[Valgrind-users] mysterious failures when using valgrind

[Peter v. H. <p.v...@om...>]

Hi,

I am developing a set of unit tests, which I would also like to run 
through valgrind from time to time. On their own the unit tests run 
absolutely fine, but when I run them through valgrind I get zillions of 
failed tests. I tracked this down to an overloaded routine (called 
exp10) which returns a __float128. The prototype is:

__float128 exp10(__float128);

When that routine is called with a negative argument, the return value 
changes when I run the code through valgrind, e.g.

-20 3fbc79ca10c9242235d511e976394d7a (without valgrind)
-20 3fbc79ca10c9240e12445f2000000000 (with valgrind)

The -20 is the argument for exp10() and was obviously converted to 
__float128, and the return value is printed here in hex format. 
Initially I thought that the lowest 64 bits were mangled, but from this 
example you can see that it is a bit more (the lowest 70 bits differ). 
When exp10 is called with a positive argument, the return value is just 
fine, e.g.

9 401cdcd6500000000000000000000000 (without valgrind)
9 401cdcd6500000000000000000000000 (with valgrind)

This is compiled on an amd64 platform using g++ 4.7.1 and valgrind 3.7.0 
(openSUSE 12.2). I also tried openSUSE 12.3 and got the same behavior 
(this is g++ 4.7.2, valgrind 3.8.1). I am totally mystified by this. 
Especially the part where positive arguments work fine and negative do 
not... Is this a valgrind bug?

Peter.