Re: [PATCH bpf-next 3/5] selftests/bpf: fix test_cpp compilation failure with clang

[Yonghong Song <yhs@fb.com>]

On 4/11/21 9:47 PM, Sedat Dilek wrote:
> On Sun, Apr 11, 2021 at 9:08 PM Yonghong Song <yhs@fb.com> wrote:
> [ ... ]
>>> BTW, did you check (llvm-)objdump output?
>>>
>>> $ /opt/llvm-toolchain/bin/llvm-objdump-12 -Dr test_cpp | grep core_extern
>>
>> This is what I got with g++ compiled test_cpp:
>>
>> $ llvm-objdump -Dr test_cpp | grep core_extern
>>     406a80: e8 5b 01 00 00                callq   0x406be0
>> <_ZL25test_core_extern__destroyP16test_core_extern>
>>     406ab9: e8 22 01 00 00                callq   0x406be0
>> <_ZL25test_core_extern__destroyP16test_core_extern>
>> 0000000000406be0 <_ZL25test_core_extern__destroyP16test_core_extern>:
>>     406be3: 74 1a                         je      0x406bff
>> <_ZL25test_core_extern__destroyP16test_core_extern+0x1f>
>>     406bef: 74 05                         je      0x406bf6
>> <_ZL25test_core_extern__destroyP16test_core_extern+0x16>
>>
> 
> What is the output when compiling with clang++ in your bpf-next environment?

$ llvm-objdump -Dr test_cpp | grep core_extern
$

So looks like all test_core_extern_*() functions are inlined.
This can be confirmed by looking at assembly code.
while for gcc, there is still the call to
   _ZL25test_core_extern__destroyP16test_core_extern
which is
   test_core_extern__destroy(test_core_extern*)

This is just a difference between compiler optimizations
between gcc and clang. We don't need to worry about this.

> 
> - Sedat -
>