[PATCH] InstCombine: Optimize (1 << X) UPred CstP2 to X UPred Log2(CstP2)

[David Majnemer]

Ping x3.


On Mon, Jun 24, 2013 at 10:44 AM, David Majnemer
<david.majnemer at gmail.com>wrote:

> Ping x2.
>
>
> On Monday, June 17, 2013, David Majnemer wrote:
>
>> Ping.
>>
>>
>> On Sun, Jun 9, 2013 at 8:15 PM, David Majnemer <david.majnemer at gmail.com>wrote:
>>
>>> On Sun, Jun 9, 2013 at 1:27 PM, David Majnemer <david.majnemer at gmail.com
>>> > wrote:
>>>
>>>> On Sun, Jun 9, 2013 at 10:37 AM, Duncan Sands <duncan.sands at gmail.com>wrote:
>>>>
>>>>> Hi David,
>>>>>
>>>>>
>>>>> On 09/06/13 14:10, David Majnemer wrote:
>>>>>
>>>>>> We may, after other optimizations, find ourselves with IR that looks
>>>>>> like:
>>>>>>
>>>>>>    %shl = shl i32 1, %y
>>>>>>    %cmp = icmp ult i32 %shl, 32
>>>>>>
>>>>>> However, we should transform it to:
>>>>>>
>>>>>>    %cmp = icmp ult i32 %y, 5
>>>>>>
>>>>>> The attached patch teaches visitICmpInstWithInstAndIntCst how to
>>>>>> perform this
>>>>>> transform.
>>>>>>
>>>>>
>>>>> while the patch looks OK, I think it could be more general.  In your
>>>>> example,
>>>>> if the icmp ult was with, say, 30 then you still could have
>>>>> transformed it.
>>>>> In the case of an icmp eq comparison, if the RHS is not a power of two
>>>>> then
>>>>> it cannot be equal to the left hand side, so you could fold the
>>>>> comparison to
>>>>> false.
>>>>>
>>>>
>>>> Thanks! I've attached an updated patch that implements additional
>>>> transforms.
>>>>
>>>
>>> And yet more transforms, it turns out we turn some unsigned comparisons
>>> into signed comparisons. Optimize these too.
>>>
>>>
>>>>
>>>>
>>>>>
>>>>> Ciao, Duncan.
>>>>> ______________________________**_________________
>>>>> llvm-commits mailing list
>>>>> llvm-commits at cs.uiuc.edu
>>>>> http://lists.cs.uiuc.edu/**mailman/listinfo/llvm-commits<http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits>
>>>>>
>>>>
>>>>
>>>
>>
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