[LLVMdev] Loop Metadata?

[Tobias Grosser]

On 02/10/2012 11:29 PM, Sebastian Pop wrote:
> On Fri, Feb 10, 2012 at 3:53 PM, Devang Patel<dpatel at apple.com>  wrote:
>> How would you tie a built-in call with a loop ?
>
> The same way as Hal was saying: tag the loop header.

So how do we tag the loop header? ;-)

If we just put the instruction in the header, in the worst case LICM 
will move it to header basic block of another loop. :-(

We could try to keep it somehow attached to the loop. E.g. by making it 
modify a scalar that is changed in each loop iteration. But than again,
some sophisticated loop fission pass may move these instructions apart.
We definitely need some conventions for such an approach. Another issue 
is, that as more semantics we put into such a loop marking instruction, 
the more probable it is that we block optimizations like dead code 
elimination of the entire loop. If we do not put any semantics, we may 
risk to loose the annotation.

Another option that I see is somehow in the direction of what Devang 
pointed out. We could make our intention of maintain information about
the loops more explicit. I could e.g. imagine having some permanent 
pass/analysis, maybe called LoopInformationManager, that is in charge of 
the loop meta data. When reading LLVM-IR it initializes itself by 
reading the loop meta-data. Other passes can call the 
LoopInformationManager for the information they are interested in. 
Passes that do not touch the loop structure need to explicitly specify 
that they preserved the loop semantics. In case this is not done, the 
LoopInformationManager removes the meta-data from the loop. Or in case 
it is informed about a change, it adapts the meta-data accordingly. Such 
that at the end, only valid meta-data will be written out.
This approach has its own drawbacks, but it may be a direction we could 
consider.

Cheers
Tobi