[clang] [llvm] [Clang] Correct __builtin_dynamic_object_size for subobject types (PR #78526)

[Bill Wendling via cfe-commits]

bwendling wrote:

> Taking a step back, while this patch is not the right direction, we can and should do better for the original example. Probably the best way to do that is to analyze the operand to `__builtin_[dynamic_]object_size` in the frontend and compute a better bound based on the form of the expression. It looks like it should be feasible to produce a tighter bound for an array-to-pointer-decay expression like `f.bar[argc]` in subobject mode, as:
> 
> * `select llvm.is.constant(argc) and (argc < 0 or argc >= 2), 0, sizeof(f.bar[argc])` for the non-dynamic case, and just
> * `select (argc < 0 or argc >= 2), 0, sizeof(f.bar[argc])` for the dynamic case.
> 
> A possibly simpler alternative would be for the frontend to pass an upper bound on the result to the LLVM builtin in mode 1, so Clang could say "I know the result will never be more than 40" and LLVM could provide either that size or the complete object size, whichever is smaller. That wouldn't give as tight a bound for the argc == 2 case, though.

It might be possible to do something like this. If retaining precise type information from the front-end is the sticking issue, it could be as simple as casting this pointer top the type of the sub-object. I'll see what I can do.

https://github.com/llvm/llvm-project/pull/78526