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From: "jchthys at yahoo dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/49148] Certain expressions take an extremely long time for no apparent reason
Date: Wed, 25 May 2011 14:45:00 -0000	[thread overview]
Message-ID: <bug-49148-4-91yuXLuZuS@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-49148-4@http.gcc.gnu.org/bugzilla/>

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49148

--- Comment #7 from John Peyton <jchthys at yahoo dot com> 2011-05-25 14:40:56 UTC ---
> Try -ffast-math (the default for ICC) which disables subnormals.

Thanks, this helped it run much faster!

I noticed it still takes longer to divide by 3.0 than by 4.0. More
specifically, the following line was fast:

     new[i] = (A[i-1] + 2*A[i] + A[i+1]) / 4.0;

The following line took about 4 times as long to run without -ffast-math:

     new[i] = (A[i-1] + A[i] + A[i+1]) / 3.0;

With -ffast-math, it was faster; however, it was still took about 2 times as
long as the division by 4.0 version. In addition, changing division by 3.0 to
multiplication by 0.333 or even 0.3 did not affect the speed.

Could this also be related to subnormal numbers, or is this just a different
issue?


  parent reply	other threads:[~2011-05-25 14:41 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-05-24 19:14 [Bug c/49148] New: " jchthys at yahoo dot com
2011-05-24 19:16 ` [Bug c/49148] " jchthys at yahoo dot com
2011-05-24 19:41 ` jchthys at yahoo dot com
2011-05-24 20:06 ` jchthys at yahoo dot com
2011-05-24 20:42 ` pinskia at gcc dot gnu.org
2011-05-24 21:47 ` [Bug rtl-optimization/49148] " jchthys at yahoo dot com
2011-05-24 21:55 ` jchthys at yahoo dot com
2011-05-25  9:56 ` rguenth at gcc dot gnu.org
2011-05-25 14:45 ` jchthys at yahoo dot com [this message]
2021-12-26 12:49 ` pinskia at gcc dot gnu.org

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