[cfe-dev] Helpers to compute sizeof/alignof/offsetof?

[Chris Lattner]

On Jan 17, 2008, at 9:05 AM, Ted Kremenek wrote:
>> The only big gotcha will be sizeof(VLA), which doesn't return a
>> constant.  I don't think there is a good way to represent this  
>> without
>> knowing the client.  The code generator (f.e.) needs to expand this
>> out to LLVM IR instructions.
>
> Agreed, clients will need to perform their own reasoning about VLAs.
> What the code generator and (say) the static analysis engine do are
> context specific.

Right.

> For the "generic" method that returns the sizeof, we may consider
> having it return a variant type that wraps an integer primitive.  The
> variant could also have a flag indicating whether or the not the
> sizeof returned is valid.  This way we can have the generic method
> return "unknown" for VLAs if clients just blindly call sizeof on VLA
> types.  For example:

How about a really simple variant: int64_t :)

Define -1 to be "invalid" and -2 to be "vla" and you're golden.

-Chris

>
>
> class SizeofVariant {
>   unsigned value;  // probably will use something other than
> unsigned, but you get the idea
>   bool IsValid;
> public:
>   SizeOfVariant(unsigned V) : value(V), IsValid(true) {}
>   explicit SizeOfVariant() : value(0), IsValid(false) {}
>
>   unsigned getValue() const { assert (IsValid); return value; }
>   bool isValid() const { return IsValid; };
>
>   operator bool() const { return isValid(); }
>   operator unsigned() const { return getValue(); }
> };
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