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A GNU Manual
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<head>
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<title>The Language (GNU Compiler Collection (GCC) Internals)</title>
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<meta name="description" content="The Language (GNU Compiler Collection (GCC) Internals)">
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<meta name="keywords" content="The Language (GNU Compiler Collection (GCC) Internals)">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Match-and-Simplify.html#Match-and-Simplify" rel="up" title="Match and Simplify">
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<link href="Funding.html#Funding" rel="next" title="Funding">
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</head>
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<body lang="en">
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<a name="The-Language"></a>
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<div class="header">
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<p>
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Previous: <a href="GIMPLE-API.html#GIMPLE-API" accesskey="p" rel="prev">GIMPLE API</a>, Up: <a href="Match-and-Simplify.html#Match-and-Simplify" accesskey="u" rel="up">Match and Simplify</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="The-Language-1"></a>
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<h3 class="section">26.2 The Language</h3>
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<a name="index-The-Language"></a>
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<p>The language to write expression simplifications in resembles other
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domain-specific languages GCC uses. Thus it is lispy. Lets start
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with an example from the match.pd file:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(simplify
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(bit_and @0 integer_all_onesp)
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@0)
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</pre></div>
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<p>This example contains all required parts of an expression simplification.
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A simplification is wrapped inside a <code>(simplify ...)</code> expression.
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That contains at least two operands - an expression that is matched
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with the GIMPLE or GENERIC IL and a replacement expression that is
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returned if the match was successful.
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</p>
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<p>Expressions have an operator ID, <code>bit_and</code> in this case. Expressions can
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be lower-case tree codes with <code>_expr</code> stripped off or builtin
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function code names in all-caps, like <code>BUILT_IN_SQRT</code>.
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</p>
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<p><code>@n</code> denotes a so-called capture. It captures the operand and lets
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you refer to it in other places of the match-and-simplify. In the
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above example it is refered to in the replacement expression. Captures
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are <code>@</code> followed by a number or an identifier.
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</p>
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<div class="smallexample">
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<pre class="smallexample">(simplify
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(bit_xor @0 @0)
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{ build_zero_cst (type); })
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</pre></div>
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<p>In this example <code>@0</code> is mentioned twice which constrains the matched
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expression to have two equal operands. Usually matches are constraint
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to equal types. If operands may be constants and conversions are involved
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matching by value might be preferred in which case use <code>@@0</code> to
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denote a by value match and the specific operand you want to refer to
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in the result part. This example also introduces
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operands written in C code. These can be used in the expression
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replacements and are supposed to evaluate to a tree node which has to
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be a valid GIMPLE operand (so you cannot generate expressions in C code).
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</p>
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<div class="smallexample">
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<pre class="smallexample">(simplify
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(trunc_mod integer_zerop@0 @1)
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(if (!integer_zerop (@1))
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@0))
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</pre></div>
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<p>Here <code>@0</code> captures the first operand of the trunc_mod expression
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which is also predicated with <code>integer_zerop</code>. Expression operands
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may be either expressions, predicates or captures. Captures
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can be unconstrained or capture expresions or predicates.
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</p>
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<p>This example introduces an optional operand of simplify,
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the if-expression. This condition is evaluated after the
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expression matched in the IL and is required to evaluate to true
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to enable the replacement expression in the second operand
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position. The expression operand of the <code>if</code> is a standard C
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expression which may contain references to captures. The <code>if</code>
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has an optional third operand which may contain the replacement
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expression that is enabled when the condition evaluates to false.
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</p>
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<p>A <code>if</code> expression can be used to specify a common condition
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for multiple simplify patterns, avoiding the need
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to repeat that multiple times:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(if (!TYPE_SATURATING (type)
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&& !FLOAT_TYPE_P (type) && !FIXED_POINT_TYPE_P (type))
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(simplify
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(minus (plus @0 @1) @0)
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@1)
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(simplify
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(minus (minus @0 @1) @0)
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(negate @1)))
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</pre></div>
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<p>Note that <code>if</code>s in outer position do not have the optional
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else clause but instead have multiple then clauses.
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</p>
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<p>Ifs can be nested.
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</p>
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<p>There exists a <code>switch</code> expression which can be used to
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chain conditions avoiding nesting <code>if</code>s too much:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(simplify
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(simple_comparison @0 REAL_CST@1)
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(switch
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/* a CMP (-0) -> a CMP 0 */
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(if (REAL_VALUE_MINUS_ZERO (TREE_REAL_CST (@1)))
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(cmp @0 { build_real (TREE_TYPE (@1), dconst0); }))
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/* x != NaN is always true, other ops are always false. */
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(if (REAL_VALUE_ISNAN (TREE_REAL_CST (@1))
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&& ! HONOR_SNANS (@1))
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{ constant_boolean_node (cmp == NE_EXPR, type); })))
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</pre></div>
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<p>Is equal to
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</p>
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<div class="smallexample">
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<pre class="smallexample">(simplify
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(simple_comparison @0 REAL_CST@1)
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(switch
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/* a CMP (-0) -> a CMP 0 */
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(if (REAL_VALUE_MINUS_ZERO (TREE_REAL_CST (@1)))
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(cmp @0 { build_real (TREE_TYPE (@1), dconst0); })
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/* x != NaN is always true, other ops are always false. */
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(if (REAL_VALUE_ISNAN (TREE_REAL_CST (@1))
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&& ! HONOR_SNANS (@1))
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{ constant_boolean_node (cmp == NE_EXPR, type); }))))
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</pre></div>
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<p>which has the second <code>if</code> in the else operand of the first.
