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262 lines
12 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Logical Operators - GNU Compiler Collection (GCC) Internals</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="GNU Compiler Collection (GCC) Internals">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Operands.html#Operands" title="Operands">
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<link rel="prev" href="Conditional-Expressions.html#Conditional-Expressions" title="Conditional Expressions">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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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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any later version published by the Free Software Foundation; with the
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Invariant Sections being ``Funding Free Software'', the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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(see below). A copy of the license is included in the section entitled
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``GNU Free Documentation License''.
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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funds for GNU development.-->
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--></style>
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</head>
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<body>
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<div class="node">
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<a name="Logical-Operators"></a>
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<p>
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Previous: <a rel="previous" accesskey="p" href="Conditional-Expressions.html#Conditional-Expressions">Conditional Expressions</a>,
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Up: <a rel="up" accesskey="u" href="Operands.html#Operands">Operands</a>
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<hr>
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</div>
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<h4 class="subsection">11.6.4 Logical Operators</h4>
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<p><a name="index-Logical-Operators-2144"></a>
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Except when they appear in the condition operand of a
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<code>GIMPLE_COND</code>, logical `and' and `or' operators are simplified
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as follows: <code>a = b && c</code> becomes
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<pre class="smallexample"> T1 = (bool)b;
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if (T1 == true)
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T1 = (bool)c;
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a = T1;
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</pre>
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<p>Note that <code>T1</code> in this example cannot be an expression temporary,
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because it has two different assignments.
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<h4 class="subsection">11.6.5 Manipulating operands</h4>
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<p>All gimple operands are of type <code>tree</code>. But only certain
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types of trees are allowed to be used as operand tuples. Basic
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validation is controlled by the function
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<code>get_gimple_rhs_class</code>, which given a tree code, returns an
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<code>enum</code> with the following values of type <code>enum
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gimple_rhs_class</code>
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<ul>
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<li><code>GIMPLE_INVALID_RHS</code>
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The tree cannot be used as a GIMPLE operand.
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<li><code>GIMPLE_TERNARY_RHS</code>
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The tree is a valid GIMPLE ternary operation.
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<li><code>GIMPLE_BINARY_RHS</code>
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The tree is a valid GIMPLE binary operation.
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<li><code>GIMPLE_UNARY_RHS</code>
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The tree is a valid GIMPLE unary operation.
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<li><code>GIMPLE_SINGLE_RHS</code>
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The tree is a single object, that cannot be split into simpler
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operands (for instance, <code>SSA_NAME</code>, <code>VAR_DECL</code>, <code>COMPONENT_REF</code>, etc).
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<p>This operand class also acts as an escape hatch for tree nodes
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that may be flattened out into the operand vector, but would need
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more than two slots on the RHS. For instance, a <code>COND_EXPR</code>
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expression of the form <code>(a op b) ? x : y</code> could be flattened
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out on the operand vector using 4 slots, but it would also
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require additional processing to distinguish <code>c = a op b</code>
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from <code>c = a op b ? x : y</code>. Something similar occurs with
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<code>ASSERT_EXPR</code>. In time, these special case tree
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expressions should be flattened into the operand vector.
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</ul>
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<p>For tree nodes in the categories <code>GIMPLE_TERNARY_RHS</code>,
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<code>GIMPLE_BINARY_RHS</code> and <code>GIMPLE_UNARY_RHS</code>, they cannot be
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stored inside tuples directly. They first need to be flattened and
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separated into individual components. For instance, given the GENERIC
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expression
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<pre class="smallexample"> a = b + c
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</pre>
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<p>its tree representation is:
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<pre class="smallexample"> MODIFY_EXPR <VAR_DECL <a>, PLUS_EXPR <VAR_DECL <b>, VAR_DECL <c>>>
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</pre>
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<p>In this case, the GIMPLE form for this statement is logically
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identical to its GENERIC form but in GIMPLE, the <code>PLUS_EXPR</code>
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on the RHS of the assignment is not represented as a tree,
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instead the two operands are taken out of the <code>PLUS_EXPR</code> sub-tree
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and flattened into the GIMPLE tuple as follows:
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<pre class="smallexample"> GIMPLE_ASSIGN <PLUS_EXPR, VAR_DECL <a>, VAR_DECL <b>, VAR_DECL <c>>
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</pre>
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<h4 class="subsection">11.6.6 Operand vector allocation</h4>
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<p>The operand vector is stored at the bottom of the three tuple
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structures that accept operands. This means, that depending on
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the code of a given statement, its operand vector will be at
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different offsets from the base of the structure. To access
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tuple operands use the following accessors
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<div class="defun">
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— GIMPLE function: unsigned <b>gimple_num_ops</b> (<var>gimple g</var>)<var><a name="index-gimple_005fnum_005fops-2145"></a></var><br>
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<blockquote><p>Returns the number of operands in statement G.
