[llvm-dev] BPF backend doesn't generate some of the symtab informations

[Hayakawa Yutaro via llvm-dev]

Hello,

I'm now trying to extract some .symtab information such as source C file name (STT_FILE) from BPF ELF
object which is compiled from following C code by clang, but it seems the generated information is different depends
on the target.

=== test.c ====
#include <stdint.h>

uint64_t test(uint64_t a) {
 return a + 1;
}
============

For example, when I specify amd64 as a backend, the clang generates the symtab like below

$ clang -target amd64 -c test.c
$ readelf -s test.o

Symbol table '.symtab' contains 4 entries:
  Num:    Value          Size Type    Bind   Vis      Ndx Name
    0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
    1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS test.c
    2: 0000000000000000     0 SECTION LOCAL  DEFAULT    2
    3: 0000000000000000    21 FUNC    GLOBAL DEFAULT    2 test

We can see the source file name or type of the symbol “test”. However, when I specify BPF as
a backend, I see output like below.

$ clang -target bpf -c test.c
$ readelf -s test.o

Symbol table '.symtab' contains 2 entries:
  Num:    Value          Size Type    Bind   Vis      Ndx Name
    0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
    1: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT    2 test

Some of the information which was generated on amd64 platform like file name or size or type of the symbol
are missing in this case.

Working environment is Ubuntu18.04-LTS and clang version is 6.0.0-1ubuntu2 (tags/RELEASE_600/final)
which is installed via apt.

I am just a beginner for LLVM, so this question might be silly, but could you give me an idea for solving
this problem or tell me what makes difference?

Regards,
Yutaro