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

[David Majnemer]

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.


>
> Ciao, Duncan.
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