huangmeng
/
Embedded_AI


			
							123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126
							<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<!-- This file documents the GNU Assembler "as".

Copyright (C) 1991-2017 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 no Invariant Sections, with no Front-Cover Texts, and with no
Back-Cover Texts.  A copy of the license is included in the
section entitled "GNU Free Documentation License".
 -->
<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
<head>
<title>Using as: Xtensa Automatic Alignment</title>

<meta name="description" content="Using as: Xtensa Automatic Alignment">
<meta name="keywords" content="Using as: Xtensa Automatic Alignment">
<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="AS-Index.html#AS-Index" rel="index" title="AS Index">
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
<link href="Xtensa-Optimizations.html#Xtensa-Optimizations" rel="up" title="Xtensa Optimizations">
<link href="Xtensa-Relaxation.html#Xtensa-Relaxation" rel="next" title="Xtensa Relaxation">
<link href="Density-Instructions.html#Density-Instructions" rel="prev" title="Density Instructions">
<style type="text/css">
<!--
a.summary-letter {text-decoration: none}
blockquote.smallquotation {font-size: smaller}
div.display {margin-left: 3.2em}
div.example {margin-left: 3.2em}
div.indentedblock {margin-left: 3.2em}
div.lisp {margin-left: 3.2em}
div.smalldisplay {margin-left: 3.2em}
div.smallexample {margin-left: 3.2em}
div.smallindentedblock {margin-left: 3.2em; font-size: smaller}
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.nocodebreak {white-space:nowrap}
span.nolinebreak {white-space:nowrap}
span.roman {font-family:serif; font-weight:normal}
span.sansserif {font-family:sans-serif; font-weight:normal}
ul.no-bullet {list-style: none}
-->
</style>


</head>

<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
<a name="Xtensa-Automatic-Alignment"></a>
<div class="header">
<p>
Previous: <a href="Density-Instructions.html#Density-Instructions" accesskey="p" rel="prev">Density Instructions</a>, Up: <a href="Xtensa-Optimizations.html#Xtensa-Optimizations" accesskey="u" rel="up">Xtensa Optimizations</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="AS-Index.html#AS-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Automatic-Instruction-Alignment"></a>
<h4 class="subsubsection">9.53.3.2 Automatic Instruction Alignment</h4>
<a name="index-alignment-of-LOOP-instructions"></a>
<a name="index-alignment-of-branch-targets"></a>
<a name="index-LOOP-instructions_002c-alignment"></a>
<a name="index-branch-target-alignment"></a>

<p>The Xtensa assembler will automatically align certain instructions, both
to optimize performance and to satisfy architectural requirements.
</p>
<p>As an optimization to improve performance, the assembler attempts to
align branch targets so they do not cross instruction fetch boundaries.
(Xtensa processors can be configured with either 32-bit or 64-bit
instruction fetch widths.)  An
instruction immediately following a call is treated as a branch target
in this context, because it will be the target of a return from the
call.  This alignment has the potential to reduce branch penalties at
some expense in code size.
This optimization is enabled by default.  You can disable it with the
&lsquo;<samp>--no-target-align</samp>&rsquo; command-line option (see <a href="Xtensa-Options.html#Xtensa-Options">Command Line Options</a>).
</p>
<p>The target alignment optimization is done without adding instructions
that could increase the execution time of the program.  If there are
density instructions in the code preceding a target, the assembler can
change the target alignment by widening some of those instructions to
the equivalent 24-bit instructions.  Extra bytes of padding can be
inserted immediately following unconditional jump and return
instructions.
This approach is usually successful in aligning many, but not all,
branch targets.
</p>
<p>The <code>LOOP</code> family of instructions must be aligned such that the
first instruction in the loop body does not cross an instruction fetch
boundary (e.g., with a 32-bit fetch width, a <code>LOOP</code> instruction
must be on either a 1 or 2 mod 4 byte boundary).  The assembler knows
about this restriction and inserts the minimal number of 2 or 3 byte
no-op instructions to satisfy it.  When no-op instructions are added,
any label immediately preceding the original loop will be moved in order
to refer to the loop instruction, not the newly generated no-op
instruction.  To preserve binary compatibility across processors with
different fetch widths, the assembler conservatively assumes a 32-bit
fetch width when aligning <code>LOOP</code> instructions (except if the first
instruction in the loop is a 64-bit instruction).
</p>
<p>Previous versions of the assembler automatically aligned <code>ENTRY</code>
instructions to 4-byte boundaries, but that alignment is now the
programmer&rsquo;s responsibility.
</p>
<hr>
<div class="header">
<p>
Previous: <a href="Density-Instructions.html#Density-Instructions" accesskey="p" rel="prev">Density Instructions</a>, Up: <a href="Xtensa-Optimizations.html#Xtensa-Optimizations" accesskey="u" rel="up">Xtensa Optimizations</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="AS-Index.html#AS-Index" title="Index" rel="index">Index</a>]</p>
</div>


</body>
</html>