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2368Re: [buildcheapeeg] BWView

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  • Doug Sutherland
    Mar 1, 2002
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      Jim,

      According to the FFTW FAQ, the enable-i386-hacks is for double
      precision transforms:

      The configure option --enable-i386-hacks enables specific
      optimizations for the Pentium and later x86 CPUs under gcc,
      which can significantly improve performance of double-
      precision transforms. <snip> These hacks provide a workaround
      to the incorrect alignment of local double variables in gcc.
      The compiler aligns these variables to multiples of 4 bytes,
      but execution is much faster (on Pentium and PentiumPro) if
      doubles are aligned to a multiple of 8 bytes.

      If we are doing only single precision transforms, perhaps
      we don't need to enable the pentium hacks? There seems to
      be some bugs in these hacks:

      In principle, these hacks are no longer required under gcc
      versions 2.95 and later, which automatically align the
      stack correctly (see -mpreferred-stack-boundary in the gcc
      manual). However, we have encountered a bug in the stack
      alignment of versions 2.95.[012] that causes FFTW's stack
      to be misaligned under some circumstances. The configure
      script automatically detects this bug and disables gcc's
      stack alignment in favor of our own hacks when
      --enable-i386-hacks is used.

      From http://www.fftw.org/doc/fftw_6.html#SEC70

      Can we skip the enable-i386-hacks for single precision?

      -- Doug
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