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126 lines
6.1 KiB
HTML
126 lines
6.1 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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<!-- This file documents the GNU Assembler "as".
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Copyright (C) 1991-2019 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
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or any later version published by the Free Software Foundation;
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with no Invariant Sections, with no Front-Cover Texts, and with no
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Back-Cover Texts. A copy of the license is included in the
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section entitled "GNU Free Documentation License".
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-->
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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>Xtensa Automatic Alignment (Using as)</title>
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<meta name="description" content="Xtensa Automatic Alignment (Using as)">
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<meta name="keywords" content="Xtensa Automatic Alignment (Using as)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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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="AS-Index.html#AS-Index" rel="index" title="AS Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Xtensa-Optimizations.html#Xtensa-Optimizations" rel="up" title="Xtensa Optimizations">
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<link href="Xtensa-Relaxation.html#Xtensa-Relaxation" rel="next" title="Xtensa Relaxation">
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<link href="Density-Instructions.html#Density-Instructions" rel="prev" title="Density Instructions">
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<style type="text/css">
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a.summary-letter {text-decoration: none}
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div.display {margin-left: 3.2em}
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div.example {margin-left: 3.2em}
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kbd {font-style: oblique}
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pre.display {font-family: inherit}
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pre.format {font-family: inherit}
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pre.smallexample {font-size: smaller}
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pre.smalllisp {font-size: smaller}
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span.nolinebreak {white-space: nowrap}
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span.roman {font-family: initial; font-weight: normal}
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span.sansserif {font-family: sans-serif; font-weight: normal}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Xtensa-Automatic-Alignment"></a>
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<div class="header">
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<p>
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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> [<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>
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</div>
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<hr>
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<a name="Automatic-Instruction-Alignment"></a>
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<h4 class="subsubsection">9.54.3.2 Automatic Instruction Alignment</h4>
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<a name="index-alignment-of-LOOP-instructions"></a>
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<a name="index-alignment-of-branch-targets"></a>
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<a name="index-LOOP-instructions_002c-alignment"></a>
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<a name="index-branch-target-alignment"></a>
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<p>The Xtensa assembler will automatically align certain instructions, both
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to optimize performance and to satisfy architectural requirements.
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</p>
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<p>As an optimization to improve performance, the assembler attempts to
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align branch targets so they do not cross instruction fetch boundaries.
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(Xtensa processors can be configured with either 32-bit or 64-bit
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instruction fetch widths.) An
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instruction immediately following a call is treated as a branch target
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in this context, because it will be the target of a return from the
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call. This alignment has the potential to reduce branch penalties at
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some expense in code size.
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This optimization is enabled by default. You can disable it with the
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‘<samp>--no-target-align</samp>’ command-line option (see <a href="Xtensa-Options.html#Xtensa-Options">Command-line Options</a>).
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</p>
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<p>The target alignment optimization is done without adding instructions
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that could increase the execution time of the program. If there are
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density instructions in the code preceding a target, the assembler can
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change the target alignment by widening some of those instructions to
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the equivalent 24-bit instructions. Extra bytes of padding can be
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inserted immediately following unconditional jump and return
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instructions.
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This approach is usually successful in aligning many, but not all,
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branch targets.
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</p>
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<p>The <code>LOOP</code> family of instructions must be aligned such that the
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first instruction in the loop body does not cross an instruction fetch
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boundary (e.g., with a 32-bit fetch width, a <code>LOOP</code> instruction
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must be on either a 1 or 2 mod 4 byte boundary). The assembler knows
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about this restriction and inserts the minimal number of 2 or 3 byte
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no-op instructions to satisfy it. When no-op instructions are added,
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any label immediately preceding the original loop will be moved in order
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to refer to the loop instruction, not the newly generated no-op
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instruction. To preserve binary compatibility across processors with
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different fetch widths, the assembler conservatively assumes a 32-bit
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fetch width when aligning <code>LOOP</code> instructions (except if the first
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instruction in the loop is a 64-bit instruction).
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</p>
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<p>Previous versions of the assembler automatically aligned <code>ENTRY</code>
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instructions to 4-byte boundaries, but that alignment is now the
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programmer’s responsibility.
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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="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> [<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>
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</div>
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</body>
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</html>
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