[Bug middle-end/64309] if (1 & (1 << n)) not simplified to if (n == 0)

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64309

--- Comment #8 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
(In reply to Marc Glisse from comment #6)
> (In reply to Marek Polacek from comment #5)
> > I don't think so.  I tried to come up with a more general transformation
> > that would simplify ((CST << n) & CST) != 0, but I haven't found anything
> 
> If both CST are (possibly different) powers of 2 it works.

Yes, but I'm not sure if it's worth it (doing it for 1, 2, 4, 8, ... instead of
just for 1).

> ((c<<n)&c)==c also works. ((pow2<<p)&(pow2<<n))!=0.

I don't see how's that a simplification.

> > yet.  So maybe just this?
> > ((1 << n) & 1) != 0 -> n == 0
> 
> That looks like what richi posted. For scalars, != 0 is useless and this
> could just be:
> (1<<n)&1 -> n==0

Yeah, it's what richi posted.