Aliasing pitfall in std::valarray

[Bastian Pranzas]

Ah, found it, thanks.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48365

Seems to refer to "The valarray array classes are defined to be free of 
certain forms of aliasing, thus allowing operations on these classes to 
be optimized." in the standard. So it seems users of that class have to 
be aware of such issues and guard against them.

So this is merely a pitfall not a bug.

I still have a question:

Could this particular pitfall not be avoided "for free" by making a 
temporary copy of the scalar(s) involved in a valarray assignment? Since 
this kind of problem only seems to happen when scalars are involved, 
making a copy should be cheap. And since the scalar is guaranteed to be 
constant throughout the operation, it might even improve the 
optimization opportunities for the compiler. As it is, the compiler 
can't possibly emit vector operations for example unless it can prove 
that no aliasing happens.

Regards,

Bastian Pranzas