[BPF] preserve debuginfo types for builtin __builtin__btf_type_id()

Authored by yonghong-song on Feb 11 2020, 4:55 PM.


[BPF] preserve debuginfo types for builtin builtinbtf_type_id()

The builtin function

u32 btf_type_id = __builtin_btf_type_id(param, 0)

can help preserve type info for the following use case:

extern void foo(..., void *data, int size);
int test(...) {
  struct t { int a; int b; int c; } d;
  d.a = ...; d.b = ...; d.c = ...;
  foo(..., &d, sizeof(d));

The function "foo" in the above only see raw data and does not
know what type of the data is. In certain cases, e.g., logging,
the additional type information will help pretty print.

This patch handles the builtin in BPF backend. It includes
an IR pass to translate the IR intrinsic to a load of
a global variable which carries the metadata, and an MI
pass to remove the intermediate load of the global variable.
Finally, in AsmPrinter pass, proper instruction are generated.

In the above example, the second argument for __builtin_btf_type_id()
is 0, which means a relocation for local adjustment,
i.e., w.r.t. bpf program BTF change, will be generated.
The value 1 for the second argument means
a relocation for remote adjustment, e.g., against vmlinux.

Differential Revision: https://reviews.llvm.org/D74572