Allow `counted_by` argument to support simple arithmetic expressions

[seanm]

Consider the following C:

struct mystruct
{
    unsigned len;
    unsigned data[] __attribute__((counted_by(len)));
};

This works with recent clang.

But it would be nice if simple arithmetic expressions could be used in the counted_by argument, like counted_by(len - 1) or counted_by(len * 4), things like that. Trying to do so gives:

<source>:4:47: error: 'counted_by' argument must be a simple declaration reference
    4 |     unsigned data[] __attribute__((counted_by(len - 1)));
      |                                               ^~~~~~~

See: https://godbolt.org/z/sx6sY1fjG

The gcc folks seemed to consider this too: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108896#c36 though from godbolt it seems they don't support it either.

There are several places in the libusb codebase that would benefit from this. The most complicated case would be counted_by((len - 2) / 2).

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Sean McBride (seanm)

Consider the following C:

struct mystruct
{
    unsigned len;
    unsigned data[] __attribute__((counted_by(len)));
};

This works with recent clang.

But it would be nice if simple arithmetic expressions could be used in the counted_by argument, like counted_by(len - 1) or counted_by(len * 4), things like that. Trying to do so gives:

<source>:4:47: error: 'counted_by' argument must be a simple declaration reference
    4 |     unsigned data[] __attribute__((counted_by(len - 1)));
      |                                               ^~~~~~~

See: https://godbolt.org/z/sx6sY1fjG

The gcc folks seemed to consider this too: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108896#c36 though from godbolt it seems they don't support it either.

There are several places in the libusb codebase that would benefit from this. The most complicated case would be counted_by((len - 2) / 2).

[shafik]

CC @rapidsna @AaronBallman