[Clp] patch for Visual C++ floating-point model problem

[Michael Hennebry]

On Sun, 10 Aug 2008, Arno Schödl wrote:

> With my floating-point model problem, I should have probably asked first which floating point model Clp/COIN assumes. This is the GCC compiler thread referring to the same problem:
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
>
> They apparently decided that GCC by default won't guarantee any rounding, similar to MSVC's /fp:fast. With the GCC -ffloat_store option, behavior is similar to MSVC /fp:precise, forcing rounding to 64 bit on assignments. I did not find this option in the COIN-Clp makefiles, and I do not recommend it because it hurts performance. If there is no other provision made that I overlooked, it seems to me that the current Clp code is incorrect, and something along the lines of my patch would be necessary.

IIRC neither C89 nor C++ guarantee much about floating point.
The only guarantees are the definitions of the values in float.h .
In particluar, x+y is not guaranteed to give the same value twice
even if x and y are unchanged, even if the value is exactly representable.

In the x86's, I think that the problem could be worked
around by either by converting all the doubles to long
doubles or by using a compiler in which double is 80 bits.

C99  might be different.

-- 
Michael   hennebry at web.cs.ndsu.NoDak.edu
"Those parts of the system that you can hit with a hammer (not advised)
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