Re: [PATCH 1/2] simplify-rtx: Simplify trunc of and of shiftrt

[Bernd Schmidt <bschmidt@redhat.com>]

On 11/10/2015 06:44 PM, Segher Boessenkool wrote:

> Yes I know.  All the rest of the code around is it like this though.
> Do you want this written in a saner way?

I won't object to leaving it as-is for now, but in the future it would 
be good to keep this in mind.

>> I'm not entirely sure what the
>> last condition here is supposed to test.
>
> It tests whether moving the truncate inside will give the same result.
> It essentially looks if it works for an x with all bits set; if that
> works, it works for any x.

Yeah, I figured afterwards that must have been the purpose of the test 
but I was thinking of other constants because of the trunc_int_for_mode 
thing. (I probably would have written "(and_const >> shift_amount) & 
~small_mask == 0" but yours should be ok too). You might want to use 
your description as a comment.

>> ... the fact that here I think you'd have to trunc_int_for_mode the AND
>> amount for the smaller mode?
>
> Ugh yes, I still have to do that for it to be valid RTL in all cases.
> Thanks for catching it.

So FAOD the patch is OK with that change.


Bernd