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BPF: simplify IR generation for __builtin_btf_type_id()

Authored by yonghong-song on Aug 3 2020, 4:12 PM.

Description

BPF: simplify IR generation for __builtin_btf_type_id()

This patch simplified IR generation for builtin_btf_type_id().
For
builtin_btf_type_id(obj, flag), previously IR builtin
looks like

if (obj is a lvalue)
  llvm.bpf.btf.type.id(obj.ptr, 1, flag)  !type
else
  llvm.bpf.btf.type.id(obj, 0, flag)  !type

The purpose of the 2nd argument is to differentiate

__builtin_btf_type_id(obj, flag) where obj is a lvalue

vs.

__builtin_btf_type_id(obj.ptr, flag)

Note that obj or obj.ptr is never used by the backend
and the obj argument is only used to derive the type.
This code sequence is subject to potential llvm CSE when

  • obj is the same .e.g., nullptr
  • flag is the same
  • metadata type is different, e.g., typedef of struct "s" and strust "s".

In the above, we don't want CSE since their metadata is different.

This patch change IR builtin to

llvm.bpf.btf.type.id(seq_num, flag)  !type

and seq_num is always increasing. This will prevent potential
llvm CSE.

Also report an error if the type name is empty for
remote relocation since remote relocation needs non-empty
type name to do relocation against vmlinux.

Differential Revision: https://reviews.llvm.org/D85174