You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
108 lines
5.0 KiB
HTML
108 lines
5.0 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
|
<html>
|
|
<!-- Copyright (C) 1988-2018 Free Software Foundation, Inc.
|
|
|
|
Permission is granted to copy, distribute and/or modify this document
|
|
under the terms of the GNU Free Documentation License, Version 1.3 or
|
|
any later version published by the Free Software Foundation; with the
|
|
Invariant Sections being "Funding Free Software", the Front-Cover
|
|
Texts being (a) (see below), and with the Back-Cover Texts being (b)
|
|
(see below). A copy of the license is included in the section entitled
|
|
"GNU Free Documentation License".
|
|
|
|
(a) The FSF's Front-Cover Text is:
|
|
|
|
A GNU Manual
|
|
|
|
(b) The FSF's Back-Cover Text is:
|
|
|
|
You have freedom to copy and modify this GNU Manual, like GNU
|
|
software. Copies published by the Free Software Foundation raise
|
|
funds for GNU development. -->
|
|
<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
|
|
<head>
|
|
<title>Size of an asm (Using the GNU Compiler Collection (GCC))</title>
|
|
|
|
<meta name="description" content="Size of an asm (Using the GNU Compiler Collection (GCC))">
|
|
<meta name="keywords" content="Size of an asm (Using the GNU Compiler Collection (GCC))">
|
|
<meta name="resource-type" content="document">
|
|
<meta name="distribution" content="global">
|
|
<meta name="Generator" content="makeinfo">
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
|
<link href="index.html#Top" rel="start" title="Top">
|
|
<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
|
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
|
<link href="Using-Assembly-Language-with-C.html#Using-Assembly-Language-with-C" rel="up" title="Using Assembly Language with C">
|
|
<link href="Alternate-Keywords.html#Alternate-Keywords" rel="next" title="Alternate Keywords">
|
|
<link href="Local-Register-Variables.html#Local-Register-Variables" rel="prev" title="Local Register Variables">
|
|
<style type="text/css">
|
|
<!--
|
|
a.summary-letter {text-decoration: none}
|
|
blockquote.indentedblock {margin-right: 0em}
|
|
blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
|
|
blockquote.smallquotation {font-size: smaller}
|
|
div.display {margin-left: 3.2em}
|
|
div.example {margin-left: 3.2em}
|
|
div.lisp {margin-left: 3.2em}
|
|
div.smalldisplay {margin-left: 3.2em}
|
|
div.smallexample {margin-left: 3.2em}
|
|
div.smalllisp {margin-left: 3.2em}
|
|
kbd {font-style: oblique}
|
|
pre.display {font-family: inherit}
|
|
pre.format {font-family: inherit}
|
|
pre.menu-comment {font-family: serif}
|
|
pre.menu-preformatted {font-family: serif}
|
|
pre.smalldisplay {font-family: inherit; font-size: smaller}
|
|
pre.smallexample {font-size: smaller}
|
|
pre.smallformat {font-family: inherit; font-size: smaller}
|
|
pre.smalllisp {font-size: smaller}
|
|
span.nolinebreak {white-space: nowrap}
|
|
span.roman {font-family: initial; font-weight: normal}
|
|
span.sansserif {font-family: sans-serif; font-weight: normal}
|
|
ul.no-bullet {list-style: none}
|
|
-->
|
|
</style>
|
|
|
|
|
|
</head>
|
|
|
|
<body lang="en">
|
|
<a name="Size-of-an-asm"></a>
|
|
<div class="header">
|
|
<p>
|
|
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>
|
|
</div>
|
|
<hr>
|
|
<a name="Size-of-an-asm-1"></a>
|
|
<h4 class="subsection">6.45.6 Size of an <code>asm</code></h4>
|
|
|
|
<p>Some targets require that GCC track the size of each instruction used
|
|
in order to generate correct code. Because the final length of the
|
|
code produced by an <code>asm</code> statement is only known by the
|
|
assembler, GCC must make an estimate as to how big it will be. It
|
|
does this by counting the number of instructions in the pattern of the
|
|
<code>asm</code> and multiplying that by the length of the longest
|
|
instruction supported by that processor. (When working out the number
|
|
of instructions, it assumes that any occurrence of a newline or of
|
|
whatever statement separator character is supported by the assembler —
|
|
typically ‘<samp>;</samp>’ — indicates the end of an instruction.)
|
|
</p>
|
|
<p>Normally, GCC’s estimate is adequate to ensure that correct
|
|
code is generated, but it is possible to confuse the compiler if you use
|
|
pseudo instructions or assembler macros that expand into multiple real
|
|
instructions, or if you use assembler directives that expand to more
|
|
space in the object file than is needed for a single instruction.
|
|
If this happens then the assembler may produce a diagnostic saying that
|
|
a label is unreachable.
|
|
</p>
|
|
<a name="index-asm-inline"></a>
|
|
<p>This size is also used for inlining decisions. If you use <code>asm inline</code>
|
|
instead of just <code>asm</code>, then for inlining purposes the size of the asm
|
|
is taken as the minimum size, ignoring how many instructions GCC thinks it is.
|
|
</p>
|
|
|
|
|
|
|
|
</body>
|
|
</html>
|