[LLVMdev] How to handle size_t in front ends?

[Talin]

Chris Lattner wrote:
> On Wed, 21 May 2008, Talin wrote:
>   
>> On a related topic: The source-level debugging descriptors require you
>> to know up front what the sizeof pointer types are. Is there any hope of
>> the frontend remaining blissfully unaware of platform details?
>>     
>
> I really don't know how to do this.  The current debug info stuff depends 
> on emitting size info into the IR.  At this point, I don't think there is 
> a good way around this.  Improvements to the design are welcome of course.
>
> -Chris
>   
As I understand this, this issue and others like it all require a 
difficult step to be taken, which is to introduce the concept of a 
constant whose value is not known until code generation time or at least 
until the compilation target is fully known. These "late bound 
constants" could then be used to implement "sizeof(type)" and other 
constants whose value is different on different targets.

On the C++ side, you would have ConstantSize(Type) which would be usable 
anywhere that you could use ConstantInt, except that you can't actually 
inspect the integer value of the constant or use it in expressions. That 
part is fairly simple; It gets more complicated if you want to try 
defining operators such as sizeof(A) + sizeof(B); This requires the IR 
to support arbitrary expressions, which I don't think you want. But most 
of the time, you don't want to add the size of A and B, you want 
sizeof({A, B}), which doesn't require any special syntax other than 
sizeof itself.

So in other words, the frontend sees the "sizeof" constant purely as a 
symbolic, opaque object, while the code generator simply converts it 
into a ConstantInt.

This makes filling in the dwarf debugging structures relatively easy as 
long as you have an LLVM type reference to use as a measuring stick. In 
fact, I'd likely make the hypothetical "DebugBuilder" API such that most 
of the info was derived from an LLVM type given as a parameter, with 
just a few additional parameters to specify the things that cannot be 
determined just from looking at the the LLVM type.

-- Talin