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

[Richard Biener]

On Tue, Nov 29, 2016 at 12:50 PM, Paolo Bonzini <bonzini@gnu.org> wrote:
>
>
> On 29/11/2016 11:16, Richard Biener wrote:
>>>> >>  (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.
>> Instead of the bit_and better put a if() condition around this.
>
> Note that the bit_and is not duplicated.  While v1 of the patch did
> and-then-shift (the source BIT_AND_EXPR was @2), here I'm doing
> shift-then-and so I would stop capturing the source BIT_AND_EXPR.

Ah, ok.

> It also matches what I'm doing for the A < 0 ? D : C pattern, so I think
> it's nicer this way.
>
> (BTW the above of course doesn't work because (if...) is only allowed at
> the top level, but I've bootstrapped and regtested already a version
> that works).
>
> Paolo