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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
An integer is simply an
int; their written form uses decimal digits.
A wide integer is an integral object whose type is
(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
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:
In a few contexts a null pointer is valid where an expression is normally
wanted. The written form of this is
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
A few other format characters are used occasionally:
There are macros to get the number of operands, the format, and the class of an expression code:
subregis not in this class.
EQ(which have class `<').
Operands of expressions are accessed using the macros
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)
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
Access to operands which are vectors is more complicated. You can use the
XVEC to get the vector-pointer itself, or the macros
XVECLEN to access the elements and length of a
XVEC (exp, idx)
XVECLEN (exp, idx)
XVECEXP (exp, idx, eltnum)
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) and other values that are used in certain types of expression. Most often they are accessed with the following macros:
memexpressions, nonzero for volatile memory references. Stored in the
volatilfield and printed as `/v'.
memexpressions, 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_structfield and printed as `/s'.
memexpressions, the alias set to which x belongs. If zero, x is not in any alias set, and may alias anything. If nonzero, x may only alias objects in the same alias set. This value is set (in a language-specific manner) by the front-end. This field is not a bit-field; it is in an integer, found as the second argument to the
regexpressions, nonzero if this register's entire life is contained in the exit test code for some loop. Stored in the
in_structfield and printed as `/s'.
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
volatilfield and printed as `/v'.
regif 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
integratedfield 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_Pis zero in this kind of use.
subregif it was made when accessing an object that was promoted to a wider mode in accord with the
PROMOTED_MODEmachine description macro (see section Storage Layout). In this case, the mode of the
subregis the declared mode of the object and the mode of
SUBREG_REGis 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_structfield and printed as `/s'.
SUBREG_PROMOTED_VAR_Pnonzero if the object being referenced is kept zero-extended and zero if it is kept sign-extended. Stored in the
unchangingfield and printed as `/u'.
memif 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
unchangingfield and printed as `/u'.
integratedfield and printed as `/i'.
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
symbol_ref, this is used as a flag for machine-specific purposes. Stored in the
volatilfield and printed as `/v'.
label_refexpressions, 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_structfield and printed as `/s'.
volatilfield and printed as `/v'.
insnin the delay slot of a branch insn, indicates that an annulling branch should be used. See the discussion under
sequencebelow. Stored in the
unchangingfield and printed as `/u'.
insnin 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_Pset, this insn will only be executed if the branch is taken. For annulled branches with
INSN_FROM_TARGET_Pclear, the insn will be executed only if the branch is not taken. When
INSN_ANNULLED_BRANCH_Pis not set, this insn will always be executed. Stored in the
in_structfield and printed as `/s'.
symbol_refif 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
unchangingfield and printed as `/u'.
call_insn, indicates that the insn represents a call to a const function. Stored in the
unchangingfield and printed as `/u'.
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_structfield and printed as `/s'.
useinsns before a
call_insnmay not be separated from the
call_insn. Stored in the
in_structfield and printed as `/s'.
These are the fields which the above macros refer to:
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.
regexpressions and in insns. In RTL dump files, it is printed as `/v'. In a
memexpression, it is 1 if the memory reference is volatile. Volatile memory references may not be deleted, reordered or combined. In a
symbol_refexpression, it is used for machine-specific purposes. In a
regexpression, 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.
memexpressions, 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
regexpressions, it is 1 if the register has its entire life contained within the test expression of some loop. In
subregexpressions, 1 means that the
subregis accessing an object that has had its mode promoted from a wider mode. In
label_refexpressions, 1 means that the referenced label is outside the innermost loop containing the insn in which the
label_refwas found. In
code_labelexpressions, 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'.
memexpressions, 1 means that the value of the expression never changes. In
subregexpressions, it is 1 if the
subregreferences 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_refexpression, 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'.
regexpression, 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
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
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
BITS_PER_UNIT bits (see section Storage Layout).
cc0(see see section Condition Code Status).
BLKmodewill not appear in RTL.
VOIDmodebecause they can be taken to have whatever mode the context requires. In debugging dumps of RTL,
VOIDmodeis expressed by the absence of any mode.
