Author mark.dickinson
Recipients loewis, mark.dickinson
Date 2008-05-22.15:01:54
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Message-id <1211468520.37.0.530631894493.issue2937@psf.upfronthosting.co.za>
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Okay; so this is definitely not a Python bug---it's a well-known
and well-documented problem with IA32 floating-point.  And I accept
that it's really not Python's responsibility to document this, either.

Nevertheless, it was a surprise to me when my (supposedly IEEE 754
compliant) Pentium 4 box produced this.  I probably shouldn't have
been surprised. I'm aware of issues with 80-bit extended precision when 
programming in C, but naively expected that Python would be largely 
immune from these, since it's always going to force intermediate results 
from (80-bit) floating-point registers into (64-bit) memory slots.

There's an excellent recent article by David Monniaux, "The pitfalls of
verifying floating-point computations.", that's available online at

http://hal.archives-ouvertes.fr/hal-00128124

that explains exactly what's going on here (it's a case of double-
rounding, as described in section 3.1.2 of that paper).

Do you think a documentation patch that added this reference, along with 
the oft-quoted "What Every Computer Scientist Should Know About 
Floating-Point Arithmetic" by David Goldberg, to Appendix B of the 
tutorial would be acceptable?

One other thing that's worth mentioning:  on Pentium 4 and later, the
gcc flags "-mfpmath=sse -msse2" appear to fix the problem, by forcing 
gcc to use the SSE floating-point unit instead of the x87-derived one.

In any case, I guess this report should be closed as 'invalid', but I 
hope that at least others who encounter this problem manage to find this 
bug report.
History
Date User Action Args
2008-05-22 15:02:03mark.dickinsonsetspambayes_score: 0.00105006 -> 0.00105006
recipients: + mark.dickinson, loewis
2008-05-22 15:02:00mark.dickinsonsetspambayes_score: 0.00105006 -> 0.00105006
messageid: <1211468520.37.0.530631894493.issue2937@psf.upfronthosting.co.za>
2008-05-22 15:01:59mark.dickinsonlinkissue2937 messages
2008-05-22 15:01:54mark.dickinsoncreate