[LLVMdev] Is address space 1 reserved?

[Pete Cooper]

> On Jan 7, 2015, at 2:10 PM, Philip Reames <listmail at philipreames.com> wrote:
> 
> 
> On 01/07/2015 12:17 PM, Pete Cooper wrote:
>> 
>>> On Jan 7, 2015, at 12:05 PM, Matt Arsenault <arsenm2 at gmail.com <mailto: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.
> I'm a bit hesitant* to do this with metadata.  At least to start with, these seem like backend specific properties.  Why not introduce some hooks into Target or Subtarget with the appropriate queries?
> 
> * Reasons for hesitancy:
> - Not sure these are purely optimizations - is dropping always legal?
It would be global metadata so can’t be dropped (or just isn’t right now so its ok anyway)
> - How do we merge such things in LTO?  
Thats a good point.
> - Forward serialization?  It might be better to define the properties better than design a reasonable scheme.
As is that.

I think at the time I proposed metadata we didn’t have TTI or anything else similar.  I would be happy to say that things like null ptr deref are defined only for address space 0, and all other address spaces can only be optimised if TTI supports it.  This means no TTI would default to not optimizing anything other than address space 0 which I think is good.  

You could also move all of the checks to TTI and define that NoTTI gives an answer for address space 0 and ignores all others.  Then you can just query TTI everywhere instead of special casing address space 0 everywhere.

Pete
>> 
>> 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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