Re: [PATCH PR85190]Adjust pointer for aligned access

[Bin.Cheng]

On Tue, Apr 10, 2018 at 3:58 PM, Bin.Cheng <amker.ch...@gmail.com> wrote:
> On Tue, Apr 10, 2018 at 2:26 PM, Jakub Jelinek <ja...@redhat.com> wrote:
>> On Tue, Apr 10, 2018 at 09:55:35AM +0000, Bin Cheng wrote:
>>> Hi Rainer, could you please help me double check that this solves the issue?
>>>
>>> Thanks,
>>> bin
>>>
>>> gcc/testsuite
>>> 2018-04-10  Bin Cheng  <bin.ch...@arm.com>
>>>
>>>       PR testsuite/85190
>>>       * gcc.dg/vect/pr81196.c: Adjust pointer for aligned access.
>>
>>> diff --git a/gcc/testsuite/gcc.dg/vect/pr81196.c 
>>> b/gcc/testsuite/gcc.dg/vect/pr81196.c
>>> index 46d7a9e..15320ae 100644
>>> --- a/gcc/testsuite/gcc.dg/vect/pr81196.c
>>> +++ b/gcc/testsuite/gcc.dg/vect/pr81196.c
>>> @@ -4,14 +4,14 @@
>>>
>>>  void f(short*p){
>>>    p=(short*)__builtin_assume_aligned(p,64);
>>> -  short*q=p+256;
>>> +  short*q=p+255;
>>>    for(;p!=q;++p,--q){
>>>      short t=*p;*p=*q;*q=t;
>>
>> This is UB then though, because p will never be equal to q.

Hmm, though it's UB in this case, is it OK for niter analysis gives
below results?

Analyzing # of iterations of loop 1
  exit condition [126, + , 18446744073709551615] != 0
  bounds on difference of bases: -126 ... -126
  result:
    # of iterations 126, bounded by 126

I don't really follow last piece of code in number_of_iterations_ne:

  /* Let nsd (step, size of mode) = d.  If d does not divide c, the loop
     is infinite.  Otherwise, the number of iterations is
     (inverse(s/d) * (c/d)) mod (size of mode/d).  */
  bits = num_ending_zeros (s);
  bound = build_low_bits_mask (niter_type,
                   (TYPE_PRECISION (niter_type)
                - tree_to_uhwi (bits)));

  d = fold_binary_to_constant (LSHIFT_EXPR, niter_type,
                   build_int_cst (niter_type, 1), bits);
  s = fold_binary_to_constant (RSHIFT_EXPR, niter_type, s, bits);

  if (!exit_must_be_taken)
    {
      /* If we cannot assume that the exit is taken eventually, record the
     assumptions for divisibility of c.  */
      assumption = fold_build2 (FLOOR_MOD_EXPR, niter_type, c, d);
      assumption = fold_build2 (EQ_EXPR, boolean_type_node,
                assumption, build_int_cst (niter_type, 0));
      if (!integer_nonzerop (assumption))
    niter->assumptions = fold_build2 (TRUTH_AND_EXPR, boolean_type_node,
                      niter->assumptions, assumption);
    }

  c = fold_build2 (EXACT_DIV_EXPR, niter_type, c, d);
  tmp = fold_build2 (MULT_EXPR, niter_type, c, inverse (s, bound));
  niter->niter = fold_build2 (BIT_AND_EXPR, niter_type, tmp, bound);
  return true;

Though infinite niters is mentioned, I don't see it's handled?

Thanks,
bin
> Sorry I already checked in, will try to correct it in another patch.
>
> Thanks,
> bin
>>
>>>    }
>>>  }
>>>  void b(short*p){
>>>    p=(short*)__builtin_assume_aligned(p,64);
>>> -  short*q=p+256;
>>> +  short*q=p+255;
>>>    for(;p<q;++p,--q){
>>>      short t=*p;*p=*q;*q=t;
>>
>> This one is fine, sure.
>>
>>         Jakub