SCmode, DCmode, XCmode, TCmode
CQImode, CHImode, CSImode, CDImode, CTImode, COImode
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
SImode on 32-bit machines.
The only modes which a machine description must support are
QImode, and the modes corresponding to
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
TImode for 16-byte structures and unions. Likewise, you can
arrange for the C type
short int to avoid using
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
enum mode_class defined in `machmode.h'. The possible
mode classes are:
CCmodeplus any modes listed in the
EXTRA_CC_MODESmacro. See section Defining Jump Instruction Patterns, also see section Condition Code Status.
Here are some C macros that relate to machine modes:
PUT_MODE (x, newmode)
GET_MODE_SIZEexcept in the case of complex modes. For them, the unit size is the size of the real or imaginary part.
The global variables
word_mode contain modes
whose classes are
MODE_INT and whose bitsizes are either
BITS_PER_WORD, respectively. On 32-bit
machines, these are
The simplest RTL expressions are those that represent constant values.
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_intand value zero, one, two or negative one will return
constm1_rtxas appropriate. Similarly, there is only one object for the integer whose value is
STORE_FLAG_VALUE. It is found in
const1_rtxwill point to the same object. If
constm1_rtxwill point to the same object.
(const_double:m addr i0 i1 ...)
HOST_BITS_PER_WIDE_INTbits 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
memexpression 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_doubleexpressions 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_MEMand 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_LOWand 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_DOUBLEand 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_DOUBLEexpression in mode mode. Similarly, the macro
CONST1_RTX (mode)refers to an expression with value 1 in mode mode and similarly for
symbol_refcontains a mode, which is usually
Pmode. Usually that is the only mode for which a symbol is directly valid.
code_labelthat 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.
label_refexpressions) combined with
minus. However, not all combinations are valid, since the assembler cannot do arbitrary arithmetic on relocatable symbols. m should be
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_sumto represent the typical two-instruction sequence used in RISC machines to reference a global memory location. m should be
Here are the RTL expression types for describing access to machine registers and to main memory.
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_REGISTERis 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
subregexpression is used. A
regexpression 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
regexpression. Some pseudo register numbers, those within the range of
LAST_VIRTUAL_REGISTERonly 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:
ARG_POINTER_REGNUMand the value of
FRAME_GROWS_DOWNWARDis 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_REGNUMis replaced with the sum of the register given by
FRAME_POINTER_REGNUMand the value
STACK_POINTER_REGNUMand the value
STACK_POINTER_REGNUM). This virtual register is replaced by the sum of the register given by
STACK_POINTER_REGNUMand the value
(subreg:m reg wordnum)
subregexpressions 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
regthat 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
subregexpressions 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
subregis to extract the individual registers of a multi-register value. Machine modes such as
TImodecan indicate values longer than a word, values which usually require two or more consecutive registers. To access one of the registers, use a
SImodeand a wordnum that says which register. Storing in a non-paradoxical
subreghas 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. 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. On a few targets,
WORDS_BIG_ENDIAN. However, most parts of the compiler treat floating point values as if they had the same endianness as integer values. This works because they handle them solely as a collection of integer values, with no particular numerical value. Only real.c and the runtime libraries care about
FLOAT_WORDS_BIG_ENDIAN. Between the combiner pass and the reload pass, it is possible to have a paradoxical
subregwhich contains a
meminstead of a
regas its first operand. After the reload pass, it is also possible to have a non-paradoxical
subregwhich contains a
mem; this usually occurs when the
memis a stack slot which replaced a pseudo register. Note that it is not valid to access a
subreg. On some machines the most significant part of a
DFmodevalue 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
DFmodevalue. 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
subregexpressions such as these from being formed. The first operand of a
subregexpression is customarily accessed with the
SUBREG_REGmacro and the second operand is customarily accessed with the
regby either the local register allocator or the reload pass.
scratchis usually present inside a
clobberoperation (see section Side Effect Expressions).
(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_intwith value zero; that is to say,
(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).
cc0; it is the value of the variable
cc0_rtx. Any attempt to create an expression of code
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
regis 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_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.
