On Sat, Jun 22, 2019 at 09:46:52AM -0600, Jeff Law wrote:
> On 6/22/19 7:55 AM, Jason Duerstock wrote:
> > More generally, we can rewrite
> >
> > if ( x > ((1 << z) -1)) { ...}
> >
> > as
> >
> > if ( x >> z ) { ... }
> >
> > This does not appear to currently be a gcc optimization. What is
> > involved in adding it?
> So first, when discussing this kind of stuff it's usually best to
> actually have a compilable example. Fragments usually are insufficient.
>
> Adding it to the RTL optimizers would be painful because of the need for
> type information (this is only valid when X is unsigned, right?)
>
> Adding it to the gimple optimizers is painful because the optimized form
> is actually de-optimized on some targets (say embedded targets with weak
> shifters) and querying the target costs/capabilities is generally
> frowned upon in the gimple optimizers.
>
> I think the combination of those two factors would tend to argue for an
> implementation in the gimple->rtl expanders. You've still got type
> information and you can query the backend for costing and capabilities.
>
> cfgexpand::expand_gimple_cond might be a good place to start.
>
> Another place to poke around would be expr:expand_expr_real_2, case
> COND_EXPR.
This is target-specific, so should just be done in the machine description?
Segher
