[LLVMdev] InstCombine Question

[Chris Lattner]

On Fri, 4 Apr 2008, David Greene wrote:
> 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
> xform.
> 09792     if (isa<ConstantPointerNull>(GEPI0) &&
> 09793         cast<PointerType>(GEPI0->getType())->getAddressSpace() == 0) {
> 09794       // Insert a new store to null instruction before the load to
> indicate
> 09795       // that this code is not reachable.  We do this instead of
> inserting
> 09796       // an unreachable instruction directly because we cannot modify
> the
> 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
> anywhere.

The &LI argument causes it to be inserted before the load.

> 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.

This xform doesn't have anything to do with block reachability.  It 
effectively transforms "load from null pointer" into an unreachable 
instruction.  Load from null pointer is undefined, so this is a safe xform 
regardless of where it occurs.

>
> 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?

Don't do undefined behavior? :)

-Chris

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