Re: [PATCH] Optimize (CST1 << A) == CST2 (PR tree-optimization/66299)

[Marc Glisse <marc.glisse@inria.fr>]

On Tue, 9 Jun 2015, Richard Biener wrote:

> On Tue, 9 Jun 2015, Richard Biener wrote:
>
>> On Tue, 9 Jun 2015, Marc Glisse wrote:
>>
>>> On Tue, 9 Jun 2015, Richard Biener wrote:
>>>
>>>>> Tweaking it so that (6<<X)==0 becomes X>=31 for TYPE_OVERFLOW_WRAPS and
>>>>> false for TYPE_OVERFLOW_UNDEFINED is probably more controversial.
>>>>
>>>> Hm, yes.  I think signed overflow != shift amount overflow, so testing the
>>>> overflow macros for this isn't valid.
>>>
>>> Would it be ok to always turn it to X>=31 then? (the value 31 is conveniently
>>> already computed in 'cand')
>>
>> I think so.
>
> Or even ((unsigned)X - 31) < 1 (I probably got that wrong) to properly
> say X>=29 && X<32, that is, preserve the implicit upper bound on X
> we have because it is used in a shift.

I don't understand in what sense this preserves the upper bound. I would 
understand storing a range for X (when it is an SSA_NAME, and it would 
require a lot of care not to propagate backwards too far), or more simply 
introducing if(X>=32) __builtin_unreachable();. But you seem to be talking 
about generating more complicated code so that if someone checks 
(6<<123)==0 it returns false?

-- 
Marc Glisse