[LLVMdev] Constant folding inttoptr i32 0 to null pointer?

[Benyei, Guy]

Well, address 0 is pretty well aligned for whatever alignment we like.

So I understand, that “load volatile i32, i32 addrspace(1)* null” is a valid memory access, accessing address 0?

Thanks
      Guy

From: David Majnemer [mailto:david.majnemer at gmail.com]
Sent: Tuesday, June 09, 2015 22:55
To: Benyei, Guy
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Constant folding inttoptr i32 0 to null pointer?



On Tue, Jun 9, 2015 at 12:32 PM, David Majnemer <david.majnemer at gmail.com<mailto:david.majnemer at gmail.com>> wrote:
'load volatile i32 addrspace(1)* null' seems fine to me.  However, it looks like instcombine will turn:
define i32 @foo() {
entry:
  %std_ld.i = load volatile i32, i32 addrspace(1)* null
  ret i32 %std_ld.i
}

into:
define i32 @foo() {
entry:
  %std_ld.i = load volatile i32, i32 addrspace(1)* null, align 536870912
  ret i32 %std_ld.i
}

which is not ok.

On second thought, I think that high alignment is benign.  It simply indicates that your backend is free to load the value as-if it were aligned to an arbitrary boundary (because it is 1 byte, 2 byte, 4 byte, etc. aligned).


On Tue, Jun 9, 2015 at 11:34 AM, Benyei, Guy <guy.benyei at intel.com<mailto:guy.benyei at intel.com>> wrote:
Thanks David,
It turns out, that the address space I was using was not 0, and yet the pointer was constant folded to null.

Here is the sequence:

Unoptimized code:

define i32 @foo() #0 {
entry:
  %address.addr.i = alloca i32, align 4
  %value.i = alloca i32, align 4
  store i32 0, i32* %address.addr.i, align 4
  %0 = load i32* %address.addr.i, align 4
  %1 = inttoptr i32 %0 to i32 addrspace(1)*
  %std_ld.i = load volatile i32 addrspace(1)* %1
  store i32 %std_ld.i, i32* %value.i, align 4
  %2 = load i32* %value.i, align 4
  ret i32 %2
}

After optimization (early CSE):

define i32 @foo() #0 {
entry:
  %std_ld.i = load volatile i32 addrspace(1)* null, align 536870912
  ret i32 %std_ld.i
}

The contant folder doesn’t seem to check for address space, it simply checks if the integer in question is zero, and folds the inttoptr to null:

Constant *llvm::ConstantFoldCastInstruction(unsigned opc, Constant *V,
                                            Type *DestTy) {

...

  if (V->isNullValue() && !DestTy->isX86_MMXTy())
    return Constant::getNullValue(DestTy);
...


Is this a bug?

Thanks
     Guy



From: David Majnemer [mailto:david.majnemer at gmail.com<mailto:david.majnemer at gmail.com>]
Sent: Tuesday, June 09, 2015 18:45
To: Benyei, Guy
Cc: llvmdev at cs.uiuc.edu<mailto:llvmdev at cs.uiuc.edu>
Subject: Re: [LLVMdev] Constant folding inttoptr i32 0 to null pointer?



On Tue, Jun 9, 2015 at 5:57 AM, Benyei, Guy <guy.benyei at intel.com<mailto:guy.benyei at intel.com>> wrote:
Hello,
It seems that ConstantFoldCastInstruction in ConstantFold.cpp folds inttoptr instruction with 0 as operand to a null pointer. It makes sense, when talking about a C-style frontend, as the C99 spec (6.3.2.3) states:

“An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant.”

On the other hand, some architectures use 0 as a valid memory location, and this constant folding seems to be possibly harmful when the code actually tries to access the memory location at address 0.
Is this behavior intentional? Do I miss something? Will a load from address null try to access address 0, or may it become an undef value?

LLVM assumes that the null pointer in address space zero can never be successfully dereferenced.  You must utilize some other address space to dereference a null pointer.


Thanks
       Guy


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