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<h3 class="section">13.7 Constant Expression Types</h3>
<p><a name="index-RTL-constants-2820"></a><a name="index-RTL-constant-expression-types-2821"></a>
The simplest RTL expressions are those that represent constant values.
<a name="index-const_005fint-2822"></a>
<dl><dt><code>(const_int </code><var>i</var><code>)</code><dd>This type of expression represents the integer value <var>i</var>. <var>i</var>
is customarily accessed with the macro <code>INTVAL</code> as in
<code>INTVAL (</code><var>exp</var><code>)</code>, which is equivalent to <code>XWINT (</code><var>exp</var><code>, 0)</code>.
<p>Constants generated for modes with fewer bits than in
<code>HOST_WIDE_INT</code> must be sign extended to full width (e.g., with
<code>gen_int_mode</code>). For constants for modes with more bits than in
<code>HOST_WIDE_INT</code> 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.
<p><a name="index-const0_005frtx-2823"></a><a name="index-const1_005frtx-2824"></a><a name="index-const2_005frtx-2825"></a><a name="index-constm1_005frtx-2826"></a>There is only one expression object for the integer value zero; it is
the value of the variable <code>const0_rtx</code>. Likewise, the only
expression for integer value one is found in <code>const1_rtx</code>, the only
expression for integer value two is found in <code>const2_rtx</code>, and the
only expression for integer value negative one is found in
<code>constm1_rtx</code>. Any attempt to create an expression of code
<code>const_int</code> and value zero, one, two or negative one will return
<code>const0_rtx</code>, <code>const1_rtx</code>, <code>const2_rtx</code> or
<code>constm1_rtx</code> as appropriate.
<p><a name="index-const_005ftrue_005frtx-2827"></a>Similarly, there is only one object for the integer whose value is
<code>STORE_FLAG_VALUE</code>. It is found in <code>const_true_rtx</code>. If
<code>STORE_FLAG_VALUE</code> is one, <code>const_true_rtx</code> and
<code>const1_rtx</code> will point to the same object. If
<code>STORE_FLAG_VALUE</code> is −1, <code>const_true_rtx</code> and
<code>constm1_rtx</code> will point to the same object.
<p><a name="index-const_005fdouble-2828"></a><br><dt><code>(const_double:</code><var>m</var> <var>i0</var> <var>i1</var><code> ...)</code><dd>This represents either a floating-point constant of mode <var>m</var> or
(on older ports that do not define
<code>TARGET_SUPPORTS_WIDE_INT</code>) an integer constant too large to fit
into <code>HOST_BITS_PER_WIDE_INT</code> bits but small enough to fit within
twice that number of bits. In the latter case, <var>m</var> will be
<code>VOIDmode</code>. For integral values constants for modes with more
bits than twice the number in <code>HOST_WIDE_INT</code> the implied high
order bits of that constant are copies of the top bit of
<code>CONST_DOUBLE_HIGH</code>. Note however that integral values are
neither inherently signed nor inherently unsigned; where necessary,
signedness is determined by the rtl operation instead.
<p>On more modern ports, <code>CONST_DOUBLE</code> only represents floating
point values. New ports define <code>TARGET_SUPPORTS_WIDE_INT</code> to
make this designation.
<p><a name="index-CONST_005fDOUBLE_005fLOW-2829"></a>If <var>m</var> is <code>VOIDmode</code>, the bits of the value are stored in
<var>i0</var> and <var>i1</var>. <var>i0</var> is customarily accessed with the macro
<code>CONST_DOUBLE_LOW</code> and <var>i1</var> with <code>CONST_DOUBLE_HIGH</code>.
<p>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
<code>REAL_VALUE_TYPE</code> (see <a href="Floating-Point.html#Floating-Point">Floating Point</a>). 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
<code>REAL_VALUE_TO_TARGET_DOUBLE</code> and friends (see <a href="Data-Output.html#Data-Output">Data Output</a>).
<p><a name="index-CONST_005fWIDE_005fINT-2830"></a><br><dt><code>(const_wide_int:</code><var>m</var> <var>nunits</var> <var>elt0</var><code> ...)</code><dd>This contains an array of <code>HOST_WIDE_INT</code>s 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
<code>TARGET_SUPPORTS_WIDE_INT</code> to be nonzero and then
<code>CONST_DOUBLE</code>s are only used to hold floating-point values. If
the target leaves <code>TARGET_SUPPORTS_WIDE_INT</code> defined as 0,
<code>CONST_WIDE_INT</code>s are not used and <code>CONST_DOUBLE</code>s are as
they were before.
<p>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.
<p><a name="index-CONST_005fWIDE_005fINT_005fVEC-2831"></a><br><dt><code>CONST_WIDE_INT_VEC (</code><var>code</var><code>)</code><dd>Returns the entire array of <code>HOST_WIDE_INT</code>s that are used to
store the value. This macro should be rarely used.
<p><a name="index-CONST_005fWIDE_005fINT_005fNUNITS-2832"></a><br><dt><code>CONST_WIDE_INT_NUNITS (</code><var>code</var><code>)</code><dd>The number of <code>HOST_WIDE_INT</code>s used to represent the number.
Note that this generally is smaller than the number of
<code>HOST_WIDE_INT</code>s implied by the mode size.
