[zokier]

If you do want the precision improvement of fma, then it makes far more sense to explicitly call fma instead of relying compiler on doing transformation that might not happen for any number of reasons. The key here is predictability, if it was actually guaranteed that expressions in the form of (x*y) + z are always done with fma, then it'd be less crazy. But now you have no way of knowing without looking at the produced assembly if fma is used or not in any particular expression.

[gnufx]

As I implied, we're normally interested in FMA for speed, not numerical properties. I don't know in what circumstances GCC wouldn't use it when vectorizing, but I haven't seen them.