[rtlanal] Do a better job of costing parallel sets containing flag-setting operations.

[Segher Boessenkool]

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