[BPF] report Invalid usage of the XADD return value" elegantly

[inclyc]

Previously report_fatal_error is used for reporting something goes wrong in the backend, but this is confusing because report_fatal_error basically means there are something unexpected & crashed in the backend.

So, turn this "crash" into an elegant error reporting. After this patch, clang can diagnose it:

bpf-crash.c:4:30: error: Invalid usage of the XADD return value
    4 | u32 next_event_id() { return __sync_fetch_and_add(&GLOBAL_EVENT_ID, 1); }
    |                              ^
1 error generated.

[eddyz87]

The doc for diagnose says:

This function returns, ..., so the caller should leave the compilation process in a self-consistent state, even though the generated code need not be correct.

In this case, we will have a register used in some later instructions, this does not seem to violate any IR-level constraints, so this change should be fine as far as I understand.

[inclyc]

In this case, we will have a register used in some later instructions, this does not seem to violate any IR-level constraints, so this change should be fine as far as I understand.

Actually if there are some errors generated, there are no binary files emitted at all. I suppose this will not affect end-users.

[eddyz87]

Actually if there are some errors generated, there are no binary files emitted at all. I suppose this will not affect end-users.

Yes, code generation stops if diagnose reports errors, I was curious if this change could lead to some incorrect machine code that leads to further error reports and it looks like it can't. So all should be good.