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Tidy up for ROLI
| author | Geogaddi\David <d.m.ronan@qmul.ac.uk> |
|---|---|
| date | Tue, 17 May 2016 18:50:19 +0100 |
| parents | 636c989477e7 |
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| 1 <html lang="en"> | |
| 2 <head> | |
| 3 <title>Stack alignment on x86 - FFTW 3.2.1</title> | |
| 4 <meta http-equiv="Content-Type" content="text/html"> | |
| 5 <meta name="description" content="FFTW 3.2.1"> | |
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| 7 <link title="Top" rel="start" href="index.html#Top"> | |
| 8 <link rel="up" href="Data-Alignment.html#Data-Alignment" title="Data Alignment"> | |
| 9 <link rel="prev" href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc" title="SIMD alignment and fftw_malloc"> | |
| 10 <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> | |
| 11 <!-- | |
| 12 This manual is for FFTW | |
| 13 (version 3.2.1, 5 February 2009). | |
| 14 | |
| 15 Copyright (C) 2003 Matteo Frigo. | |
| 16 | |
| 17 Copyright (C) 2003 Massachusetts Institute of Technology. | |
| 18 | |
| 19 Permission is granted to make and distribute verbatim copies of | |
| 20 this manual provided the copyright notice and this permission | |
| 21 notice are preserved on all copies. | |
| 22 | |
| 23 Permission is granted to copy and distribute modified versions of | |
| 24 this manual under the conditions for verbatim copying, provided | |
| 25 that the entire resulting derived work is distributed under the | |
| 26 terms of a permission notice identical to this one. | |
| 27 | |
| 28 Permission is granted to copy and distribute translations of this | |
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| 30 modified versions, except that this permission notice may be | |
| 31 stated in a translation approved by the Free Software Foundation. | |
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| 44 --></style> | |
| 45 </head> | |
| 46 <body> | |
| 47 <div class="node"> | |
| 48 <p> | |
| 49 <a name="Stack-alignment-on-x86"></a> | |
| 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>, | |
| 51 Up: <a rel="up" accesskey="u" href="Data-Alignment.html#Data-Alignment">Data Alignment</a> | |
| 52 <hr> | |
| 53 </div> | |
| 54 | |
| 55 <h4 class="subsection">3.1.2 Stack alignment on x86</h4> | |
| 56 | |
| 57 <p>On the Pentium and subsequent x86 processors, there is a substantial | |
| 58 performance penalty if double-precision variables are not stored | |
| 59 8-byte aligned; a factor of two or more is not unusual. | |
| 60 Unfortunately, the stack (the place that local variables and | |
| 61 subroutine arguments live) is not guaranteed by the Intel ABI to be | |
| 62 8-byte aligned. | |
| 63 | |
| 64 <p>Recent versions of <code>gcc</code> (as well as most other compilers, we are | |
| 65 told, such as Intel's, Metrowerks', and Microsoft's) are able to keep | |
| 66 the stack 8-byte aligned; <code>gcc</code> does this by default (see | |
| 67 <code>-mpreferred-stack-boundary</code> in the <code>gcc</code> documentation). | |
| 68 If you are not certain whether your compiler maintains stack alignment | |
| 69 by default, it is a good idea to make sure. | |
| 70 | |
| 71 <p>Unfortunately, <code>gcc</code> only <em>preserves</em> the stack | |
| 72 alignment—as a result, if the stack starts off misaligned, it will | |
| 73 always be misaligned, with a disastrous effect on performance (in | |
| 74 double precision). To prevent this, FFTW includes hacks to align its | |
| 75 own stack if necessary, so it should perform well even if you call it | |
| 76 from a program with a misaligned stack. Currently, our hacks support | |
| 77 <code>gcc</code> and the Intel C compiler; if you use another compiler you | |
| 78 are on your own. Fortunately, recent versions of glibc (on GNU/Linux) | |
| 79 provide a properly-aligned starting stack, but this was not the case | |
| 80 with a number of older versions, and we are not certain of the | |
| 81 situation on other operating systems. Hopefully, as time goes by this | |
| 82 will become less of a concern. | |
| 83 | |
| 84 <!-- --> | |
| 85 </body></html> | |
| 86 |