[cfe-dev] Addition of function parameter attribute to help __builtin_object_size

[George Burgess IV via cfe-dev]

Hello friends,

I plan to add a function parameter attribute called carry_size (name
suggestions welcome). The goal of this attribute is to help Clang/LLVM
answer `__builtin_object_size(Ptr, Type)` calls when the object pointed to
by Ptr isn't visible in the caller of __builtin_object_size.

An example of its use:
#define CS(N) __attribute__((carry_size(N)))
void __attribute__((noinline)) Func(void *const p CS(0)) {
  printf("%d\n", __builtin_object_size(p, 0));
}

int main() {
  char c[10];
  Func(c); // prints "10\n"
}

For more info, please see the doc here:
https://docs.google.com/document/d/14TwNLxFL_jhiYn7_kppNUDiaTopRAIY5bc-B8NcnKMM/edit?usp=sharing

Thanks!
George
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