Re: [PATCH] Fix PR47594: Sign extend constants while translating to Graphite

[Sebastian Pop <sebpop@gmail.com>]

On Tue, Jul 26, 2011 at 03:22, Richard Guenther <rguenther@suse.de> wrote:
> On Mon, 25 Jul 2011, Sebastian Pop wrote:
>
>> "Bug 47594 - gfortran.dg/vect/vect-5.f90 execution test fails when
>> compiled with -O2 -fgraphite-identity"
>>
>> The problem is due to the fact that Graphite generates this loop:
>>
>>     for (scat_3=0;scat_3<=4294967295*scat_1+T_51-1;scat_3++) {
>>       S6(scat_1,scat_3);
>>     }
>>
>> that has a "-1" encoded as an unsigned "4294967295".  This constant
>> comes from the computation of the number of iterations "M - I" of
>> the inner loop:
>>
>>         do I = 1, N
>>           do J = I, M
>>             A(J,2) = B(J)
>>           end do
>>         end do
>>
>> The patch fixes the problem by sign-extending the constants for the
>> step of a chain of recurrence in scan_tree_for_params_right_scev.
>>
>> The same patter could occur for multiplication by a scalar, like in
>> "-1 * N" and so the patch also fixes these cases in
>> scan_tree_for_params.
>
> That certainly feels odd (again).  How does it end up being unsigned
> in the first place?

We got this expression from niter.  niter analysis turns all expressions
into unsigned types before starting computations.  I tried to see if we
could improve niter, but that would be a major work.  I also thought
about using PPL or ISL to implement niter for graphite.

> Randomly sign-extending stuff looks bogus to me.
> Does graphite operate on infinite precision signed integers?  Or
> does it operate on twos-complement fixed precision integers?

Graphite represents constants using mpz_t.

Sebastian