Re: [PATCH PR84648]Adjust loop exit conditions for loop-until-wrap cases.

[bin.cheng]

------------------------------------------------------------------
Sender:Richard Biener <richard.guent...@gmail.com>
Sent at:2018 Nov 13 (Tue) 23:03
To:bin.cheng <bin.ch...@linux.alibaba.com>
Cc:GCC Patches <gcc-patches@gcc.gnu.org>
Subject:Re: [PATCH PR84648]Adjust loop exit conditions for loop-until-wrap 
cases.

> 
> On Sun, Nov 11, 2018 at 9:02 AM bin.cheng <bin.ch...@linux.alibaba.com> wrote:
>>
>> Hi,
>> This patch fixes PR84648 by adjusting exit conditions for loop-until-wrap 
>> cases.
>> It only handles simple cases in which IV.base are constants because we rely 
>> on
>> current niter analyzer which doesn't handle parameterized bound in wrapped
>> case.  It could be relaxed in the future.
>>
>> Bootstrap and test on x86_64 in progress.
> 
> Please use TYPE_MIN/MAX_VALUE or wi::min/max_value consistently.
> Either tree_int_cst_equal (iv0->base, TYPE_MIN_VALUE (type)) or
> wide_int_to_tree (niter_type, wi::max_value (TYPE_PRECISION (type),
> TYPE_SIGN (type))).
> 
> Also
> 
> +  iv0->base = low;
> +  iv0->step = fold_convert (niter_type, integer_one_node);
> 
> build_int_cst (niter_type, 1);
> 
> +  iv1->base = high;
> +  iv1->step = integer_zero_node;
> 
> build_int_cst (niter_type, 0);
Fixed, thanks for reviewing.

> 
> With the code, what happens to signed IVs?  I suppose we figure out things
> earlier by means of undefined overflow?
The code takes advantage of signed undefined overflow and handle it as wrap.
In the reported test case, we have following IL:
  <bb 2> :
  goto <bb 4>; [INV]

  <bb 3> :
  i_4 = i_2 + 1;

  <bb 4> :
  # i_2 = PHI <0(2), i_4(3)>
  i.0_1 = (signed int) i_2;
  if (i.0_1 >= 0)
    goto <bb 3>; [INV]
  else
    goto <bb 5>; [INV]

So the IV is actually transformed into signed int, we rely on scev to understand
type conversion correctly and generate (int){0, 1} for i.0_1.  Is this 
reasonable?

Updated patch attached, bootstrap and test on x86_64.

Thanks,
bin

2018-11-11  Bin Cheng  <bin.ch...@linux.alibaba.com>

        PR tree-optimization/84648
        * tree-ssa-loop-niter.c (adjust_cond_for_loop_until_wrap): New.
        (number_of_iterations_cond): Adjust exit cond for loop-until-wrap case
        by calling adjust_cond_for_loop_until_wrap.

2018-11-11  Bin Cheng  <bin.ch...@linux.alibaba.com>

        PR tree-optimization/84648
        * gcc.dg/tree-ssa/pr84648.c: New test.
        * gcc.dg/pr68317.c: Add warning check on overflow.

pr84648-20181114.txt
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