Somewhat OT: gcc, x86, -ffast-math, and Linux

From: Daniel Forrest
Date: Fri Mar 26 2004 - 15:56:04 EST

I've tried Googling for an answer on this, but have come up empty and
I think it likely that someone here probably knows the answer...

We are testing and breaking in 6 racks of compute nodes, each rack
containing 30 1U boxes, each box containing 2 x 2.8GHz Xeon CPUs.
Each rack contains identical hardware (single purchase) with the
exception that one rack has double the memory per node. The 6 racks
are located in six different labs across our campus. It is available
to me only as a "black box" queueing system.

I am running one of our applications that has been compiled using gcc
with the -ffast-math option. I am finding that the identical program
using the same input data files is producing different results on
different machines. However, the differences are all less than the
precision of a single-precision floating point number. By this I mean
that if the results (which are written to 15 digits of precision) are
only compared to 7 digits then the results are the same. Also, most
of the time the 15 digit values are the same.

My question is this: Why aren't the results always the same? What is
the -ffast-math option doing? How are the excess bits of precision
dealt with during context switches? Shouldn't the same binary with
the same inputs produce the same output on identical hardware?

I have run the same test with the program compiled without -ffast-math
enabled and the results are always identical.

Any insight would be appreciated.

Daniel K. Forrest Laboratory for Molecular and
forrest@xxxxxxxxxxxxx Computational Genomics
University of Wisconsin, Madison
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