what did they not implement from 754? Given the design of the processor i can imagine them being very aggressive with assumptions around exception handling / traps - iirc this is actually the original reason why Tomasulo algorithm exists because the first out-of-order processors didn't actually make any guarantees about the order that you would observe these side effects.

Alpha punts handling of denormals, infinities and NaNs to software emulation, but that wasn't particularly unusual: some sparc CPUs and early implementations of Arm VFP floating point did the same.

Looking at the alpha architecture manual, the fp emulation traps are imprecise, which imposes constraints on codegen to make it work right: the "trap shadow" extends from the potentially trapping insn until a following trap barrier, and in the shadow you mustn't e.g. use a register more than once as a destination, have a branch, or modify registers that are inputs to any insns in the shadow. (The idea is that the hardware will have already executed some of the insns in the shadow by the time it realises it needs to trap, and the handler has to be able to emulate the trapping insn and resume execution at the insn just after that, so it will re-execute all the insns in the shadow.) That's obviously pretty inconvenient for codegen, so I wouldn't be surprised if the compiler provided some kind of fast-math mode where it didn't trap and you just had to avoid generating denormals, infinities, etc.

I think making the fp using code have to be written carefully to work with the software emulation of edge cases is unusual -- I don't think either sparc or arm imposed that requirement, and instead trap precisely, or at least before anything happens where it would matter that the fp insn is emulated late.