From: Robert Dewar <dewar@gnat.com>
To: Joost VandeVondele <jv244@hermes.cam.ac.uk>
Cc: gcc@gcc.gnu.org
Subject: Re: (a+b)+c should be replaced by a+(b+c)
Date: Thu, 25 Mar 2004 14:45:00 -0000 [thread overview]
Message-ID: <4062D287.7050100@gnat.com> (raw)
In-Reply-To: <Pine.SOL.4.58.0403250711490.18567@yellow.csi.cam.ac.uk>
Joost VandeVondele wrote:
> good compilers (e.g. xlf90) will (at -O4) do higher order transforms of
> the loop to introduce blocking, independent FMAs, ... that makes this
> little piece of code about 100 times faster at O4 than O2 (what about
> LNO/SSA?). This can only be done if you allow (a+b)+c -> a+(b+c). It is
> basically what any optimized blas routine will do. Matrix multiply is a
> trivial example, if you want blas performance, call blas. There are many
> other kernels like this in e.g. scientific code that are not blas. You
> can't expect a scientist to hand unroll and block any kernel to the
> appropriate depth for any machine. There need to be a compiler option to
> do this. This can only be done if you allow (a+b)+c -> a+(b+c).
Can you really deduce this freedom from later versions of the Fortran
standard?
next prev parent reply other threads:[~2004-03-25 12:37 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2004-03-25 12:03 Joost VandeVondele
2004-03-25 14:45 ` Robert Dewar [this message]
2004-03-25 15:07 ` Joost VandeVondele
2004-03-25 15:18 ` Robert Dewar
2004-03-25 15:32 ` Joost VandeVondele
2004-03-25 15:59 ` Scott Robert Ladd
2004-03-25 16:18 ` Jakub Jelinek
2004-03-25 16:38 ` Scott Robert Ladd
2004-03-25 19:47 ` Laurent GUERBY
2004-03-25 20:16 ` Scott Robert Ladd
2004-03-26 2:51 ` Gabriel Paubert
2004-03-26 3:17 ` Jakub Jelinek
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