<p><a name="index-CONST_005fWIDE_005fINT_005fELT-2833"></a><br><dt><code>CONST_WIDE_INT_NUNITS (</code><var>code</var><code>,</code><var>i</var><code>)</code><dd>Returns the <code>i</code>th element of the array. Element 0 is contains
the low order bits of the constant.
<p><a name="index-const_005ffixed-2834"></a><br><dt><code>(const_fixed:</code><var>m</var><code> ...)</code><dd>Represents a fixed-point constant of mode <var>m</var>.
The operand is a data structure of type <code>struct fixed_value</code> and
is accessed with the macro <code>CONST_FIXED_VALUE</code>. The high part of
data is accessed with <code>CONST_FIXED_VALUE_HIGH</code>; the low part is
accessed with <code>CONST_FIXED_VALUE_LOW</code>.
<p><a name="index-const_005fvector-2835"></a><br><dt><code>(const_vector:</code><var>m</var><code> [</code><var>x0</var> <var>x1</var><code> ...])</code><dd>Represents a vector constant. The square brackets stand for the vector
containing the constant elements. <var>x0</var>, <var>x1</var> and so on are
the <code>const_int</code>, <code>const_double</code> or <code>const_fixed</code> elements.
<p>The number of units in a <code>const_vector</code> is obtained with the macro
<code>CONST_VECTOR_NUNITS</code> as in <code>CONST_VECTOR_NUNITS (</code><var>v</var><code>)</code>.
<p>Individual elements in a vector constant are accessed with the macro
<code>CONST_VECTOR_ELT</code> as in <code>CONST_VECTOR_ELT (</code><var>v</var><code>, </code><var>n</var><code>)</code>
where <var>v</var> is the vector constant and <var>n</var> is the element
desired.
<p><a name="index-const_005fstring-2836"></a><br><dt><code>(const_string </code><var>str</var><code>)</code><dd>Represents a constant string with value <var>str</var>. Currently this is
used only for insn attributes (see <a href="Insn-Attributes.html#Insn-Attributes">Insn Attributes</a>) since constant
strings in C are placed in memory.
<p><a name="index-symbol_005fref-2837"></a><br><dt><code>(symbol_ref:</code><var>mode</var> <var>symbol</var><code>)</code><dd>Represents the value of an assembler label for data. <var>symbol</var> is
a string that describes the name of the assembler label. If it starts
with a ‘<samp><span class="samp">*</span></samp>’, the label is the rest of <var>symbol</var> not including
the ‘<samp><span class="samp">*</span></samp>’. Otherwise, the label is <var>symbol</var>, usually prefixed
with ‘<samp><span class="samp">_</span></samp>’.
<p>The <code>symbol_ref</code> contains a mode, which is usually <code>Pmode</code>.
Usually that is the only mode for which a symbol is directly valid.
<p><a name="index-label_005fref-2838"></a><br><dt><code>(label_ref:</code><var>mode</var> <var>label</var><code>)</code><dd>Represents the value of an assembler label for code. It contains one
operand, an expression, which must be a <code>code_label</code> or a <code>note</code>
of type <code>NOTE_INSN_DELETED_LABEL</code> that appears in the instruction
sequence to identify the place where the label should go.
<p>The reason for using a distinct expression type for code label
references is so that jump optimization can distinguish them.
<p>The <code>label_ref</code> contains a mode, which is usually <code>Pmode</code>.
Usually that is the only mode for which a label is directly valid.
<p><a name="index-const-2839"></a><br><dt><code>(const:</code><var>m</var> <var>exp</var><code>)</code><dd>Represents a constant that is the result of an assembly-time
arithmetic computation. The operand, <var>exp</var>, is an expression that
contains only constants (<code>const_int</code>, <code>symbol_ref</code> and
<code>label_ref</code> expressions) combined with <code>plus</code> and
<code>minus</code>. However, not all combinations are valid, since the
assembler cannot do arbitrary arithmetic on relocatable symbols.
<p><var>m</var> should be <code>Pmode</code>.
<p><a name="index-high-2840"></a><br><dt><code>(high:</code><var>m</var> <var>exp</var><code>)</code><dd>Represents the high-order bits of <var>exp</var>, usually a
<code>symbol_ref</code>. The number of bits is machine-dependent and is
normally the number of bits specified in an instruction that initializes
the high order bits of a register. It is used with <code>lo_sum</code> to
represent the typical two-instruction sequence used in RISC machines to
reference a global memory location.
<p><var>m</var> should be <code>Pmode</code>.
</dl>
<p><a name="index-CONST0_005fRTX-2841"></a><a name="index-CONST1_005fRTX-2842"></a><a name="index-CONST2_005fRTX-2843"></a>The macro <code>CONST0_RTX (</code><var>mode</var><code>)</code> refers to an expression with
value 0 in mode <var>mode</var>. If mode <var>mode</var> is of mode class
<code>MODE_INT</code>, it returns <code>const0_rtx</code>. If mode <var>mode</var> is of
mode class <code>MODE_FLOAT</code>, it returns a <code>CONST_DOUBLE</code>
expression in mode <var>mode</var>. Otherwise, it returns a
<code>CONST_VECTOR</code> expression in mode <var>mode</var>. Similarly, the macro
<code>CONST1_RTX (</code><var>mode</var><code>)</code> refers to an expression with value 1 in
mode <var>mode</var> and similarly for <code>CONST2_RTX</code>. The
<code>CONST1_RTX</code> and <code>CONST2_RTX</code> macros are undefined
for vector modes.
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