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108 lines
5.0 KiB
HTML
108 lines
5.0 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1988-2018 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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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Size of an asm (Using the GNU Compiler Collection (GCC))</title>
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<meta name="description" content="Size of an asm (Using the GNU Compiler Collection (GCC))">
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<meta name="keywords" content="Size of an asm (Using the GNU Compiler Collection (GCC))">
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<meta name="distribution" content="global">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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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="Using-Assembly-Language-with-C.html#Using-Assembly-Language-with-C" rel="up" title="Using Assembly Language with C">
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<link href="Alternate-Keywords.html#Alternate-Keywords" rel="next" title="Alternate Keywords">
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<link href="Local-Register-Variables.html#Local-Register-Variables" rel="prev" title="Local Register Variables">
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</head>
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<body lang="en">
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<a name="Size-of-an-asm"></a>
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<div class="header">
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<p>
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Previous: <a href="Explicit-Register-Variables.html#Explicit-Register-Variables" accesskey="p" rel="prev">Explicit Register Variables</a>, Up: <a href="Using-Assembly-Language-with-C.html#Using-Assembly-Language-with-C" accesskey="u" rel="up">Using Assembly Language with C</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="Size-of-an-asm-1"></a>
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<h4 class="subsection">6.45.6 Size of an <code>asm</code></h4>
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<p>Some targets require that GCC track the size of each instruction used
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in order to generate correct code. Because the final length of the
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code produced by an <code>asm</code> statement is only known by the
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assembler, GCC must make an estimate as to how big it will be. It
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does this by counting the number of instructions in the pattern of the
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<code>asm</code> and multiplying that by the length of the longest
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instruction supported by that processor. (When working out the number
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of instructions, it assumes that any occurrence of a newline or of
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whatever statement separator character is supported by the assembler —
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typically ‘<samp>;</samp>’ — indicates the end of an instruction.)
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</p>
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<p>Normally, GCC’s estimate is adequate to ensure that correct
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code is generated, but it is possible to confuse the compiler if you use
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pseudo instructions or assembler macros that expand into multiple real
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instructions, or if you use assembler directives that expand to more
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space in the object file than is needed for a single instruction.
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If this happens then the assembler may produce a diagnostic saying that
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a label is unreachable.
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</p>
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<a name="index-asm-inline"></a>
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<p>This size is also used for inlining decisions. If you use <code>asm inline</code>
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instead of just <code>asm</code>, then for inlining purposes the size of the asm
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is taken as the minimum size, ignoring how many instructions GCC thinks it is.
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</p>
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</body>
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</html>
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