[LLVMdev] Null pointers with a non-0 representation

[Micah Villmow]

I ran into the same problem in OpenCL with Local/Private address spaces and I believe it was discussed here in the past. So I would suggest looking through the archives as I believe some of your questions are answered there.
This post might be a good starting point:
http://lists.cs.uiuc.edu/pipermail/llvmdev/2011-October/044101.html

Also the SPIR provisional spec as of last November deals with this issue if I remember correctly, so maybe you can discuss with Boaz Ouriel if he is still at Intel Israel about how they went about handling this problem, or look at the SPIR related posts from last year.

Hope this helps,
Micah

From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On Behalf Of Kuperstein, Michael M
Sent: Sunday, November 17, 2013 1:15 AM
To: LLVMdev at cs.uiuc.edu
Subject: [LLVMdev] Null pointers with a non-0 representation

I feel a bit silly asking this, but here goes.

The C spec  does not require the null pointer to be implemented as an integer with the 0 bit-pattern. The implementation may arbitrarily choose any other (integer, but not necessarily) representation, as long as it's distinct from any legal pointer. The only requirement is that casting an integer 0 to a pointer type results in a null pointer.
LLVM IR appears to have similar the casting behavior.  That is, if I try to create a pointer with integer value 0 (e.g. by constructing an inttoptr constant expression with a i32 0 argument), it gets immediately folded into a null pointer constant. Furthermore, a PtrToInt from a null pointer results in 0 (which is even "stricter" than the C spec.)

So, that raises a few question:

a)      Are those the desired semantics? I guess if clang generates this kind of IntToPtr instructions (as opposed to directly resolving the cast to a null pointer) this is unavoidable. But otherwise, why does LLVM treat null pointers and pointers with the integer value 0 as equivalent?
It's completely natural for most platforms, but isn't necessarily the right thing to do.

b)      Assuming this is really desirable, should the fact that casting an integer 0 to a pointer type results in a null pointer be documented in the langref (as part of the inttoptr/ptrtoint documentation)? Or is it too "low-level"?

c)       Let's say I want to create a pointer with the numeric value 0, distinct from the null pointer, because 0 really does represent a valid pointer on my platform. Should this possible at the IR level? If it should be, then how?

Anyone has any inputs, except laughing me out of the (virtual) room? :-)

Thanks,
   Michael

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