio stream, and returns a single RTL object.
Reading RTL from a file is very slow. This is no currently not a problem because reading RTL occurs only as part of building the compiler.
People frequently have the idea of using RTL stored as text in a file as an interface between a language front end and the bulk of GNU CC. This idea is not feasible.
GNU CC was designed to use RTL internally only. Correct RTL for a given program is very dependent on the particular target machine. And the RTL does not contain all the information about the program.
The proper way to interface GNU CC to a new language front end is with the "tree" data structure. There is no manual for this data structure, but it is described in the files `tree.h' and `tree.def'.
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