[llvm-dev] Failure to turn a div by power of 2 into a single shift

[Philip Reames via llvm-dev]

I'd missed the fact that j wasn't just being decremented.  This isn't as 
easy as I said.

Philip

On 03/03/2016 02:36 PM, Philip Reames via llvm-dev wrote:
> SCEV should be able to easily prove that j is positive here.  I'm not 
> sure where the right place to use that information would be in this 
> case.  Sanjoy, can you comment?
>
> Philip
>
> On 03/03/2016 02:06 PM, Haicheng Wu wrote:
>>
>> Hello,
>>
>> I have  a simple loop like below
>>
>> int I, j;
>>
>> for (j = n; j > 1; j = i) {
>>
>>   i = j / 2;
>>
>> }
>>
>> The signed division can be safely turned into a single shift since j 
>> is known to be positive from the loop guard.  LLVM currently cannot 
>> find out j is positive and compiles the above division into 3 
>> instructions.  Any thoughts on where to fix this?
>>
>> Thank you in advance,
>>
>> Haicheng
>>
>
>
>
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