[LLVMdev] Is address space 1 reserved?

[Pete Cooper]

> On Jan 7, 2015, at 12:05 PM, Matt Arsenault <arsenm2 at gmail.com> wrote:
> 
> 
>> On Jan 7, 2015, at 2:55 PM, Philip Reames <listmail at philipreames.com <mailto:listmail at philipreames.com>> wrote:
>> 
>> 
>> On 01/07/2015 11:52 AM, Matt Arsenault wrote:
>>> 
>>>> On Jan 7, 2015, at 2:25 PM, Owen Anderson <resistor at mac.com <mailto:resistor at mac.com>> wrote:
>>>> 
>>>> I'm not aware of any such restriction, and I know of several LLVM based systems that use address space 1 for something other than that.
>>>> 
>>>> -Owen
>>> 
>>> Yes, this would be a problem for us. We use 1 for a normal address space where 0 is invalid. However, we also have a problem where some other address spaces do want 0 to be a valid address, which just sort of don’t work correctly now.
>> If you have an example with a null in a non-0 address space being mishandled, please file a bug.  We'll fix them as we find them.  
> 
> I think the problems aren’t so much that accessing 0 doesn’t work (although I imagine there are problems with that), but expectations of comparison with null. The main problem I’m aware of is comparisons with null pointers. The first global object in addrspace(3) will have the address of 0, so if a user does if (x != NULL), it will not behave as expected. For C I think this is supposed to be fixed by changing the value of NULL to -1, but I don’t think that is currently supported. That is also complicated because the null value is different for different address spaces, and I think the actual null pointer value must be 0 for C++. It doesn’t really turn up often in real code so I don’t think anybody has really spent time thinking about how to properly solve this.
Actually, we had a similar discussion a while ago about this: http://lists.cs.uiuc.edu/pipermail/llvmdev/2013-August/064624.html <http://lists.cs.uiuc.edu/pipermail/llvmdev/2013-August/064624.html>

In the link I gave, I proposed using global metadata to describe address spaces.  Its useful, for example, to know that an address space is always to constant memory, i.e., the CL model.

I think later in the conversation we also thought about defining the relationships between address spaces in a similar method to tbaa on types.  Then you could do address space AA.

Pete
> 
> -Matt
> 
> 
>>> 
>>> -Matt
>>> 
>>> 
>>> 
>>>> 
>>>> On Jan 7, 2015, at 1:18 PM, Philip Reames <listmail at philipreames.com <mailto:listmail at philipreames.com>> wrote:
>>>> 
>>>>> On the review for http://reviews.llvm.org/D6808 <http://reviews.llvm.org/D6808>, majnemer <http://reviews.llvm.org/p/majnemer/> commented that:
>>>>> "Address space 1 has a special meaning in LLVM, it's identical to address space 0 except for the fact that "null" may be dereferenced. You might want to consider a different address space."
>>>>> 
>>>>> This is the first I've heard of this and I can't find any documentation about it being reserved, either in general, or specifically for x86.  Can anyone clarify?
>>>>> 
>>>>> The only address spaces with special meanings I know of are:
>>>>> - 0 (the normal address space, null is not dereferencable)
>>>>> - 256 - TLS, GS relative addressing
>>>>> - 257 - FS relative addressing
>>>>> 
>>>>> Philip
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>> 
> 
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