[LLVMdev] InstCombine Question

David Greene dag at cray.com
Fri Apr 4 09:38:48 PDT 2008

I am confused by this bit of code in instcombine:

09789   if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(Op)) {
09790     const Value *GEPI0 = GEPI->getOperand(0);
09791     // TODO: Consider a target hook for valid address spaces for this 
09792     if (isa<ConstantPointerNull>(GEPI0) &&
09793         cast<PointerType>(GEPI0->getType())->getAddressSpace() == 0) {
09794       // Insert a new store to null instruction before the load to 
09795       // that this code is not reachable.  We do this instead of 
09796       // an unreachable instruction directly because we cannot modify 
09797       // CFG.
09798       new StoreInst(UndefValue::get(LI.getType()),
09799                     Constant::getNullValue(Op->getType()), &LI);
09800       return ReplaceInstUsesWith(LI, UndefValue::get(LI.getType()));
09801     }
09802   } 

First, what happens to the StoreInst?  It looks like it is not attached 

Second, I am seeing this happen in a block that is definitely reachable.
Later on the null GEP is removed because it isInstructionTriviallyDead.
But the undef store to null remains since the block is in fact reachable.
This is obviously bad.

So how does the undef store to null appear in the IR when it isn't attached
anywhere and how can I get rid of it?

What is this code trying to do, anyway?


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