Re: [PATCH] improve folding of expressions that move a single bit around

[Jeff Law <law at redhat dot com>]

On 11/28/2016 06:10 AM, Paolo Bonzini wrote:
> On 27/11/2016 00:28, Marc Glisse wrote:
> > On Sat, 26 Nov 2016, Paolo Bonzini wrote:
> >
> > > --- match.pd    (revision 242742)
> > > +++ match.pd    (working copy)
> > > @@ -2554,6 +2554,19 @@
> > >   (cmp (bit_and@2 @0 integer_pow2p@1) @1)
> > >   (icmp @2 { build_zero_cst (TREE_TYPE (@0)); })))
> > >
> > > +/* If we have (A & C) != 0 ? D : 0 where C and D are powers of 2,
> > > +   convert this into a shift of (A & C).  */
> > > +(simplify
> > > + (cond
> > > +  (ne (bit_and@2 @0 integer_pow2p@1) integer_zerop)
> > > +  integer_pow2p@3 integer_zerop)
> > > + (with {
> > > +    int shift = wi::exact_log2 (@3) - wi::exact_log2 (@1);
> > > +  }
> > > +  (if (shift > 0)
> > > +   (lshift (convert @2) { build_int_cst (integer_type_node, shift); })
> > > +   (convert (rshift @2 { build_int_cst (integer_type_node, -shift);
> > > })))))
> >
> > What happens if @1 is the sign bit, in a signed type? Do we get an
> > arithmetic shift right?
>
> It shouldn't happen because the canonical form of a sign bit test is A <
> 0 (that's the pattern immediately after).  However I can add an "if" if
> preferred, or change the pattern to do the AND after the shift.
But are we absolutely sure it'll be in canonical form every time?   Is 
there a pattern in in match.pd which turns an (x & SIGN_BIT) tests into 
canonical form?


>  (bit_and
>   (if (shift > 0)
>    (lshift (convert @0) { build_int_cst (integer_type_node, shift); })
>    (convert (rshift @0 { build_int_cst (integer_type_node, -shift); })))
>   @3)
>
> What do you think?
Shouldn't be necessary if we're working on unsigned types.  May be 
necessary on signed types unless we're 100% sure sign bit tests will be 
canonicalized.

jeff