[LLVMdev] how to eliminate dead infinite loops?

Andrew Clinton andrew at sidefx.com
Wed Nov 24 13:23:13 PST 2010

On 11/24/2010 03:36 PM, Nick Lewycky wrote:
> Andrew Clinton wrote:
>> Most of my programs contain loops that the LoopDeletion pass is unable
>> to remove.  It appears that the following code in LoopDeletion.cpp:152
>> is the culprit:
>>     ScalarEvolution&  SE = getAnalysis<ScalarEvolution>();
>>     const SCEV *S = SE.getMaxBackedgeTakenCount(L);
>>     if (isa<SCEVCouldNotCompute>(S))
>>       return Changed;
>> So, LoopDeletion thinks my loops might be infinite so it does not delete
>> them - even if they do not write to any of the function return values.
>> Is there a way to flag a loop as non-infinite?  Or will I need to create
>> my own modified loop deletion pass that can eliminate potentially
>> infinite loops.  Is it correct just to remove the above code to allow
>> deletion of infinite loops?
> I think what you actually want is the ADCE pass (opt -adce). The 
> reason we turned ADCE off by default is that it kept deleting infinite 
> loops, and the LoopDeletion pass was written as a safe replacement.
> Nick

This looks sort of like what I'm looking for.  The only problem is that 
ADCE is treating all terminator instructions as live:

   // Collect the set of "root" instructions that are known live.
   for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
     if (isa<TerminatorInst>(I.getInstructionIterator()) ||
         isa<DbgInfoIntrinsic>(I.getInstructionIterator()) ||
         I->mayHaveSideEffects()) {

Is this correct?  I would have thought that branch instructions should 
not be initially live.

FYI, removing the ScalarEvolution conditional in the LoopDeletion code 
(and copying the algorithm to my own loop deletion pass) seems to 
correctly handle the elimination of infinite loops.  However I'd still 
like to find a way to do this with standard passes if possible.


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