IVOPT improvement patch

[Xinliang David Li]

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

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.
>