Hi,
I think the behavior you described can be expected in some cases. It is
most likely not a difference in floating point representation(thanks to
ieee754). different instruction sequences are expected to produce
different results thanks to rounding errors, especially in your case
where the result is used over and over. the errors grow with each
step!!! (if you haven't seen it before) You have to check out Goldberg
1992 "What every scientist should know about floating point" he shows
you how to estimate an upper bound on the rounding error. The bound
depends on the number and type of and ordering of flops. That is why
different sequences can produce different results. Also for curiosity
sake you might take a close look at the fpu control word, and verify
that the same mode is being used on both builds. Setting the control
word is something that the compiler is responsible for but not part of
the ieee754 standard. It is conceivable(but unlikely) that the FPU is
being configured differently on the two systems. (see Intel "Programming
with the x87 fpu"), specifically the rounding control field.
HTH
--
Burlen Loring
Information Technologist III
Space Science Center
Institute for the Study of Earth, Oceans, and Space
University of New Hampshire
39 College Road, Durham, NH 03824
Phone: 603-862-1140
Cristea Bogdan wrote:
Hi
I use gcc compiler in order to study the behavior of some iterative
maps (x_n=f(x_{n-1})) using single floating point precision (following
IEEE 754 standard). I have noticed that using different compiler
options, the output sequence could be very different. My programs are
written in C++ using 'float' type and I use 'gcc version 4.1.2
20061115 (prerelease) (Debian 4.1.1-21)'. Compilation options are:
'-Wall -O3 -march=pentium4' or '-Wall'. The same program compiled with
gcc 4.2.2 gives the same results regardless of the compilation options
(on an athlon64 processor).
Is this a bug of the compiler or the floating point representation
differs from one compiler version to the other?