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The <code>switch</code> expression takes <code>if</code> expressions as
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operands (which may not have else clauses) and as a last operand
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a replacement expression which should be enabled by default if
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no other condition evaluated to true.
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</p>
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<p>Captures can also be used for capturing results of sub-expressions.
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</p>
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<div class="smallexample">
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<pre class="smallexample">#if GIMPLE
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(simplify
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(pointer_plus (addr@2 @0) INTEGER_CST_P@1)
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(if (is_gimple_min_invariant (@2)))
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{
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poly_int64 off;
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tree base = get_addr_base_and_unit_offset (@0, &off);
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off += tree_to_uhwi (@1);
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/* Now with that we should be able to simply write
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(addr (mem_ref (addr @base) (plus @off @1))) */
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build1 (ADDR_EXPR, type,
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build2 (MEM_REF, TREE_TYPE (TREE_TYPE (@2)),
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build_fold_addr_expr (base),
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build_int_cst (ptr_type_node, off)));
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})
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#endif
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</pre></div>
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<p>In the above example, <code>@2</code> captures the result of the expression
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<code>(addr @0)</code>. For outermost expression only its type can be captured,
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and the keyword <code>type</code> is reserved for this purpose. The above
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example also gives a way to conditionalize patterns to only apply
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to <code>GIMPLE</code> or <code>GENERIC</code> by means of using the pre-defined
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preprocessor macros <code>GIMPLE</code> and <code>GENERIC</code> and using
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preprocessor directives.
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</p>
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<div class="smallexample">
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<pre class="smallexample">(simplify
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(bit_and:c integral_op_p@0 (bit_ior:c (bit_not @0) @1))
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(bit_and @1 @0))
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</pre></div>
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<p>Here we introduce flags on match expressions. The flag used
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above, <code>c</code>, denotes that the expression should
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be also matched commutated. Thus the above match expression
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is really the following four match expressions:
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</p>
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<div class="smallexample">
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<pre class="smallexample"> (bit_and integral_op_p@0 (bit_ior (bit_not @0) @1))
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(bit_and (bit_ior (bit_not @0) @1) integral_op_p@0)
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(bit_and integral_op_p@0 (bit_ior @1 (bit_not @0)))
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(bit_and (bit_ior @1 (bit_not @0)) integral_op_p@0)
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</pre></div>
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<p>Usual canonicalizations you know from GENERIC expressions are
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applied before matching, so for example constant operands always
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come second in commutative expressions.
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</p>
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<p>The second supported flag is <code>s</code> which tells the code
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generator to fail the pattern if the expression marked with
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<code>s</code> does have more than one use. For example in
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</p>
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<div class="smallexample">
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<pre class="smallexample">(simplify
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(pointer_plus (pointer_plus:s @0 @1) @3)
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(pointer_plus @0 (plus @1 @3)))
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</pre></div>
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<p>this avoids the association if <code>(pointer_plus @0 @1)</code> is
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used outside of the matched expression and thus it would stay
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live and not trivially removed by dead code elimination.
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</p>
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<p>More features exist to avoid too much repetition.
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</p>
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<div class="smallexample">
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<pre class="smallexample">(for op (plus pointer_plus minus bit_ior bit_xor)
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(simplify
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(op @0 integer_zerop)
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@0))
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</pre></div>
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<p>A <code>for</code> expression can be used to repeat a pattern for each
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operator specified, substituting <code>op</code>. <code>for</code> can be
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nested and a <code>for</code> can have multiple operators to iterate.
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</p>
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<div class="smallexample">
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<pre class="smallexample">(for opa (plus minus)
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opb (minus plus)
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(for opc (plus minus)
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(simplify...
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</pre></div>
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<p>In this example the pattern will be repeated four times with
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<code>opa, opb, opc</code> being <code>plus, minus, plus</code>,
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<code>plus, minus, minus</code>, <code>minus, plus, plus</code>,
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<code>minus, plus, minus</code>.
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</p>
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<p>To avoid repeating operator lists in <code>for</code> you can name
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them via
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</p>
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<div class="smallexample">
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<pre class="smallexample">(define_operator_list pmm plus minus mult)
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</pre></div>
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<p>and use them in <code>for</code> operator lists where they get expanded.
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</p>
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<div class="smallexample">
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<pre class="smallexample">(for opa (pmm trunc_div)
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(simplify...
