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

[David Majnemer]

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.
>>> ______________________________**_________________
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>>> llvm-commits at cs.uiuc.edu
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>>>
>>
>>
>
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