[LLVMdev] Suggestion: Support union types in IR

[Talin]

Chris Lattner wrote:
> On May 5, 2009, at 8:09 PM, Talin wrote:
>
>   
>> I wanted to mention, by the way, that my need/desire for this hasn't
>> gone away :)
>>
>> And my wish list still includes support for something like uintptr_t  
>> - a
>> primitive integer type that is defined to always be the same size as a
>> pointer, however large or small that may be on different platforms.  
>> (So
>> that the frontend doesn't need to know how big a pointer is and can
>> generate the same IR that works on both 32-bit and 64-bit platforms.)
>>     
>
> Why not just use a pointer, such as i8*?
>   
Suppose I have an STL-like container that has a 'begin' and 'end' 
pointer. Now I want to find the size() of the container. Since you 
cannot subtract pointers in LLVM IR, you have to cast them to an integer 
type first. But what integer type do you cast them to? I suppose you 
could simply always cast them to i64, and hope that the backend will 
generate efficient code for the subtraction, but I have no way of 
knowing this.

Now, I'm going to anticipate what I think will be your next argument, 
which is that at some point I must know the size of the result since I 
am assigning the result of size() to some interger variable eventually. 
Which is true, however, if the size of that eventual variable is smaller 
than a pointer, then I want to check it for overflow before I do the 
assignment. I don't want to just do a blind bitcast and have the top 
bits be lopped off.

The problem of checking for overflow when assigning from an integer of 
unknown size to an integer of known size is left as an exercise for the 
reader.
> -Chris
>