Hi! On Mon, Jun 19, 2017 at 02:46:59PM +0100, Richard Earnshaw (lists) wrote: > Many parallel set insns are of the form of a single set that also sets > the condition code flags. In this case the cost of such an insn is > normally the cost of the part that doesn't set the flags, since updating > the condition flags is simply a side effect. > > At present all such insns are treated as having unknown cost (ie 0) and > combine assumes that such insns are infinitely more expensive than any > other insn sequence with a non-zero cost.
That's not what combine does: it optimistically assumes any combination with unknown costs is an improvement. > This patch addresses this problem by allowing insn_rtx_cost to ignore > the condition setting part of a PARALLEL iff there is exactly one > comparison set and one non-comparison set. If the only set operation is > a comparison we still use that as the basis of the insn cost. I'll test this on a zillion archs, see what the effect is. Have you considered costing general parallels as well? Segher
