[Bug rtl-optimization/54300] [4.7, 4.8, 4.9 Regression] regcprop incorrectly looks through parallel register swap operation

[rearnsha at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54300

--- Comment #11 from Richard Earnshaw <rearnsha at gcc dot gnu.org> ---
(In reply to Eric Botcazou from comment #10)
> > and regcprop substitues d19 for d18 in insn 27, missing the fact that insn
> > 73 is swapping the two values (thus clobbering the old d19 value).
> 
> It's probably fooled by single_set returning the first set of the insn
> because of the REG_UNUSED note.  I wonder whether it's a generic pitfall of
> single_set.

Hmm, interesting.  Perhaps single_set should not do this if the dead set
clobbers an input.