> It looks like this is an existing (potential) problem,
> but default_noce_conversion_profitable_p uses seq_cost, which in turn
> uses insn_cost.  And insn_cost has an optional target hook behind it,
> which allows for costing based on insn attributes etc.  For a true
> apples-with-apples comparison we should use insn_cost here too.

Good point, fixed that.

> I think the detail that COSTS_N_INSNS (2) is the default is useful here.
> (In other words, I'd forgotten by the time I'd poked around other bits of
> ifcvt and was about to ask why we didn't cost the condition “properly”.)
> So how about something like:
> 
>     The original costs already include a base cost of COSTS_N_INSNS (2):
>     one instruction for the compare (which we will be needing either way)
>     and one instruction for the branch.

Yes, this is much clearer. I went with that wording in the attached v2.

Regards
  Robin