very confused about single_set

[Zdenek Dvorak]

Hello,

> 1) single_set is built on top of single_set2.
> 2) single_set_2 uses REG_UNUSED notes.
> 3) tree-ssa-loop-ivops.c:seq_cost uses single_set.
> 
> I can find no indication that rtl dfa is run here to provide the
> information for single_set to produce the correct answer.

it is not.  Nevertheless, the notes are only relevant for insns with
more than one set, which are not common enough to care about them.
(also, seq_cost will disappear soon).

Zdenek