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</p></blockquote></div>
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<div class="defun">
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— GIMPLE function: tree <b>gimple_op</b> (<var>gimple g, unsigned i</var>)<var><a name="index-gimple_005fop-2146"></a></var><br>
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<blockquote><p>Returns operand <code>I</code> from statement <code>G</code>.
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</p></blockquote></div>
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<div class="defun">
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— GIMPLE function: tree * <b>gimple_ops</b> (<var>gimple g</var>)<var><a name="index-gimple_005fops-2147"></a></var><br>
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<blockquote><p>Returns a pointer into the operand vector for statement <code>G</code>. This
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is computed using an internal table called <code>gimple_ops_offset_</code>[].
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This table is indexed by the gimple code of <code>G</code>.
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<p>When the compiler is built, this table is filled-in using the
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sizes of the structures used by each statement code defined in
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gimple.def. Since the operand vector is at the bottom of the
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structure, for a gimple code <code>C</code> the offset is computed as sizeof
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(struct-of <code>C</code>) - sizeof (tree).
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<p>This mechanism adds one memory indirection to every access when
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using <code>gimple_op</code>(), if this becomes a bottleneck, a pass can
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choose to memoize the result from <code>gimple_ops</code>() and use that to
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access the operands.
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</p></blockquote></div>
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<h4 class="subsection">11.6.7 Operand validation</h4>
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<p>When adding a new operand to a gimple statement, the operand will
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be validated according to what each tuple accepts in its operand
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vector. These predicates are called by the
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<code>gimple_</code><var>name</var><code>_set_...()</code>. Each tuple will use one of the
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following predicates (Note, this list is not exhaustive):
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<div class="defun">
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— GIMPLE function: bool <b>is_gimple_val</b> (<var>tree t</var>)<var><a name="index-is_005fgimple_005fval-2148"></a></var><br>
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<blockquote><p>Returns true if t is a "GIMPLE value", which are all the
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non-addressable stack variables (variables for which
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<code>is_gimple_reg</code> returns true) and constants (expressions for which
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<code>is_gimple_min_invariant</code> returns true).
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</p></blockquote></div>
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<div class="defun">
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— GIMPLE function: bool <b>is_gimple_addressable</b> (<var>tree t</var>)<var><a name="index-is_005fgimple_005faddressable-2149"></a></var><br>
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<blockquote><p>Returns true if t is a symbol or memory reference whose address
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can be taken.
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</p></blockquote></div>
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<div class="defun">
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— GIMPLE function: bool <b>is_gimple_asm_val</b> (<var>tree t</var>)<var><a name="index-is_005fgimple_005fasm_005fval-2150"></a></var><br>
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<blockquote><p>Similar to <code>is_gimple_val</code> but it also accepts hard registers.
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</p></blockquote></div>
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<div class="defun">
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— GIMPLE function: bool <b>is_gimple_call_addr</b> (<var>tree t</var>)<var><a name="index-is_005fgimple_005fcall_005faddr-2151"></a></var><br>
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<blockquote><p>Return true if t is a valid expression to use as the function
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called by a <code>GIMPLE_CALL</code>.
