combiner: how to compute cost for bit insertion?

[Segher Boessenkool]

On Mon, Jul 10, 2017 at 05:10:03PM +0200, Georg-Johann Lay wrote:
> Any ideas for a sane approach?

You could change insn_rtx_cost to actually calculate the cost of the
insn, not just set_src_cost of a single set.  This will need checking
on a great many targets, not in the least because most target's cost
functions are, uh, not so good.  Big project, but hopefully well worth
the effort.

Or you change your cost function to on a QImode shift assume it is an
insert instruction.  Not correct of course (but neither is the currently
calculated cost), but perhaps it gives good results in practice?


Segher