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Re: IVOPT improvement patch


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.


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.

Attachment: gcc-pr45119-4.patch
Description: Text document


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