Pmode. If there are any
addressofexpressions left in the function after CSE, reg is forced into the stack and the
addressofexpression is replaced with a
plusexpression for the address of its stack slot.
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_double and m is a mode of class
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
Pmodeitem minus the number of bits set by the
highcode (see section Constant Expression Types). m should be
(minus:m x y)
plusbut represents subtraction.
(compare:m x y)
(cc0)is used, it is
VOIDmode. Otherwise it is some mode in class
CCmode. See section Condition Code Status. Normally, x and y must have the same mode. Otherwise,
compareis valid only if the mode of x is in class
MODE_INTand y is a
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
VOIDmodeconstants 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.
(mult:m x y)
(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
(div:m x y)
(truncate:m1 (div:m2 x (sign_extend:m2 y)))
(udiv:m x y)
divbut represents unsigned division.
(mod:m x y)
(umod:m x y)
udivbut 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)
smax, but the values are interpreted as unsigned integers.
(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
QImoderegardless of m.
(lshiftrt:m x c)
(ashiftrt:m x c)
ashiftbut for right shift. Unlike the case for left shift, these two operations are distinct.
(rotate:m x c)
(rotatert:m x c)
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
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
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
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
(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
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
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)
gtbut does unsigned comparison, on fixed-point numbers only.
(lt:m x y)
(ltu:m x y)
gtubut test for "less than".
(ge:m x y)
(geu:m x y)
gtubut test for "greater than or equal".
(le:m x y)
(leu:m x y)
gtubut test for "less than or equal".
(if_then_else cond then else)
if_then_elseexpressions 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_ENDIANsays 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
extvpattern (see section Standard Pattern Names For Generation) and is usually a full-word integer mode, which is the default if none is specified. The mode of pos is machine-specific and is also specified in the
extvpattern. 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_extractbut 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
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
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.
Declaration expression codes do not represent arithmetic operations but rather state assertions about their operands.
(strict_low_part (subreg:m (reg:n r) 0))
setexpression. In addition, the operand of this expression must be a non-paradoxical
subregexpression. The presence of
strict_low_partsays 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)
cc0. If lval is a
mem, it has a machine mode; then x must be valid for that mode. If lval is a
regwhose 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
subregwhose 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
subreg, then the part of the register specified by the machine mode of the
subregis 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
compareexpression 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_refexpression (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
(plus:SI (pc) y), where y may be a
mem; these unusual patterns are used to represent jumps through branch tables. If lval is neither
(pc), the mode of lval must not be
VOIDmodeand the mode of x must be valid for the mode of lval. lval is customarily accessed with the
SET_DESTmacro and x with the
returnexpression code is never used. Inside an
if_then_elseexpression, represents the value to be placed in
pcto 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)
memexpression 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_MODEto 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.
memexpression. 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
clobberexpressions 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
parallelare each a
clobberexpression whose arguments are
match_scratch(see section RTL Template) expressions, the combiner phase can add the appropriate
clobberexpressions 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
clobberexpression for a register appears inside a
parallelwith 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
scratchexpression; 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
scratchinstead of a pseudo-register because this will allow the combiner phase to add the
clobberwhen required. You do this by coding (
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.
regexpression. During the reload phase, an insn that has a
useas pattern can carry a reg_equal note. These
useinsns will be deleted before the reload phase exits. 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
useinsns will be deleted before the delayed branch scheduling phase exits.
(parallel [x0 x1 ...])
parallelis a vector of expressions. x0, x1 and so on are individual side effect expressions--expressions of code
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
paralleland expect the result of one
setto 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
parallelwhose elements are the operands needed to output the resulting assembler code--often
memor 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 ...])
sequenceRTX is never placed in an actual insn during RTL generation. It represents the sequence of insns that result from a
define_expandbefore those insns are passed to
emit_insnto insert them in the chain of insns. When actually inserted, the individual sub-insns are separated out and the
sequenceis 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_Pis 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_Pindicates 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:
(unspec [operands ...] index)
(unspec_volatile [operands ...] index)
unspec_volatileis used for volatile operations and operations that may trap;
unspecis used for other operations. These codes may appear inside a
patternof an insn, inside a
parallel, or inside an expression.
