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

[Segher Boessenkool]

On Mon, Jun 19, 2017 at 03:28:20PM +0100, Richard Earnshaw (lists) wrote:
> > That's not what combine does: it optimistically assumes any combination
> > with unknown costs is an improvement.
> 
> So try this testcase on ARM.
> 
> unsigned long x, y, z;
> int b;
> void test()
> {
>    b = __builtin_sub_overflow (y,z, &x);
> }
> 
> 
> Without the patch, combine rips apart a compare and subtract insn
> because it sees it as having cost zero and substitutes it with separate
> compare and subtract insns.

> The combine log before the patch shows:
> 
> allowing combination of insns 10 and 51
> original costs 0 + 8 = 0
> replacement costs 4 + 12 = 16

Yes, this is a good example of a case where your patch helps.  Thanks.

> So it is clearly deciding that the original costs are greater than the
> replacement costs.

No: it allows any combination with unknown cost (either old or new cost).
See combine_validate_cost.

> >> 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?
> 
> I thought about it but concluded that there's no generically correct
> answer.  It might be the max of all the individual sets or it might be
> the sum, or it might be somewhere in between.  For example on ARM the
> load/store multiple operations are expressed as parallels, but their
> cost will depend on how many loads/stores happen in parallel within the
> hardware.
> 
> I think we'd need a new back-end hook to handle the other cases sensibly.

And in general make insn_rtx_cost do something more sane than just looking
at a set_src_cost, yeah.

The problem is changing any of this without regressing some targets.
Of course we are in stage 1 now ;-)


Segher