>> I wonder how to deal with the situation
>> where CHREC_RIGHT changes? Whenever we call chrec_fold_plus
>> with CHRECs of the same loop we get { a + b, step_a + step_b }_loop
>> which will invalidate nowrap bounds?
>>
>> Likewise if you start with
>> { init, +, 1 }_0 and a bound of N, adding 5 requires to adjust N?
>> So even changing CHREC_LEFT is problematic.
>
> Hmm, but the bound would/should then still be N because its derived from the
> IV
> type, even after adding 5? Or maybe I misunderstood?
Ah I think I got your point now. The bound "N" is obtained in
scev_probably_wraps which takes place after all the chrec plus/... things
have already happened (I hope :)).
> Right now the conjunction of two nowrap bounds happens through MIN. If we
> add
> one chrec to another, leading to a laxer bound, indeed no bounds update
> happens. We would pessimize the bound, perhaps significantly so?, and
> version
> with it. I'll try to come up with a test case for this. During testing of
> the
> patch I haven't been particularly lucky with building test cases that show
> the
> specific behavior I want to test, though...
Looks like "AI" is better than me (at that?)... it's even simple but not sure
the example demonstrates a lot:
__attribute__ ((noipa)) void
f (int *a, unsigned char s1, unsigned char s2, int n)
{
unsigned char i1 = s1, i2 = s2;
for (int j = 0; j < n; j++)
{
a[j + i1 - i2] += 1;
i1++;
i2++;
}
}
We have chrecs for i1 and i2 here that are being combined during analysis:
(instantiate_scev
(instantiate_below = 5 -> 3)
(evolution_loop = 1)
(chrec = {(int) s1_14(D), +, 2}_1)
(res = {(int) s1_14(D), +, 2}_1))
(instantiate_scev
(instantiate_below = 5 -> 3)
(evolution_loop = 1)
(chrec = {(int) s2_15(D), +, 1}_1)
(res = {(int) s2_15(D), +, 1}_1))
and eventually something like this:
(instantiate_scev
(instantiate_below = 5 -> 3)
(evolution_loop = 1)
(chrec = {a_18(D) + ((long unsigned int) s1_14(D) - (long unsigned int)
s2_15(D)) * 4, +, 4}_1)
(res = {a_18(D) + ((long unsigned int) s1_14(D) - (long unsigned int)
s2_15(D)) * 4, +, 4}_1))
I'm seeing:
adding no-wrap assumption: (unsigned int) ~MAX_EXPR <s1_14(D), s2_15(D)> >=
(unsigned int) n_17(D)
I guess that's 255 - max (s1, s2) >= n? And if we choose s1 = s2 they cancel
each other out we could actually vectorize but don't because the assumption is
not "smart" enough. I'd say that's missed optimization (if at all)?
--
Regards
Robin
