From: Doug Sutherland (wearable_at_earthlink.net)
Date: 2002-03-01 09:27:39
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
This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:39 BST