llvm dev - Mar 2016 - Failure to turn a div by power of 2 into a single shift

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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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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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
>>
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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