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Re: GCC beaten by ICC in stupid trig test!
- From: Scott Robert Ladd <coyote at coyotegulch dot com>
- To: Roger Sayle <roger at eyesopen dot com>
- Cc: gcc at gcc dot gnu dot org
- Date: Wed, 24 Mar 2004 19:21:08 -0500
- Subject: Re: GCC beaten by ICC in stupid trig test!
- References: <Pine.LNX.firstname.lastname@example.org>
Roger Sayle wrote:
May I remind everyone that the subject title "GCC beaten by ICC in
stupid trig test!" refers to a posting by Scott Robert Ladd...
This thread has wandered a tad far from it's original topic, hasn't it?
I also note that my original subject did not contain the word "stupid",
which was added by someone who with some badly mistaken notions and a
Back to the original topic: Doing a "fair" comparison between ICC and
GCC is problematic at best, given that neither compiler provides
complete documentation about their various options. For example, it
appears the ICC does "unsafe" math by default, leading me to suspect
that I should use ICC's -mp or -mp1 switches when comparing against GCC.
But I'm not certain if "icc -mp" is *really* equivalent to a plain "gcc"
At least in the case of GCC, I can study the source code to find every
instance where -ffast-math affects code generation... however, the
average compiler user has neither the skills or time to examine the
compiler source code for indications of its behavior.
What a numerical programmer needs to know is: How, exactly, do all of
these switches affect accuracy?
Accuracy benchmarks are few and far between, and many are hoary old
codes translated badly from antiquated versions of Fortran. I've found
that most of these "accuracy" benchmarks produce identical results with
and without -ffast-math; when there are differences, it is trivial, and
in one case, -ffast-math actually *improved* accuracy.
Of course, the definition of "accuracy" is somewhat nebulous. For some
programs, it is important that identical results be produced on any
platform; for other programs, accuracy reflects precision.
And, of course, most programmers forget the mathematical rules about
significant digits in source data. If I multiply 1.5 by 3.1415927, the
answer is 4.7, not 4.71238905, unless I know for a fact that 1.5 is
exact, and not some measurement of a value between 1.49 and 1.51 (for
Ah, but I now enter the realm of interval arithmetic, and am drifting
from my own topic.
Hence my desire to write a new accuracy benchmark, something I'm doing
whenever I have some of that elusive "free time." ;)
Scott Robert Ladd
Coyote Gulch Productions (http://www.coyotegulch.com)
Software Invention for High-Performance Computing