Re: [gsoc 2014] moving fold-const patterns to gimple

[Prathamesh Kulkarni <bilbotheelffriend@gmail.com>]

On Fri, Mar 14, 2014 at 9:25 PM, Marc Glisse <marc.glisse@inria.fr> wrote:
> On Fri, 14 Mar 2014, Prathamesh Kulkarni wrote:
>
>> I had a look at PR 14753
>> (http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14753) from the first
>> link. I have tried to implement those transforms (attached patch,
>> stage-1 compiled).
>> I have written the transforms to operate on GENERIC.
>
>
> Why not directly gimple or the .pd file?
>
>
>> Is that correct ?
>> The patterns mentioned in the links were:
>> a) (X >> CST1) >= CST2 -> X >= CST2 << CST1
>> however, an expression Y >= CST gets folded to Y > CST - 1
>> so the transform I wrote:
>> (X >> CST1) > CST2 -> X > CST2 << CST1
>
>
> That's not the same, try X=1, CST1=1, CST2=0.
Ah yes. Shall following be correct ?
(X >> CST1) > CST2 -> X > ( (CST2 + 1) << CST1 ) - 1
Works correctly for X=1, CST1 = 1, CST2 = 0

(X >> CST1) > CST2
=> (X >> CST1) >= (CST2 + 1)   // this pattern is mentioned in PR
=> X >= (CST2 + 1) << CST1
=> X > ((CST2 + 1) << CST1) - 1
>
>
>> b) (X & ~CST) == 0 -> X <= CST
>
>
> Uh, that can't be true for all constants, only some with a very specific
> shape (7 is 2^3-1).
Agreed. Shall the pattern be folded if CST is 2^(n-1) ?
>
> --
> Marc Glisse