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Re: IVOPT improvement patch


On Fri, Jul 30, 2010 at 11:46 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Fri, Jul 30, 2010 at 10:54 AM, Xinliang David Li <davidxl@google.com> wrote:
>> Why is start offset not 1 to begin with? Let's assume it is correct,
>> there are a couple of problems in this patch:
>>
>> 1) when the precision of the HOST_WIDE_INT is the same as the bitsize
>> of the address_mode, max_offset = (HOST_WIDE_INT) 1 << width will
>> produce a negative number
>> 2) last_off should be initialized to 0 to match the original behavior
>> 3) The i&& guard will make sure the loop terminates, but the offset
>> compuation will be wrong -- i<<1 will first overflows to a negative
>> number, then gets truncated to zero, ?that means when this happens,
>> the last_off will be negative when the loop terminates.
>>
>> David
>
> I don't know exactly what get_address_cost is supposed to do. Here is
> a new patch which avoids overflow and speeds up finding max/min offsets.
>


The code is wrong for -m32 on 64bit host. We should start with
the maximum and minimum offsets like:

      width = GET_MODE_BITSIZE (address_mode) - 1;
      if (width > (HOST_BITS_PER_WIDE_INT - 1))
        width = HOST_BITS_PER_WIDE_INT - 1;
      addr = gen_rtx_fmt_ee (PLUS, address_mode, reg1, NULL_RTX);

      for (i = width; i; i--)
        {
          off = -((HOST_WIDE_INT) 1 << i);
          XEXP (addr, 1) = gen_int_mode (off, address_mode);
          if (memory_address_addr_space_p (mem_mode, addr, as))
            break;
        }
      data->min_offset = off;

      for (i = width; i; i--)
        {
          off = ((HOST_WIDE_INT) 1 << i) - 1;
          XEXP (addr, 1) = gen_int_mode (off, address_mode);
          if (memory_address_addr_space_p (mem_mode, addr, as))
            break;
        }
      data->max_offset = off;

Here is the updated patch.


H.J.
---
> H.J.
> ---
>>
>> On Fri, Jul 30, 2010 at 10:27 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>> On Fri, Jul 30, 2010 at 9:58 AM, Xinliang David Li <davidxl@google.com> wrote:
>>>> There is a problem in this patch -- when i wraps to zero and terminate
>>>> the loop, the maxoffset computed will be zero which is wrong.
>>>>
>>>> My previous patch won't have this problem.
>>>
>>> Your patch changed the start offset. ?Here is the updated patch.
>>>
>>>
>>> H.J.
>>>>
>>>> David
>>>>
>>>> On Fri, Jul 30, 2010 at 9:49 AM, Xinliang David Li <davidxl@google.com> wrote:
>>>>> This looks fine to me -- Zdenek or other reviewers --- is this one ok?
>>>>>
>>>>> Thanks,
>>>>>
>>>>> David
>>>>>
>>>>> On Fri, Jul 30, 2010 at 8:45 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>>>> On Thu, Jul 29, 2010 at 6:04 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>>>>> It looks strange:
>>>>>>>
>>>>>>> + ? ? ?width = (GET_MODE_BITSIZE (address_mode) < ?HOST_BITS_PER_WIDE_INT - 1)
>>>>>>> + ? ? ? ? ?? GET_MODE_BITSIZE (address_mode) : HOST_BITS_PER_WIDE_INT - 1;
>>>>>>> ? ? ? addr = gen_rtx_fmt_ee (PLUS, address_mode, reg1, NULL_RTX);
>>>>>>> - ? ? ?for (i = start; i <= 1 << 20; i <<= 1)
>>>>>>> + ? ? ?for (i = 1; i < width; i++)
>>>>>>> ? ? ? ?{
>>>>>>> - ? ? ? ? XEXP (addr, 1) = gen_int_mode (i, address_mode);
>>>>>>> + ? ? ? ? ?HOST_WIDE_INT offset = (1ll << i);
>>>>>>> + ? ? ? ? XEXP (addr, 1) = gen_int_mode (offset, address_mode);
>>>>>>> ? ? ? ? ?if (!memory_address_addr_space_p (mem_mode, addr, as))
>>>>>>> ? ? ? ? ? ?break;
>>>>>>> ? ? ? ?}
>>>>>>>
>>>>>>> HOST_WIDE_INT may be long or long long. "1ll" isn't always correct.
>>>>>>> I think width can be >= 31. Depending on HOST_WIDE_INT,
>>>>>>>
>>>>>>> HOST_WIDE_INT offset = -(1ll << i);
>>>>>>>
>>>>>>> may have different values. The whole function looks odd to me.
>>>>>>>
>>>>>>>
>>>>>>
>>>>>> Here is a different approach to check address overflow.
>>>>>>
>>>>>>
>>>>>> --
>>>>>> H.J.
>>>>>> --
>>>>>> 2010-07-29 ?H.J. Lu ?<hongjiu.lu@intel.com>
>>>>>>
>>>>>> ? ? ? ?PR bootstrap/45119
>>>>>> ? ? ? ?* tree-ssa-loop-ivopts.c (get_address_cost): Re-apply revision
>>>>>> ? ? ? ?162652. ?Check address overflow.
>>>>>>
>>>>>
>>>>
>>>
>>>
>>>
>>> --
>>> H.J.
>>>
>>
>
>
>
> --
> H.J.
>



-- 
H.J.

Attachment: gcc-pr45119-5.patch
Description: Text document


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