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216 lines
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216 lines
13 KiB
HTML
4 years ago
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<title>Constants - GNU Compiler Collection (GCC) Internals</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals">
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<link rel="up" href="RTL.html#RTL" title="RTL">
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<link rel="prev" href="Machine-Modes.html#Machine-Modes" title="Machine Modes">
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<link rel="next" href="Regs-and-Memory.html#Regs-and-Memory" title="Regs and Memory">
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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(see below). A copy of the license is included in the section entitled
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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</head>
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<body>
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<div class="node">
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<a name="Constants"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Regs-and-Memory.html#Regs-and-Memory">Regs and Memory</a>,
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Previous: <a rel="previous" accesskey="p" href="Machine-Modes.html#Machine-Modes">Machine Modes</a>,
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Up: <a rel="up" accesskey="u" href="RTL.html#RTL">RTL</a>
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<hr>
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</div>
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<h3 class="section">13.7 Constant Expression Types</h3>
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<p><a name="index-RTL-constants-2820"></a><a name="index-RTL-constant-expression-types-2821"></a>
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The simplest RTL expressions are those that represent constant values.
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<a name="index-const_005fint-2822"></a>
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<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>
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is customarily accessed with the macro <code>INTVAL</code> as in
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<code>INTVAL (</code><var>exp</var><code>)</code>, which is equivalent to <code>XWINT (</code><var>exp</var><code>, 0)</code>.
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<p>Constants generated for modes with fewer bits than in
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<code>HOST_WIDE_INT</code> must be sign extended to full width (e.g., with
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<code>gen_int_mode</code>). For constants for modes with more bits than in
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<code>HOST_WIDE_INT</code> the implied high order bits of that constant are
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copies of the top bit. Note however that values are neither
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inherently signed nor inherently unsigned; where necessary, signedness
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is determined by the rtl operation instead.
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<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
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the value of the variable <code>const0_rtx</code>. Likewise, the only
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expression for integer value one is found in <code>const1_rtx</code>, the only
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expression for integer value two is found in <code>const2_rtx</code>, and the
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only expression for integer value negative one is found in
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<code>constm1_rtx</code>. Any attempt to create an expression of code
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<code>const_int</code> and value zero, one, two or negative one will return
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<code>const0_rtx</code>, <code>const1_rtx</code>, <code>const2_rtx</code> or
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<code>constm1_rtx</code> as appropriate.
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<p><a name="index-const_005ftrue_005frtx-2827"></a>Similarly, there is only one object for the integer whose value is
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<code>STORE_FLAG_VALUE</code>. It is found in <code>const_true_rtx</code>. If
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<code>STORE_FLAG_VALUE</code> is one, <code>const_true_rtx</code> and
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<code>const1_rtx</code> will point to the same object. If
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<code>STORE_FLAG_VALUE</code> is −1, <code>const_true_rtx</code> and
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<code>constm1_rtx</code> will point to the same object.
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<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
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(on older ports that do not define
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<code>TARGET_SUPPORTS_WIDE_INT</code>) an integer constant too large to fit
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into <code>HOST_BITS_PER_WIDE_INT</code> bits but small enough to fit within
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twice that number of bits. In the latter case, <var>m</var> will be
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<code>VOIDmode</code>. For integral values constants for modes with more
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bits than twice the number in <code>HOST_WIDE_INT</code> the implied high
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order bits of that constant are copies of the top bit of
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<code>CONST_DOUBLE_HIGH</code>. Note however that integral values are
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neither inherently signed nor inherently unsigned; where necessary,
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signedness is determined by the rtl operation instead.
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<p>On more modern ports, <code>CONST_DOUBLE</code> only represents floating
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point values. New ports define <code>TARGET_SUPPORTS_WIDE_INT</code> to
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make this designation.
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<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
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<var>i0</var> and <var>i1</var>. <var>i0</var> is customarily accessed with the macro
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<code>CONST_DOUBLE_LOW</code> and <var>i1</var> with <code>CONST_DOUBLE_HIGH</code>.
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<p>If the constant is floating point (regardless of its precision), then
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the number of integers used to store the value depends on the size of
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<code>REAL_VALUE_TYPE</code> (see <a href="Floating-Point.html#Floating-Point">Floating Point</a>). The integers
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represent a floating point number, but not precisely in the target
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machine's or host machine's floating point format. To convert them to
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the precise bit pattern used by the target machine, use the macro
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<code>REAL_VALUE_TO_TARGET_DOUBLE</code> and friends (see <a href="Data-Output.html#Data-Output">Data Output</a>).