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</p></blockquote></div>
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<div class="defun">
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— GIMPLE function: bool <b>is_gimple_mem_ref_addr</b> (<var>tree t</var>)<var><a name="index-is_005fgimple_005fmem_005fref_005faddr-2152"></a></var><br>
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<blockquote><p>Return true if t is a valid expression to use as first operand
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of a <code>MEM_REF</code> expression.
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</p></blockquote></div>
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<div class="defun">
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— GIMPLE function: bool <b>is_gimple_constant</b> (<var>tree t</var>)<var><a name="index-is_005fgimple_005fconstant-2153"></a></var><br>
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<blockquote><p>Return true if t is a valid gimple constant.
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</p></blockquote></div>
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<div class="defun">
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— GIMPLE function: bool <b>is_gimple_min_invariant</b> (<var>tree t</var>)<var><a name="index-is_005fgimple_005fmin_005finvariant-2154"></a></var><br>
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<blockquote><p>Return true if t is a valid minimal invariant. This is different
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from constants, in that the specific value of t may not be known
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at compile time, but it is known that it doesn't change (e.g.,
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the address of a function local variable).
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</p></blockquote></div>
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<div class="defun">
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— GIMPLE function: bool <b>is_gimple_ip_invariant</b> (<var>tree t</var>)<var><a name="index-is_005fgimple_005fip_005finvariant-2155"></a></var><br>
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<blockquote><p>Return true if t is an interprocedural invariant. This means that t
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is a valid invariant in all functions (e.g. it can be an address of a
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global variable but not of a local one).
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</p></blockquote></div>
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<div class="defun">
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— GIMPLE function: bool <b>is_gimple_ip_invariant_address</b> (<var>tree t</var>)<var><a name="index-is_005fgimple_005fip_005finvariant_005faddress-2156"></a></var><br>
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<blockquote><p>Return true if t is an <code>ADDR_EXPR</code> that does not change once the
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program is running (and which is valid in all functions).
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</p></blockquote></div>
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<h4 class="subsection">11.6.8 Statement validation</h4>
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<div class="defun">
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— GIMPLE function: bool <b>is_gimple_assign</b> (<var>gimple g</var>)<var><a name="index-is_005fgimple_005fassign-2157"></a></var><br>
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<blockquote><p>Return true if the code of g is <code>GIMPLE_ASSIGN</code>.
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</p></blockquote></div>
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<div class="defun">
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— GIMPLE function: bool <b>is_gimple_call</b> (<var>gimple g</var>)<var><a name="index-is_005fgimple_005fcall-2158"></a></var><br>
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<blockquote><p>Return true if the code of g is <code>GIMPLE_CALL</code>.
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</p></blockquote></div>
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<div class="defun">
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— GIMPLE function: bool <b>is_gimple_debug</b> (<var>gimple g</var>)<var><a name="index-is_005fgimple_005fdebug-2159"></a></var><br>
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<blockquote><p>Return true if the code of g is <code>GIMPLE_DEBUG</code>.
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</p></blockquote></div>
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<div class="defun">
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— GIMPLE function: bool <b>gimple_assign_cast_p</b> (<var>const_gimple g</var>)<var><a name="index-gimple_005fassign_005fcast_005fp-2160"></a></var><br>
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<blockquote><p>Return true if g is a <code>GIMPLE_ASSIGN</code> that performs a type cast
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operation.
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</p></blockquote></div>
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<div class="defun">
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— GIMPLE function: bool <b>gimple_debug_bind_p</b> (<var>gimple g</var>)<var><a name="index-gimple_005fdebug_005fbind_005fp-2161"></a></var><br>
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<blockquote><p>Return true if g is a <code>GIMPLE_DEBUG</code> that binds the value of an
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expression to a variable.
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</p></blockquote></div>
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<div class="defun">
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— GIMPLE function: bool <b>is_gimple_omp</b> (<var>gimple g</var>)<var><a name="index-is_005fgimple_005fomp-2162"></a></var><br>
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<blockquote><p>Return true if g is any of the OpenMP codes.
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</p></blockquote></div>
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</body></html>
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