[LLVMdev] Dereferencing null pointers

[Hal Finkel]

----- Original Message -----
> From: "Thomas F Raoux" <thomas.f.raoux at intel.com>
> To: llvmdev at cs.uiuc.edu
> Sent: Thursday, December 11, 2014 1:03:39 PM
> Subject: [LLVMdev] Dereferencing null pointers
> 
> Hi,
> 
> I would like to understand better the meaning of constant null
> pointer in LLVM IR.
> 
> 
> 
> Can the optimizer assume that dereferencing a null pointer is always
> unreachable? Or is it only the case for address space 0? Is it ok to
> have null pointer be a valid pointer for an address space other than
> 0?

Correct, it is valid to have a null pointer dereference in address spaces other than 0.

> 
> 
> 
> In InstCombine pass in InstCombiner::visitLoadInst(LoadInst &LI) I
> see that we replace load of null pointer by unreachable only for
> address space 0. But there is also code doing the following
> transformation for all the address spaces:
> 
> // load (select (cond, null, P)) -> load P
> 
> if(isa<ConstantPointerNull>(SI->getOperand(1)) &&
> 
> LI.getPointerAddressSpace() == 0) {
> 
> LI.setOperand(0, SI->getOperand(2));
> 
> return &LI;
> 
> }
> 
> 
> 
> Is this a bug? Would the correct behavior be to check that the
> pointers’ address space is 0?

But it does, no? it has && LI.getPointerAddressSpace() == 0

 -Hal

> 
> 
> 
> Cheers,
> 
> Thomas
> 
> 
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory