The simplest RTL expressions are those that represent constant values.
(const_int i) ¶This type of expression represents the integer value i. i
is customarily accessed with the macro INTVAL as in
INTVAL (exp), which is equivalent to XWINT (exp, 0).
Constants generated for modes with fewer bits than in
HOST_WIDE_INT must be sign extended to full width (e.g., with
gen_int_mode). For constants for modes with more bits than in
HOST_WIDE_INT the implied high order bits of that constant are
copies of the top bit. Note however that values are neither
inherently signed nor inherently unsigned; where necessary, signedness
is determined by the rtl operation instead.
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 i0 i1 …) ¶This represents either a floating-point constant of mode m or
(on older ports that do not define
TARGET_SUPPORTS_WIDE_INT) an integer constant too large to fit
into HOST_BITS_PER_WIDE_INT bits but small enough to fit within
twice that number of bits. In the latter case, m will be
VOIDmode. For integral values constants for modes with more
bits than twice the number in HOST_WIDE_INT the implied high
order bits of that constant are copies of the top bit of
CONST_DOUBLE_HIGH. Note however that integral values are
neither inherently signed nor inherently unsigned; where necessary,
signedness is determined by the rtl operation instead.
On more modern ports, CONST_DOUBLE only represents floating
point values. New ports define TARGET_SUPPORTS_WIDE_INT to
make this designation.
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 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 Output of Data).
The host dependency for the number of integers used to store a double
value makes it problematic for machine descriptions to use expressions
of code const_double and therefore a syntactic alias has been
provided:
(const_double_zero:m)
standing for:
(const_double:m 0 0 …)
for matching the floating-point value zero, possibly the only useful one.
(const_wide_int:m nunits elt0 …) ¶This contains an array of HOST_WIDE_INTs that is large enough
to hold any constant that can be represented on the target. This form
of rtl is only used on targets that define
TARGET_SUPPORTS_WIDE_INT to be nonzero and then
CONST_DOUBLEs are only used to hold floating-point values. If
the target leaves TARGET_SUPPORTS_WIDE_INT defined as 0,
CONST_WIDE_INTs are not used and CONST_DOUBLEs are as
they were before.
The values are stored in a compressed format. The higher-order 0s or -1s are not represented if they are just the logical sign extension of the number that is represented.
CONST_WIDE_INT_VEC (code) ¶Returns the entire array of HOST_WIDE_INTs that are used to
store the value. This macro should be rarely used.
CONST_WIDE_INT_NUNITS (code) ¶The number of HOST_WIDE_INTs used to represent the number.
Note that this generally is smaller than the number of
HOST_WIDE_INTs implied by the mode size.
CONST_WIDE_INT_ELT (code,i) ¶Returns the ith element of the array. Element 0 is contains
the low order bits of the constant.
(const_fixed:m …) ¶Represents a fixed-point constant of mode m.
The operand is a data structure of type struct fixed_value and
is accessed with the macro CONST_FIXED_VALUE. The high part of
data is accessed with CONST_FIXED_VALUE_HIGH; the low part is
accessed with CONST_FIXED_VALUE_LOW.
(const_poly_int:m [c0 c1 …]) ¶Represents a poly_int-style polynomial integer with coefficients
c0, c1, …. The coefficients are wide_int-based
integers rather than rtxes. CONST_POLY_INT_COEFFS gives the
values of individual coefficients (which is mostly only useful in
low-level routines) and const_poly_int_value gives the full
poly_int value.
(const_vector:m [x0 x1 …]) ¶Represents a vector constant. The values in square brackets are
elements of the vector, which are always const_int,
const_wide_int, const_double or const_fixed
expressions.
Each vector constant v is treated as a specific instance of an arbitrary-length sequence that itself contains ‘CONST_VECTOR_NPATTERNS (v)’ interleaved patterns. Each pattern has the form:
{ base0, base1, base1 + step, base1 + step * 2, … }
The first three elements in each pattern are enough to determine the values of the other elements. However, if all steps are zero, only the first two elements are needed. If in addition each base1 is equal to the corresponding base0, only the first element in each pattern is needed. The number of determining elements per pattern is given by ‘CONST_VECTOR_NELTS_PER_PATTERN (v)’.
For example, the constant:
{ 0, 1, 2, 6, 3, 8, 4, 10, 5, 12, 6, 14, 7, 16, 8, 18 }
is interpreted as an interleaving of the sequences:
{ 0, 2, 3, 4, 5, 6, 7, 8 }
{ 1, 6, 8, 10, 12, 14, 16, 18 }
where the sequences are represented by the following patterns:
base0 == 0, base1 == 2, step == 1 base0 == 1, base1 == 6, step == 2
In this case:
CONST_VECTOR_NPATTERNS (v) == 2 CONST_VECTOR_NELTS_PER_PATTERN (v) == 3
Thus the first 6 elements (‘{ 0, 1, 2, 6, 3, 8 }’) are enough
to determine the whole sequence; we refer to them as the “encoded”
elements. They are the only elements present in the square brackets
for variable-length const_vectors (i.e. for
const_vectors whose mode m has a variable number of
elements). However, as a convenience to code that needs to handle
both const_vectors and parallels, all elements are
present in the square brackets for fixed-length const_vectors;
the encoding scheme simply reduces the amount of work involved in
processing constants that follow a regular pattern.
Sometimes this scheme can create two possible encodings of the same vector. For example { 0, 1 } could be seen as two patterns with one element each or one pattern with two elements (base0 and base1). The canonical encoding is always the one with the fewest patterns or (if both encodings have the same number of patterns) the one with the fewest encoded elements.
‘const_vector_encoding_nelts (v)’ gives the total number of
encoded elements in v, which is 6 in the example above.
CONST_VECTOR_ENCODED_ELT (v, i) accesses the value
of encoded element i.
‘CONST_VECTOR_DUPLICATE_P (v)’ is true if v simply contains repeated instances of ‘CONST_VECTOR_NPATTERNS (v)’ values. This is a shorthand for testing ‘CONST_VECTOR_NELTS_PER_PATTERN (v) == 1’.
‘CONST_VECTOR_STEPPED_P (v)’ is true if at least one pattern in v has a nonzero step. This is a shorthand for testing ‘CONST_VECTOR_NELTS_PER_PATTERN (v) == 3’.
CONST_VECTOR_NUNITS (v) gives the total number of elements
in v; it is a shorthand for getting the number of units in
‘GET_MODE (v)’.
The utility function const_vector_elt gives the value of an
arbitrary element as an rtx. const_vector_int_elt gives
the same value as a wide_int.
(const_string str) ¶Represents a constant string with value str. Currently this is used only for insn attributes (see Instruction Attributes) since constant strings in C are placed in memory.
(symbol_ref:mode symbol) ¶Represents the value of an assembler label for data. symbol is a string that describes the name of the assembler label. If it starts with a ‘*’, the label is the rest of symbol not including the ‘*’. Otherwise, the label is symbol, usually prefixed with ‘_’.
The symbol_ref contains a mode, which is usually Pmode.
Usually that is the only mode for which a symbol is directly valid.
(label_ref:mode label) ¶Represents the value of an assembler label for code. It contains one
operand, an expression, which must be a code_label or a note
of type NOTE_INSN_DELETED_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.
The label_ref contains a mode, which is usually Pmode.
Usually that is the only mode for which a label is directly valid.
(const:m exp) ¶Represents a constant that is the result of an assembly-time
arithmetic computation. The operand, exp, contains only
const_int, symbol_ref, label_ref or unspec
expressions, combined with plus and minus. Any such
unspecs are target-specific and typically represent some form
of relocation operator. m should be a valid address mode.
(high:m exp) ¶Represents the high-order bits of exp.
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 large immediate values and/or link-time constants such
as global memory addresses. In the latter case, m is Pmode
and exp is usually a constant expression involving symbol_ref.
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. If mode mode is of
mode class MODE_FLOAT, it returns a CONST_DOUBLE
expression in mode mode. Otherwise, it returns a
CONST_VECTOR 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. The
CONST1_RTX and CONST2_RTX macros are undefined
for vector modes.