https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88233
Segher Boessenkool <segher at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |ASSIGNED
Last reconfirmed| |2018-11-28
Assignee|unassigned at gcc dot gnu.org |segher at gcc dot
gnu.org
Ever confirmed|0 |1
--- Comment #1 from Segher Boessenkool <segher at gcc dot gnu.org> ---
Confirmed.
We start with
insn_cost 12 for 6: r121:TI=[r124:DI]
REG_DEAD r124:DI
insn_cost 4 for 9: r122:DF=r121:TI#0
insn_cost 4 for 10: r123:DF=r121:TI#8
REG_DEAD r121:TI
What we really want is a 6+9+10 combination. But combine does not currently
try to do such a thing (both 9 and 10 link back via the same reg). It does
try to do 6+9, but that needs to do a 2->2 combine, which it doesn't do because
it started off with a move insn, it needs one in the end as well, and even if
we didn't have such restrictions for 2->2, the insn_costs for the insns you end
up with is bigger than that for what you started with (only when also combining
it with insn 10 it becomes a lot lower).
So, I'll see if I can easily make it try 6+9+10 here.
