[PATCH] D68822: [WIP][BPF] Support external globals

[Yonghong Song via Phabricator via llvm-commits]

yonghong-song created this revision.
yonghong-song added reviewers: ast, anakryiko.
Herald added subscribers: llvm-commits, hiraditya.
Herald added a project: LLVM.

Emit types for all globals. Previously, the types for external
variables and globals with section names are omitted.

For external variables, also provide additional information
about the purpose of the extern's through section name.
For example, for the following example,

  -bash-4.4$ cat t.c
  extern int a __attribute__((section("bpf_linux")));
  extern int b __attribute__((section("bpf_curr_module")));
  extern int c __attribute__((section("bpf_cross_module")));
  int test() {
    return a + b + c; 
  }
  -bash-4.4$ clang -target bpf -g -O2 -S t.c

Generated BTF_KIND_VARs:

      .long   66                      # BTF_KIND_VAR(id = 4)
      .long   234881024               # 0xe000000
      .long   0
      .long   18
  <=== 1 << 4 | 2   extern linux symbols
      .long   68                      # BTF_KIND_VAR(id = 5)
      .long   234881024               # 0xe000000
      .long   0
      .long   34 
  <=== 2 << 4 | 2   current module to-be-patched symbols
      .long   70                      # BTF_KIND_VAR(id = 6)
      .long   234881024               # 0xe000000
      .long   0
      .long   2
  <=== 0 << 4 | 2   extern cross bpf module symbols

The current u32 is defined as below in uapi/linux/btf.h:
enum {

  BTF_VAR_STATIC = 0,
  BTF_VAR_GLOBAL_ALLOCATED,

};
struct btf_var {

  __u32   linkage;

};

We may need to add macros to access linkage and other information.
We need to carve bits with some reserved.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D68822

Files:
  llvm/lib/Target/BPF/BTF.h
  llvm/lib/Target/BPF/BTFDebug.cpp

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