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<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
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to hold any constant that can be represented on the target. This form
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of rtl is only used on targets that define
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<code>TARGET_SUPPORTS_WIDE_INT</code> to be nonzero and then
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<code>CONST_DOUBLE</code>s are only used to hold floating-point values. If
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the target leaves <code>TARGET_SUPPORTS_WIDE_INT</code> defined as 0,
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<code>CONST_WIDE_INT</code>s are not used and <code>CONST_DOUBLE</code>s are as
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they were before.
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<p>The values are stored in a compressed format. The higher-order
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0s or -1s are not represented if they are just the logical sign
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extension of the number that is represented.
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<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
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store the value. This macro should be rarely used.
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<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.
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Note that this generally is smaller than the number of
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<code>HOST_WIDE_INT</code>s implied by the mode size.
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<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
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the low order bits of the constant.
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<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>.
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The operand is a data structure of type <code>struct fixed_value</code> and
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is accessed with the macro <code>CONST_FIXED_VALUE</code>. The high part of
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data is accessed with <code>CONST_FIXED_VALUE_HIGH</code>; the low part is
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accessed with <code>CONST_FIXED_VALUE_LOW</code>.
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<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
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containing the constant elements. <var>x0</var>, <var>x1</var> and so on are
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the <code>const_int</code>, <code>const_double</code> or <code>const_fixed</code> elements.
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<p>The number of units in a <code>const_vector</code> is obtained with the macro
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<code>CONST_VECTOR_NUNITS</code> as in <code>CONST_VECTOR_NUNITS (</code><var>v</var><code>)</code>.
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<p>Individual elements in a vector constant are accessed with the macro
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<code>CONST_VECTOR_ELT</code> as in <code>CONST_VECTOR_ELT (</code><var>v</var><code>, </code><var>n</var><code>)</code>
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where <var>v</var> is the vector constant and <var>n</var> is the element
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desired.
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<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
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used only for insn attributes (see <a href="Insn-Attributes.html#Insn-Attributes">Insn Attributes</a>) since constant
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strings in C are placed in memory.
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<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
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a string that describes the name of the assembler label. If it starts
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with a ‘<samp><span class="samp">*</span></samp>’, the label is the rest of <var>symbol</var> not including
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the ‘<samp><span class="samp">*</span></samp>’. Otherwise, the label is <var>symbol</var>, usually prefixed
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with ‘<samp><span class="samp">_</span></samp>’.
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<p>The <code>symbol_ref</code> contains a mode, which is usually <code>Pmode</code>.
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Usually that is the only mode for which a symbol is directly valid.
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<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
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operand, an expression, which must be a <code>code_label</code> or a <code>note</code>
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of type <code>NOTE_INSN_DELETED_LABEL</code> that appears in the instruction
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sequence to identify the place where the label should go.
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<p>The reason for using a distinct expression type for code label
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references is so that jump optimization can distinguish them.
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<p>The <code>label_ref</code> contains a mode, which is usually <code>Pmode</code>.
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Usually that is the only mode for which a label is directly valid.
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<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
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arithmetic computation. The operand, <var>exp</var>, is an expression that
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contains only constants (<code>const_int</code>, <code>symbol_ref</code> and
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<code>label_ref</code> expressions) combined with <code>plus</code> and
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<code>minus</code>. However, not all combinations are valid, since the
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assembler cannot do arbitrary arithmetic on relocatable symbols.
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<p><var>m</var> should be <code>Pmode</code>.
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<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
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<code>symbol_ref</code>. The number of bits is machine-dependent and is
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normally the number of bits specified in an instruction that initializes
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the high order bits of a register. It is used with <code>lo_sum</code> to
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represent the typical two-instruction sequence used in RISC machines to
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reference a global memory location.
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<p><var>m</var> should be <code>Pmode</code>.
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</dl>
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<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
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value 0 in mode <var>mode</var>. If mode <var>mode</var> is of mode class
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<code>MODE_INT</code>, it returns <code>const0_rtx</code>. If mode <var>mode</var> is of
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mode class <code>MODE_FLOAT</code>, it returns a <code>CONST_DOUBLE</code>
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expression in mode <var>mode</var>. Otherwise, it returns a
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<code>CONST_VECTOR</code> expression in mode <var>mode</var>. Similarly, the macro
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<code>CONST1_RTX (</code><var>mode</var><code>)</code> refers to an expression with value 1 in
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mode <var>mode</var> and similarly for <code>CONST2_RTX</code>. The
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<code>CONST1_RTX</code> and <code>CONST2_RTX</code> macros are undefined
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for vector modes.
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</body></html>
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