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From: "olegendo at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/64309] if (1 & (1 << n)) not simplified to if (n == 0)
Date: Mon, 15 Dec 2014 19:08:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64309

--- Comment #4 from Oleg Endo <olegendo at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #1)
> Confirmed.  Sth like
> 
>  (simplify
>   (ne (bit_and (lshift integer_onep @0) integer_onep) integer_zerop)
>   (eq @0 { build_zero_cst (TREE_TYPE (@0)); })
> 
> with eventually also covering if ((1 & (1<< n)) == 0) -> if (n & 1 == 0)
> 
> You can extend this to cover the other cases you mention.

I thought you might suggest something like this. :)
While the transform for the if (...) is probably going to be beneficial for all
the targets, I'm not so sure about the 'return ((1 << 1) & (1 << n));' variant,
though.  On some targets a shift+and might be cheaper than cmp+cstore.  Is
there any way to get that information during tree optimization?  If not, it
might be better to do that transformation on the RTL.