[LLVMdev] Detecting counted loops

[Chris Lattner]

On Feb 24, 2009, at 8:46 AM, Andrew Haley wrote:

> I need to be able to detect a well-behaved loop.  (i.e one where exp1
> assigns a value to an int i, exp2 compares i with a loop constant,
> exp3 adjusts i by a loop constant, and the inner block has no
> assignments to i.)
>
> I need this because in Sun's Java VM garbage collection only takes
> place at safepoints, so a potentially unbounded loop must call
> safepoint() some time.  However, safepoints are potentially very
> costly, so we don't want to have them except where it is absolutely
> necessary.  Any loop that is known to be of finite duration does not
> need to call safepoint().
>
> I presume that LLVM can fairly easily find well-behaved loops in order
> for it to do loop optimizations.  I don't want to have to do the
> analysis in my front-end if I can possibly avoid it.  Is there some
> logic in LLVM that I can call which will tell me where well-behaved
> loops are?  Or, perhaps, I might have to write a special-purpose
> optimization pass to do this.

Hi Andrew,

Check out the LoopInfo pass and the ScalarEvolution code in include/ 
llvm/Analysis.  ScalarEvolution::getIterationCount is probably what  
you want.

-Chris