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1 <html lang="en">
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2 <head>
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3 <title>Stack alignment on x86 - FFTW 3.2.1</title>
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4 <meta http-equiv="Content-Type" content="text/html">
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5 <meta name="description" content="FFTW 3.2.1">
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6 <meta name="generator" content="makeinfo 4.8">
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7 <link title="Top" rel="start" href="index.html#Top">
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8 <link rel="up" href="Data-Alignment.html#Data-Alignment" title="Data Alignment">
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9 <link rel="prev" href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc" title="SIMD alignment and fftw_malloc">
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10 <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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11 <!--
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12 This manual is for FFTW
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13 (version 3.2.1, 5 February 2009).
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14
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15 Copyright (C) 2003 Matteo Frigo.
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16
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17 Copyright (C) 2003 Massachusetts Institute of Technology.
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18
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19 Permission is granted to make and distribute verbatim copies of
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20 this manual provided the copyright notice and this permission
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21 notice are preserved on all copies.
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22
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23 Permission is granted to copy and distribute modified versions of
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24 this manual under the conditions for verbatim copying, provided
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25 that the entire resulting derived work is distributed under the
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26 terms of a permission notice identical to this one.
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27
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28 Permission is granted to copy and distribute translations of this
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29 manual into another language, under the above conditions for
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30 modified versions, except that this permission notice may be
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31 stated in a translation approved by the Free Software Foundation.
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32 -->
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34 <style type="text/css"><!--
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44 --></style>
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45 </head>
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46 <body>
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47 <div class="node">
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48 <p>
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49 <a name="Stack-alignment-on-x86"></a>
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50 Previous: <a rel="previous" accesskey="p" href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>,
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51 Up: <a rel="up" accesskey="u" href="Data-Alignment.html#Data-Alignment">Data Alignment</a>
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52 <hr>
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53 </div>
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54
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55 <h4 class="subsection">3.1.2 Stack alignment on x86</h4>
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56
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57 <p>On the Pentium and subsequent x86 processors, there is a substantial
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58 performance penalty if double-precision variables are not stored
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59 8-byte aligned; a factor of two or more is not unusual.
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60 Unfortunately, the stack (the place that local variables and
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61 subroutine arguments live) is not guaranteed by the Intel ABI to be
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62 8-byte aligned.
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63
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64 <p>Recent versions of <code>gcc</code> (as well as most other compilers, we are
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65 told, such as Intel's, Metrowerks', and Microsoft's) are able to keep
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66 the stack 8-byte aligned; <code>gcc</code> does this by default (see
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67 <code>-mpreferred-stack-boundary</code> in the <code>gcc</code> documentation).
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68 If you are not certain whether your compiler maintains stack alignment
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69 by default, it is a good idea to make sure.
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70
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71 <p>Unfortunately, <code>gcc</code> only <em>preserves</em> the stack
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72 alignment—as a result, if the stack starts off misaligned, it will
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73 always be misaligned, with a disastrous effect on performance (in
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74 double precision). To prevent this, FFTW includes hacks to align its
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75 own stack if necessary, so it should perform well even if you call it
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76 from a program with a misaligned stack. Currently, our hacks support
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77 <code>gcc</code> and the Intel C compiler; if you use another compiler you
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78 are on your own. Fortunately, recent versions of glibc (on GNU/Linux)
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79 provide a properly-aligned starting stack, but this was not the case
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80 with a number of older versions, and we are not certain of the
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81 situation on other operating systems. Hopefully, as time goes by this
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82 will become less of a concern.
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83
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84 <!-- -->
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85 </body></html>
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