[LLVMdev] Suggestion: Support union types in IR

[Talin]

Chris Lattner wrote:
> On Dec 30, 2008, at 12:41 PM, Talin wrote:
>   
>> I've been thinking about how to represent unions or "disjoint types"  
>> in LLVM IR. At the moment, the only way I know to achieve this right  
>> now is to create a struct that is as large as the largest type in  
>> the union and then bitcast it to access the fields contained within.  
>> However, that requires that the frontend know the sizes of all of  
>> the various low-level types (the "size_t" problem, which has been  
>> discussed before), otherwise you get problems trying to mix pointer  
>> and non-pointer types.
>>     
>
> That's an interesting point.  As others have pointed out, we've  
> resisted having a union type because it isn't strictly needed for the  
> current set of front-ends.  If a front-end is trying to generate  
> target-independent IR though, I can see the utility.  The "gep trick"  
> won't work for type generation.
>
>   
>> It seems to me that adding a union type to the IR would be a logical  
>> extension to the language. The syntax for declaring a union would be  
>> similar to that of declaring a struct. To access a union member, you  
>> would use GetElementPointer, just as if it were a struct. The only  
>> difference is that in this case, the GEP doesn't actually modify the  
>> address, it merely returns the input argument as a different type.  
>> In all other ways, unions would be treated like structs, except that  
>> the size of the union would always be the size of the largest  
>> member, and all of the fields within the union would be located  
>> located at relative offset zero.
>>     
>
> Yes, your proposal makes sense, for syntax, I'd suggest:  u{ i32, float}
>
>   
>> Unions could of course be combined with other types:
>>
>>    {{int|float}, bool} *
>>    n = getelementptr i32 0, i32 0, i32 1
>>
>> So in the above example, the GEP returns a pointer to the float field.
>>     
>
> I don't have a specific problem with adding this.  The cost of doing  
> so is that it adds (a small amount of) complexity to a lot of places  
> that walk the type graphs.  The only pass that I predict will be  
> difficult to update to handle this is the BasicAA pass, which reasons  
> about symbolic (not concrete) offsets and should return mustalias in  
> the appropriate cases.  Also, to validate this, I think llvm-gcc  
> should start generating this for C unions where possible.
>
> If you're interested in implementing this and seeing all the details  
> of the implementation through to the end, I don't see significant  
> problems.  I think adding a simple union type would make more sense  
> than adding first-class support for a *discriminated* union.
>   
So, I spent a little bit of time looking at how to code this. Rather 
than defining a brand-new LLVM type, I think the best approach is to 
generalize the existing StructType. We can change the "isPacked" flag 
(which is stored in the SubclassData of the type) to a 3-valued enum 
called StructLayout, which has 3 members: UnpackedLayout, PackedLayout, 
and UnionLayout.

After that, it's mostly a case of hunting down everywhere that calls 
"isPacked", and adding the appropriate case. :)
> -Chris
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