Mercurial > hg > batch-feature-extraction-tool

<html lang="en">
<head>
<title>Stack alignment on x86 - FFTW 3.2.1</title>
<meta http-equiv="Content-Type" content="text/html">
<meta name="description" content="FFTW 3.2.1">
<meta name="generator" content="makeinfo 4.8">
<link title="Top" rel="start" href="index.html#Top">
<link rel="up" href="Data-Alignment.html#Data-Alignment" title="Data Alignment">
<link rel="prev" href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc" title="SIMD alignment and fftw_malloc">
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
<!--
This manual is for FFTW
(version 3.2.1, 5 February 2009).

Copyright (C) 2003 Matteo Frigo.

Copyright (C) 2003 Massachusetts Institute of Technology.

     Permission is granted to make and distribute verbatim copies of
     this manual provided the copyright notice and this permission
     notice are preserved on all copies.

     Permission is granted to copy and distribute modified versions of
     this manual under the conditions for verbatim copying, provided
     that the entire resulting derived work is distributed under the
     terms of a permission notice identical to this one.

     Permission is granted to copy and distribute translations of this
     manual into another language, under the above conditions for
     modified versions, except that this permission notice may be
     stated in a translation approved by the Free Software Foundation.
   -->
<meta http-equiv="Content-Style-Type" content="text/css">
<style type="text/css"><!--
  pre.display { font-family:inherit }
  pre.format  { font-family:inherit }
  pre.smalldisplay { font-family:inherit; font-size:smaller }
  pre.smallformat  { font-family:inherit; font-size:smaller }
  pre.smallexample { font-size:smaller }
  pre.smalllisp    { font-size:smaller }
  span.sc    { font-variant:small-caps }
  span.roman { font-family:serif; font-weight:normal; }
  span.sansserif { font-family:sans-serif; font-weight:normal; }
--></style>
</head>
<body>
<div class="node">
<p>
<a name="Stack-alignment-on-x86"></a>
Previous:&nbsp;<a rel="previous" accesskey="p" href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>,
Up:&nbsp;<a rel="up" accesskey="u" href="Data-Alignment.html#Data-Alignment">Data Alignment</a>
<hr>
</div>

<h4 class="subsection">3.1.2 Stack alignment on x86</h4>

<p>On the Pentium and subsequent x86 processors, there is a substantial
performance penalty if double-precision variables are not stored
8-byte aligned; a factor of two or more is not unusual.
Unfortunately, the stack (the place that local variables and
subroutine arguments live) is not guaranteed by the Intel ABI to be
8-byte aligned.

   <p>Recent versions of <code>gcc</code> (as well as most other compilers, we are
told, such as Intel's, Metrowerks', and Microsoft's) are able to keep
the stack 8-byte aligned; <code>gcc</code> does this by default (see
<code>-mpreferred-stack-boundary</code> in the <code>gcc</code> documentation).
If you are not certain whether your compiler maintains stack alignment
by default, it is a good idea to make sure.

   <p>Unfortunately, <code>gcc</code> only <em>preserves</em> the stack
alignment&mdash;as a result, if the stack starts off misaligned, it will
always be misaligned, with a disastrous effect on performance (in
double precision).  To prevent this, FFTW includes hacks to align its
own stack if necessary, so it should perform well even if you call it
from a program with a misaligned stack.  Currently, our hacks support
<code>gcc</code> and the Intel C compiler; if you use another compiler you
are on your own.  Fortunately, recent versions of glibc (on GNU/Linux)
provide a properly-aligned starting stack, but this was not the case
with a number of older versions, and we are not certain of the
situation on other operating systems.  Hopefully, as time goes by this
will become less of a concern.

<!--  -->
</body></html>
author	Geogaddi\David <d.m.ronan@qmul.ac.uk>
date	Wed, 04 May 2016 11:02:59 +0100
parents	25bf17994ef1
children