Re: [Valgrind-users] precision problem

[Michael P. <md...@tr...>]

jody writes:

> Hi
>  I'm using valgrind-3.2.1 on fedora 6.
>
> I have encounter a situation where computations with doubles
> give different result when the application is run normally or under valgrind.
>
>
>
> Here is a very simple application showing the problem,  vgt.c:

Your test program behaves differently based on the implementation of
"double".  In particular, the 1950 result depends on the intermediate
results being calculated using 80-bit floats, as is traditional on the
stack of x86 FPUs.  If the compiled code uses 64-bit floats, which
will happen on x86-64 (and most other) machines, if you use gcc's
-ffloat-store flag on x86 machines, or if you use Valgrind, it
produces the 1949 result.

According to http://valgrind.org/help/projects.html, the Vex library
that underlies Valgrind only supports 64-bit floating-point
arithmetic, so there is probably no way to get the 80-bit results
within Valgrind.

Michael Poole