[LLVMdev] Casting between address spaces and address space semantics

[Matthijs Kooijman]

Hi Mon Ping,

> If I remember correctly, I was also not fond of passing another 
> TargetAddrSpace reference to the TargetData object.  I was hoping that we 
> could encode the information as a target description string like we do for 
> ABI information.   I just don't want to end up with too many objects that 
> describe the machine. One can argue that we shouldn't pollute the 
> TargetData since it describes the ABI  alignment, size, and object 
> layoutbut I feel that this data fits naturally there.  If you and other 
> people feel it is cleaner with a separate pass, I'm fine with it.

Perhaps encoding it in TargetData makes sense. However, I was avoiding this
for now, since Chris commented a while back that he wanted to have it in
TargetData "only if absolutely required".

However, thinking of this a bit more I do see your point about TargetData.
Another interesting advantage is that it would be a lot easier to make things
consistent between clang and LLVM, by simply using the TargetData string
that gets embedded in the module.

So, I guess embedding this info in TargetData makes more sense. How would this
look like? I would think of something like:

as1:<2:=3:>4:!3

This would mean address space 1 is a subset of 2, equivalent to 3, a superset
of 4 and disjoint with 3. A number of these could be present in a TargetData
string, to fully describe the situation. Any relations not described mean
disjoint. Relations can also be implicitely defined, ie, as1:<2-as2:<3 also
implies as1:<3. 

I'm not sure if the > should be present, since that's always reversible. Also,
! is probably not so nice for disjoint, any other suggestions?

As to implementing this, I'm thinking of a equivalency table (for every
address space, store the equivalent address space with the lowest id) and for
each of those lowest id spaces in each equivalency group store a set of
address spaces that are a subset. This set should be complete, so when the
string says A > B > C, the set should store both B and C as subsets of A.

This allows for resolving the relation between two address spaces by two
lookups in the equivalency table and (one or) two lookups in the subset table.
No results in the subset table means the relation is disjoint, then.

Any comments on this? Chris, would this be acceptable?

> I want to treat my next point with some delicacy as I don't want to start a 
> religious war.  I just want to get clarification from the community on the 
> use of multiple inheritance for the case of Phases like 
> AllDisjointAddrspaces.  From what I can gather, the use of multiple 
> inheritance is to separate the interface (TargetAddrSpace) to access data 
> from the interface of the phase (ImmutablePhase).  In this case, will we 
> ever create a concrete class from TargetAddrSpace that doesn't also derive 
> from ImmutablePass? If not, I don't think is worth using multiple 
> inheritance in this case.
I think you are right here, changing the inheritance in this way also works
fine.

>>> For the case of a GetElementPointer, we are replacing a bitcast to a
>>> pointer, getelem with a getelem bitcast.  The assumption is the latter
>>> bitcast will hopefully go away when we iterate through those uses.
>> Uh? Is this a comment about what the current code or my patch does, or what
>> it should do? I don't understand what you mean here.
> My comment was more on what I thought the patch did and I wanted to confirm 
> that it will cleanup newly generated bit cast that are created.
In that case, yes, the newly generated bitcasts should be iteratively cleaned
up whenever possible.

>> True, anyone actually using address space should make sure that this info
>> is correct anyway. So, no need for an unknown default?
> That is my feeling.
Ok.

> No, you got  my point even though my example is not a good one. If the 
> address calculation was using a variable, I don't think we can fold it into 
> the GEP and we might lose this information.
Ie, a variable that is stored to, you mean? In that case, the address space is
probably propagated until the store instruction. Perhaps it can even be
propagated through the store instruction, so it stores to a bitcasted pointer
(ie, bitcast i32 addrspace(2)* * to i32 addrspace(1) * *). I suspect other
parts of instcombine might handle this from here and change the variable's
type to i32 addrspace(1), if possible. I guess this is something for later on.

> The point I was trying to make is that the information needs to be
> propagated through any address calculation when possible.
Yes, but that shouldn't be too hard to add to the existing code.

Gr.

Matthijs
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