Re: [PATCH 0/3] Fix PR47654 and PR49649

[Richard Guenther <rguenther at suse dot de>]

On Thu, 7 Jul 2011, Sebastian Pop wrote:

> Hi,
> 
> First there are two cleanup patches independent of the fix:
> 
>   Start counting nesting level from 0.
>   Do not compute twice type, lb, and ub.
> 
> Then the patch that fixes PR47654:
> 
>   Fix PR47654: Compute LB and UB of a CLAST expression.
> 
> One of the reasons we cannot determine the IV type only from the
> polyhedral representation is that as in the testcase of PR47654, we
> are asked to generate an induction variable going from 0 to 127.  That
> could be represented with a "char".  However the upper bound
> expression of the loop generated by CLOOG is min (127, 51*scat_1 + 50)
> and that would overflow if we use a "char" type.  To evaluate a type
> in which the expression 51*scat_1 + 50 does not overflow, we have to
> compute an upper and lower bound for the expression.
> 
> To fix the problem exposed by Tobias:
> 
> > for (i = 0 ; i < 2; i++)
> >  for (j = i ; j < i + 1; j++)
> >    for (k = j ; k < j + 1; k++)
> >      for (m = k ; m < k + 1; m++)
> >        for (n = m ; n < m + 1; n++)
> >          A[0] += A[n];
> > 
> > I am a little bit afraid that we will increase the type size by an
> > order of magnitude (or at least one bit) for each nesting level.
> 
> instead of computing the lb and ub of scat_1 in "51*scat_1 + 50" based
> on the type of scat_1 (that we already code generated when building
> the outer loop), we use the polyhedral representation to get an
> accurate lb and ub for scat_1.
> 
> When translating the substitutions of a user statement using this
> precise method, like for example S5 in vect-pr43423.c:
> 
>   for (scat_1=0;scat_1<=min(T_3-1,T_4-1);scat_1++) {
>     S5(scat_1);
> 
> we get a type that is too precise: based on the interval [0,99] we get
> the type "unsigned char" when the type of scat_1 is "int", misleading
> the vectorizer due to the insertion of spurious casts:
> 
> #  Access function 0: (int) {(<unnamed-unsigned:8>) graphite_IV.7_56, +, 1}_3;
> #)
> affine dependence test not usable: access function not affine or constant.
> 
> So we have to keep around the previous code gcc_type_for_clast_* that
> computes the type of an expression as the max precision of the
> components of that expression, and use that when computing the types
> of substitution expressions.
> 
> The patches passed together a full bootstrap and test on amd64-linux.
> Ok for trunk?

The idea sounds good to me and the middle-end-like looking pieces
look good.  I'd appreciate a 2nd look from Tobias.

Thanks,
Richard.