[chrisseaton]

If the explicit fused instruction (so VFMADD..., we're not talking micro-code fusing, are we?) doesn't have the same semantics as a separate multiply and add then it's a compiler bug to select it, isn't it?

I think if you're talking micro-code fusing then Intel does guarantee it has exactly the same semantics as a separate multiply and add.

Interesting fact - I believe modern Intel architectures actually only have fused multiply add. If you do just a multiply it'll do a fused multiply and add zero.