[llvm-commits] [llvm] r49928 - /llvm/trunk/lib/Analysis/ScalarEvolution.cpp

[Nick Lewycky]

Dale Johannesen wrote:
> That seems to work equally well, but results in noticeably worse code,  
> 7 instructions instead of 4 in the preheader in my example test.   
> Could you explain why what we did was wrong?

Yes, your code would miscompile in the case where the loop test was 
false to begin with and thus never executes at all.

See PR2003 for the discussion and an example of what goes wrong in the 
ULT case. I don't have an example handy using UGT (and I find them hard 
to come up with).

Nick

> On Apr 18, 2008, at 9:27 PM, Nick Lewycky wrote:
> 
>> Hi Dale,
>>
>> I don't think this is right. The correct fix is to replace
>> getNegativeSCEV with getNotSCEV.
>>
>> Nick
>>
>> Dale Johannesen wrote:
>>> Author: johannes
>>> Date: Fri Apr 18 16:38:31 2008
>>> New Revision: 49928
>>>
>>> URL: http://llvm.org/viewvc/llvm-project?rev=49928&view=rev
>>> Log:
>>> Fix a scalar evolution bug.  Reversing everything
>>> does not work because of 0; 2>0 but -2U is also >0.
>>>
>>>
>>> Modified:
>>>    llvm/trunk/lib/Analysis/ScalarEvolution.cpp
>>>
>>> Modified: llvm/trunk/lib/Analysis/ScalarEvolution.cpp
>>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ScalarEvolution.cpp?rev=49928&r1=49927&r2=49928&view=diff
>>>
>>> = 
>>> = 
>>> = 
>>> = 
>>> = 
>>> = 
>>> = 
>>> = 
>>> = 
>>> =====================================================================
>>> --- llvm/trunk/lib/Analysis/ScalarEvolution.cpp (original)
>>> +++ llvm/trunk/lib/Analysis/ScalarEvolution.cpp Fri Apr 18 16:38:31  
>>> 2008
>>> @@ -1980,8 +1980,7 @@
>>>     break;
>>>   }
>>>   case ICmpInst::ICMP_UGT: {
>>> -    SCEVHandle TC = HowManyLessThans(SE.getNegativeSCEV(LHS),
>>> -                                     SE.getNegativeSCEV(RHS), L,  
>>> false);
>>> +    SCEVHandle TC = HowFarToZero(SE.getMinusSCEV(LHS, RHS), L);
>>>     if (!isa<SCEVCouldNotCompute>(TC)) return TC;
>>>     break;
>>>   }
>>>
>>>
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