[PATCH] D74668: [Clang][BPF] implement __builtin_btf_type_id() builtin function

[Yonghong Song via Phabricator via cfe-commits]

yonghong-song added a comment.

In D74668#2019558 <https://reviews.llvm.org/D74668#2019558>, @anakryiko wrote:

> what's the use case for flag==0 (no relocation)? why using built-in at all in such case? Also flag==1 means relocate to local BTF ID or remote (kernel) BTF ID? Do you plan to add flag=2 as well to cover both cases? Or am I misunderstanding the meaning of this flag?


Originally, I thought flag = 0 for the following use case:

  e.g., they just want to know the type of a particular local structure for pretty print purpose.
  Note that currently only types for global/extern variables, function parameters are recorded in btf.
    int test() {
       struct { int a; int b; ...} ctx;
       btf_id = __builtin_btf_type_id(ctx, 0);
       bpf_seq_write(seq, &btf_id, sizeof(btf_id));
       bpf_seq_write(seq, &ctx, sizeof(ctx));
       ...
    }

But obviously without relocation, this will not work with btf deduplication, future static linking etc.

flag 1: for relocation. My original thinking is for vmlinux relocation.

        

I think you brought a good point about local relocation, so will need
to change the flag to:

  flag 0 : local relocation
  flag 1:  vmlinux relocation

Two more relocation types will be generated:

  BTF_TYPE_ID_LOCAL
  BTF_TYPE_ID_REMOTE


Repository:
  rG LLVM Github Monorepo

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https://reviews.llvm.org/D74668