On Fri, 13 Oct 2017, Marc Glisse wrote:
> On Fri, 13 Oct 2017, Richard Biener wrote:
>
> > On Thu, 12 Oct 2017, Jakub Jelinek wrote:
> >
> > > Hi!
> > >
> > > Marc in the PR mentioned that it is not really good that the recommended
> > > rotate pattern is recognized only during forwprop1 and later, which is
> > > after
> > > einline and that inlining or early opts could have changed stuff too much
> > > so
> > > that we wouldn't recogize it anymore.
> >
> > Hmm, but the only thing functions see is inlining early optimized bodies
> > into them and then constant propagation performed, so I don't see how
> > we could confuse the pattern in a way to be indetectable. Also
>
> For instance, in
>
> unsigned f(unsigned x, int y) { return (x << y) | (x >> (-y & 31)); }
> unsigned g(unsigned x) { return f(x << 2, 3); }
>
> if we inline f into g before optimizing f, we might combine (x << 2) << 3 into
> x << 5, which would make it harder to recognize the rotation. For some reason
> that's not happening, but similar scenarios seemed possible to me. But if the
> inliner calls early optimizations (including forwprop) on the callee before
> inlining it, then my comment is indeed nonsense.
We are always inlining early optimized bodies (unless it is the recursion
edge in a cyclic callgraph). So we're effectively re-optimizing any
code we inline as well.
> > early inlining is performed on early optimized bodies so cost metrics
> > see rotates, not the unrecognized form.
>
> Ah, so there's a hidden forwprop pass in einline?
No. The early optimization pipeline runs on each function separately
and the processing is done in an order that optimizes callees before
callers. The first step is to perform inlining into the current
function, then we optimize the result.
> unsigned f(unsigned x, int y) {
> unsigned z = x << y;
> return z | (x >> (-y & 31));
> }
> unsigned g(unsigned x, int y) { return f(x, y); }
>
> After einline, g has rotate but not f, that's rather confusing.
It indeed is ;) Non-IPA pass dumps do not show function bodies
at a "single point in time".
Richard.
