[llvm-commits] [llvm] r42075 - in /llvm/trunk: lib/Transforms/Scalar/LICM.cpp test/Transforms/LICM/2007-09-17-PrompteValue.ll

[Devang Patel]

On Sep 18, 2007, at 7:02 AM, Dan Gohman wrote:

>> +      // If GEP base is NULL then the calculated address  used by  
>> Store or
>> +      // Load instruction is invalid. Do not promote this value  
>> because
>> +      // it may expose load and store instruction that are covered  
>> by
>> +      // condition which may not yet folded.
>> +      if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(V))
>> +        if (isa<ConstantPointerNull>(GEP->getOperand(0)))
>> +          PointerOk = false;
>> +
>
> It looks like there's a larger problem here. The testcase's loop body
> may never be executed, so it isn't safe for LICM to hoist/sink
> anything out to an unguarded location, null pointer constants or
> otherwise.

We intend to split LICM into three passes - sink, hoist and promote- 
values. Irrespective of sinking and hoisting, promote-values should  
not promote values that are unsafe. Avoiding hoisting GEP here will  
solve this test case, but promote-values bug will be exposed when it  
is supplied manually hoisted GEP.

promote-values is not in business to identify dead loops. And when  
loop conditionals are runtime dependent it is not possible to  
determine it at compile time at all. The fix is to check unsafe stores  
and loads before promoting value.

-
Devang