(addr_vec:m [lr0 lr1 ...])
label_refexpressions. The mode m specifies how much space is given to each address; normally m would be
(addr_diff_vec:m base [lr0 lr1 ...] min max flags)
label_refexpressions and so is base. The mode m specifies how much space is given to each address-difference. min and max are set up by branch shortening and hold a label with a minimum and a maximum address, respectively. flags indicates the relative position of base, min and max to the cointaining insn and of min and max to base. See rtl.def for details.
Four special side-effect expression codes appear as memory addresses.
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
DFmodevalue and use the result to address a
pre_decbut a different value. The value represented here is the value x has before being decremented.
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
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
(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
asm statement has multiple output values, its insn has
set RTX's inside of 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
statements; others represent labels to jump to or various sorts of
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
XINT, but instead three special macros are
The first insn in the chain is obtained by calling
last insn is the result of calling
get_last_insn. Within the
chain delimited by these insns, the
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
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
which it is contained. Similar rules apply for
This means that the above invariants are not necessarily true for insns
sequence expressions. Specifically, if insn is the
first insn in a
NEXT_INSN (PREV_INSN (insn))
is the insn containing the
sequence expression, as is the value
PREV_INSN (NEXT_INSN (insn)) is insn is the last
insn in the
sequence expression. You can use these expressions
to find the containing
Every insn has one of the following six expression codes:
insnis used for instructions that do not jump and do not do function calls.
sequenceexpressions are always contained in insns with code
insneven if one of those insns should jump or do function calls. Insns with code
insnhave four additional fields beyond the three mandatory ones listed above. These four are described in a table below.
jump_insnis used for instructions that may jump (or, more generally, may contain
label_refexpressions). If there is an instruction to return from the current function, it is recorded as a
jump_insninsns have the same extra fields as
insninsns, accessed in the same way and in addition contain a field
JUMP_LABELwhich is defined once jump optimization has completed. For simple conditional and unconditional jumps, this field contains the
code_labelto which this insn will (possibly conditionally) branch. In a more complex jump,
JUMP_LABELrecords 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
call_insnis 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_insninsns have the same extra fields as
insninsns, accessed in the same way and in addition contain a field
CALL_INSN_FUNCTION_USAGE, which contains a list (chain of
clobberexpressions that denote hard registers used or clobbered by the called function. A register specified in a
clobberin this list is modified after the execution of the
call_insn, while a register in a
clobberin the body of the
call_insnis clobbered before the insn completes execution.
clobberexpressions in this list augment registers specified in
CALL_USED_REGISTERS(see section Basic Characteristics of Registers).
code_labelinsn 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_NUMBERis 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_labelappears in an RTL expression, it normally appears within a
label_refwhich represents the address of the label, as a number. The field
LABEL_NUSESis only defined once the jump optimization phase is completed and contains the number of times this label is referenced in the current function.
volatilefunctions, which do not return (e.g.,
exit). They contain no information beyond the three standard fields.
noteinsns are used to represent additional debugging and declarative information. They contain two nonstandard fields, an integer which is accessed with the macro
NOTE_LINE_NUMBERand a string accessed with
NOTE_LINE_NUMBERis positive, the note represents the position of a source line and
NOTE_SOURCE_FILEis the source file name that the line came from. These notes control generation of line number data in the assembler output. Otherwise,
NOTE_LINE_NUMBERis not really a line number but a code with one of the following values (and
NOTE_SOURCE_FILEmust contain a null pointer):
CODE_LABELis associated with the given region.
forloop. They enable the loop optimizer to find loops quickly.
continuestatements jump to.
returnstatements jump to (on machine where a single instruction does not suffice for returning). This note may be deleted by jump optimization.
setjmpor a related function.
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
QImode if both are required. The common
subexpression elimination pass sets the mode of an insn to
when it is the first insn in a block that has already been processed.
