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148 lines
6.1 KiB
HTML
148 lines
6.1 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Disable Insn Alternatives - 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="Constraints.html#Constraints" title="Constraints">
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<link rel="prev" href="Machine-Constraints.html#Machine-Constraints" title="Machine Constraints">
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<link rel="next" href="Define-Constraints.html#Define-Constraints" title="Define Constraints">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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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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(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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(b) The FSF's Back-Cover Text is:
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</head>
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<body>
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<div class="node">
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<a name="Disable-Insn-Alternatives"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Define-Constraints.html#Define-Constraints">Define Constraints</a>,
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Previous: <a rel="previous" accesskey="p" href="Machine-Constraints.html#Machine-Constraints">Machine Constraints</a>,
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Up: <a rel="up" accesskey="u" href="Constraints.html#Constraints">Constraints</a>
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<hr>
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</div>
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<h4 class="subsection">16.8.6 Disable insn alternatives using the <code>enabled</code> attribute</h4>
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<p><a name="index-enabled-3364"></a>
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There are three insn attributes that may be used to selectively disable
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instruction alternatives:
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<dl>
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<dt><code>enabled</code><dd>Says whether an alternative is available on the current subtarget.
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<br><dt><code>preferred_for_size</code><dd>Says whether an enabled alternative should be used in code that is
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optimized for size.
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<br><dt><code>preferred_for_speed</code><dd>Says whether an enabled alternative should be used in code that is
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optimized for speed.
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</dl>
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<p>All these attributes should use <code>(const_int 1)</code> to allow an alternative
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or <code>(const_int 0)</code> to disallow it. The attributes must be a static
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property of the subtarget; they cannot for example depend on the
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current operands, on the current optimization level, on the location
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of the insn within the body of a loop, on whether register allocation
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has finished, or on the current compiler pass.
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<p>The <code>enabled</code> attribute is a correctness property. It tells GCC to act
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as though the disabled alternatives were never defined in the first place.
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This is useful when adding new instructions to an existing pattern in
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cases where the new instructions are only available for certain cpu
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architecture levels (typically mapped to the <code>-march=</code> command-line
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option).
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<p>In contrast, the <code>preferred_for_size</code> and <code>preferred_for_speed</code>
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attributes are strong optimization hints rather than correctness properties.
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<code>preferred_for_size</code> tells GCC which alternatives to consider when
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adding or modifying an instruction that GCC wants to optimize for size.
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<code>preferred_for_speed</code> does the same thing for speed. Note that things
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like code motion can lead to cases where code optimized for size uses
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alternatives that are not preferred for size, and similarly for speed.
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<p>Although <code>define_insn</code>s can in principle specify the <code>enabled</code>
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attribute directly, it is often clearer to have subsiduary attributes
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for each architectural feature of interest. The <code>define_insn</code>s
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can then use these subsiduary attributes to say which alternatives
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require which features. The example below does this for <code>cpu_facility</code>.
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<p>E.g. the following two patterns could easily be merged using the <code>enabled</code>
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attribute:
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<pre class="smallexample">
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(define_insn "*movdi_old"
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[(set (match_operand:DI 0 "register_operand" "=d")
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(match_operand:DI 1 "register_operand" " d"))]
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"!TARGET_NEW"
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"lgr %0,%1")
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(define_insn "*movdi_new"
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[(set (match_operand:DI 0 "register_operand" "=d,f,d")
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(match_operand:DI 1 "register_operand" " d,d,f"))]
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"TARGET_NEW"
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"@
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lgr %0,%1
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ldgr %0,%1
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lgdr %0,%1")
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</pre>
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<p>to:
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<pre class="smallexample">
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(define_insn "*movdi_combined"
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[(set (match_operand:DI 0 "register_operand" "=d,f,d")
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(match_operand:DI 1 "register_operand" " d,d,f"))]
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""
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"@
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lgr %0,%1
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ldgr %0,%1
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lgdr %0,%1"
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[(set_attr "cpu_facility" "*,new,new")])
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</pre>
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<p>with the <code>enabled</code> attribute defined like this:
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<pre class="smallexample">
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(define_attr "cpu_facility" "standard,new" (const_string "standard"))
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(define_attr "enabled" ""
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(cond [(eq_attr "cpu_facility" "standard") (const_int 1)
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(and (eq_attr "cpu_facility" "new")
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(ne (symbol_ref "TARGET_NEW") (const_int 0)))
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(const_int 1)]
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(const_int 0)))
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</pre>
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</body></html>
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