On 19/06/17 15:08, Segher Boessenkool wrote: > 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.
Actually the logic is int reject = old_cost > 0 && new_cost > old_cost; So reject will never be true if old cost is zero. R. > >> 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 >
