Re: Move some bit and binary optimizations in simplify and match

[Richard Biener <richard.guenther@gmail.com>]

On Wed, Oct 14, 2015 at 12:45 PM, Marc Glisse <marc.glisse@inria.fr> wrote:
> On Wed, 14 Oct 2015, Richard Biener wrote:
>
>>> +/* Fold (a * (1 << b)) into (a << b)  */
>>> +(simplify
>>> + (mult:c @0 (convert? (lshift integer_onep@1 @2)))
>>> +  (if (! FLOAT_TYPE_P (type)
>>> +        && tree_nop_conversion_p (type, TREE_TYPE (@2)))
>>> +   (lshift @0 (convert @2))))
>>>
>>> You don't need/want to convert @2 (fold-const doesn't convert, does it?),
>>> and you don't need to check for tree_nop_conversion_p.
>>
>>
>> I think for long x and x * (long)(1u << b) you need to do because the
>> result for b == 33 would be different.
>
>
> - that check should be with TREE_TYPE (@1)

of course

> - 1u << 33 is undefined, isn't it?

Is it?  I thought it were fine for unsigned.  Not sure if we should exploit this
undefinedness here.  Btw, if it were a truncating conversion then the
resulting shift could be invalid if @2 is too big (but not too big for the
wider shift).  So either way I think we should only allow nop conversions
here (as fold-const.c did).

Richard.

> x * (int)(1ul << b), which for b=33 should yield 0, would give the undefined
> x << b so some check does seem needed indeed.
>
> --
> Marc Glisse