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

[Paolo Bonzini]

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.

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