Here is a table of the extra fields of
sequence. If it is a
parallel, each element of the
parallelmust be one these codes, except that
parallelexpressions cannot be nested and
addr_diff_vecare not permitted inside a
addr_diff_vecexpression. Matching is also never attempted on insns that result from an
asmstatement. These contain at least one
asm_operandsexpression. The function
asm_noperandsreturns 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.
insn_listexpressions) giving information about dependencies between instructions within a basic block. Neither a jump nor a label may come between the related insns.
insn_listexpressions) giving miscellaneous information about the insn. It is often information pertaining to the registers used in this insn.
LOG_LINKS field of an insn is a chain of
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
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
dependencies have mode
REG_DEP_OUTPUT, and data dependencies have
REG_NOTES field of an insn is a chain similar to the
LOG_LINKS field but it includes
expr_list expressions in
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
The first operand op of the note is data whose meaning depends on
the kind of note.
REG_NOTE_KIND (x) returns the kind of
register note. Its counterpart, the macro
(x, newkind) sets the register note type of x to be
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
These register notes annotate inputs to an insn:
REG_DEADnote would be redundant and is usually not present until after the reload pass, but no code relies on this fact.
REG_NONNEGnote is added to insns only if the machine description has a `decrement_and_branch_until_zero' pattern.
clobberinsn 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_CONFLICTnote attached, and a final insn that copies the output to itself with an attached
REG_EQUALnote giving the expression being computed. This block is encapsulated with
REG_RETVALnotes on the first and last insns, respectively.
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:
strict_low_partexpression, the note refers to the register that is contained in
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. A
REG_EQUIVnote is also used on an instruction which copies a register parameter into a pseudo-register at entry to a function, if there is a stack slot where that parameter could be stored. Although other insns may set the pseudo-register, it is valid for the compiler to replace the pseudo-register by stack slot throughout the function, provided the compiler ensures that the stack slot is properly initialized by making the replacement in the initial copy instruction as well. This is used on machines for which the calling convention allocates stack space for register parameters. See
REG_PARM_STACK_SPACEin section Passing Function Arguments on the Stack. 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_EQUALis 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_EQUIVnotes are used by register allocation to indicate that there is an available substitute expression (either a constant or a
memexpression 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_EQUIVnote 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_EQUALnote. In the early stages of register allocation, a
REG_EQUALnote is changed into a
REG_EQUIVnote 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_EQUALnotes and passes subsequent to register allocation need only check for
REG_DEADnote, 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.
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_EQUALnote will also usually be attached to this insn to provide the expression being computed by the sequence.
REG_RETVAL: it is placed on the first insn of a multi-insn sequence, and it points to the last one.
cc0, the insns which set and use
cc0set and use
cc0are adjacent. However, when branch delay slot filling is done, this may no longer be true. In this case a
REG_CC_USERnote will be placed on the insn setting
cc0to point to the insn using
REG_CC_SETTERnote will be placed on the insn using
cc0to point to the insn setting
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
These notes describe information gathered from gcov profile data. They
are stored in the
REG_NOTES field of an insn as an
For convenience, the machine mode in an
expr_list is printed using these symbolic codes in debugging dumps.
The only difference between the expression codes
expr_list is that the first operand of an
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
These insns must satisfy special rules, and their bodies must use a special
RTL expression code,
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
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
For a subroutine that returns a value whose mode is not
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.
on these machines should have a body which is a
that contains both the
call expression and
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
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.
regobject to represent it, and therefore only a single machine mode.
symbol_refobject referring to it.
const_intexpression with value 0, only one with value 1, and only one with value -1. Some other integer values are also stored uniquely.
const_doubleexpression with value 0 for each floating point mode. Likewise for values 1 and 2.
scratchappears 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
scratchis seen it is distinct from all others that are seen.
memobject 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.
asmstatement has multiple output operands, a distinct
asm_operandsexpression 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_operandsexpressions come from the same statement, so all optimizations must carefully preserve the sharing if they copy the vector at all.
unshare_all_rtlin `emit-rtl.c', after which the above rules are guaranteed to be followed.
copy_rtx_if_shared, which is a subroutine of
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 not currently a problem since 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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