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</pre></div>
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<p>So this example iterates over <code>plus</code>, <code>minus</code>, <code>mult</code>
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and <code>trunc_div</code>.
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</p>
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<p>Using operator lists can also remove the need to explicitely write
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a <code>for</code>. All operator list uses that appear in a <code>simplify</code>
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or <code>match</code> pattern in operator positions will implicitely
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be added to a new <code>for</code>. For example
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</p>
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<div class="smallexample">
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<pre class="smallexample">(define_operator_list SQRT BUILT_IN_SQRTF BUILT_IN_SQRT BUILT_IN_SQRTL)
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(define_operator_list POW BUILT_IN_POWF BUILT_IN_POW BUILT_IN_POWL)
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(simplify
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(SQRT (POW @0 @1))
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(POW (abs @0) (mult @1 { built_real (TREE_TYPE (@1), dconsthalf); })))
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</pre></div>
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<p>is the same as
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</p>
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<div class="smallexample">
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<pre class="smallexample">(for SQRT (BUILT_IN_SQRTF BUILT_IN_SQRT BUILT_IN_SQRTL)
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POW (BUILT_IN_POWF BUILT_IN_POW BUILT_IN_POWL)
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(simplify
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(SQRT (POW @0 @1))
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(POW (abs @0) (mult @1 { built_real (TREE_TYPE (@1), dconsthalf); }))))
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</pre></div>
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<p><code>for</code>s and operator lists can include the special identifier
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<code>null</code> that matches nothing and can never be generated. This can
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be used to pad an operator list so that it has a standard form,
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even if there isn’t a suitable operator for every form.
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</p>
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<p>Another building block are <code>with</code> expressions in the
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result expression which nest the generated code in a new C block
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followed by its argument:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(simplify
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(convert (mult @0 @1))
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(with { tree utype = unsigned_type_for (type); }
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(convert (mult (convert:utype @0) (convert:utype @1)))))
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</pre></div>
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<p>This allows code nested in the <code>with</code> to refer to the declared
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variables. In the above case we use the feature to specify the
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type of a generated expression with the <code>:type</code> syntax where
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<code>type</code> needs to be an identifier that refers to the desired type.
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Usually the types of the generated result expressions are
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determined from the context, but sometimes like in the above case
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it is required that you specify them explicitely.
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</p>
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<p>As intermediate conversions are often optional there is a way to
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avoid the need to repeat patterns both with and without such
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conversions. Namely you can mark a conversion as being optional
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with a <code>?</code>:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(simplify
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(eq (convert@0 @1) (convert? @2))
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(eq @1 (convert @2)))
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</pre></div>
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<p>which will match both <code>(eq (convert @1) (convert @2))</code> and
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<code>(eq (convert @1) @2)</code>. The optional converts are supposed
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to be all either present or not, thus
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<code>(eq (convert? @1) (convert? @2))</code> will result in two
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patterns only. If you want to match all four combinations you
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have access to two additional conditional converts as in
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<code>(eq (convert1? @1) (convert2? @2))</code>.
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</p>
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<p>Predicates available from the GCC middle-end need to be made
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available explicitely via <code>define_predicates</code>:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(define_predicates
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integer_onep integer_zerop integer_all_onesp)
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</pre></div>
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<p>You can also define predicates using the pattern matching language
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and the <code>match</code> form:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(match negate_expr_p
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INTEGER_CST
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(if (TYPE_OVERFLOW_WRAPS (type)
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|| may_negate_without_overflow_p (t))))
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(match negate_expr_p
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(negate @0))
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</pre></div>
|
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<p>This shows that for <code>match</code> expressions there is <code>t</code>
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available which captures the outermost expression (something
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not possible in the <code>simplify</code> context). As you can see
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<code>match</code> has an identifier as first operand which is how
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you refer to the predicate in patterns. Multiple <code>match</code>
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for the same identifier add additional cases where the predicate
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matches.
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</p>
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<p>Predicates can also match an expression in which case you need
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to provide a template specifying the identifier and where to
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get its operands from:
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</p>
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<div class="smallexample">
|
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<pre class="smallexample">(match (logical_inverted_value @0)
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(eq @0 integer_zerop))
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(match (logical_inverted_value @0)
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(bit_not truth_valued_p@0))
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</pre></div>
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<p>You can use the above predicate like
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</p>
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<div class="smallexample">
|
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<pre class="smallexample">(simplify
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(bit_and @0 (logical_inverted_value @0))
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{ build_zero_cst (type); })
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</pre></div>
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<p>Which will match a bitwise and of an operand with its logical
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inverted value.
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</p>
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<hr>
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<div class="header">
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<p>
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Previous: <a href="GIMPLE-API.html#GIMPLE-API" accesskey="p" rel="prev">GIMPLE API</a>, Up: <a href="Match-and-Simplify.html#Match-and-Simplify" accesskey="u" rel="up">Match and Simplify</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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