Re: how to check if target supports andnot instruction ?

[Marc Glisse <marc dot glisse at inria dot fr>]

On Thu, 13 Oct 2016, Prathamesh Kulkarni wrote:

> On 12 October 2016 at 14:43, Richard Biener <rguenther@suse.de> wrote:
> > On Wed, 12 Oct 2016, Marc Glisse wrote:
> >
> > > On Wed, 12 Oct 2016, Prathamesh Kulkarni wrote:
> > >
> > > > I was having a look at PR71636 and added the following pattern to match.pd:
> > > > x & ((1U << b) - 1) -> x & ~(~0U << b)
> > > > However the transform is useful only if the target supports "andnot"
> > > > instruction.
> > >
> > > rth was selling the transformation as a canonicalization, which is beneficial
> > > when there is an andnot instruction, and neutral otherwise, so it could be
> > > done always.
> >
> > Well, its three instructions to three instructions and a more expensive
> > constant(?).  ~0U might not be available as immediate for the shift
> > instruction and 1U << b might be available as a bit-set instruction ...
> > (vs. the andnot).

True, I hadn't thought of bit-set.

> > So yes, we might decide to canonicalize to andnot (and decide that
> > three binary to two binary and one unary op is "better").
> >
> > So no excuse to explore the target specific .pd fragment idea ... :/
> Hi,
> I have attached patch that adds the transform.
> Does that look OK ?

Why bit_not of build_zero_cst instead of build_all_ones_cst, as suggested 
in the PR? If we only do the transformation when (1<<b)-1 is fed to a 
bit_and, then we probably want to require that it has a single use (maybe 
even the shift).

> I am not sure how to write test-cases for it though.
> For the test-case:
> unsigned f(unsigned x, unsigned b)
> {
>  unsigned t1 = 1U << b;
>  unsigned t2 = t1 - 1;
>  unsigned t3 = x & t2;
>  return t3;
> }
>
> forwprop dump shows:
> Applying pattern match.pd:523, gimple-match.c:47419
> gimple_simplified to _6 = 4294967295 << b_1(D);
> _8 = ~_6;
> t3_5 = x_4(D) & _8;
>
> I could scan for "_6 = 4294967295 << b_1(D);"  however I suppose
> ~0 would depend on width of int and not always be 4294967295 ?
> Or should I scan for "_6 = 4294967295 << b_1(D);"
> and add /* { dg-require-effective int32 } */  to the test-case ?

You could check that you have ~, or that you don't have " 1 << ".

--
Marc Glisse