From: Yonghong Song <yhs@fb.com>
To: <sedat.dilek@gmail.com>
Cc: <bpf@vger.kernel.org>, Andrii Nakryiko <andrii@kernel.org>,
Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>,
<kernel-team@fb.com>, Nick Desaulniers <ndesaulniers@google.com>
Subject: Re: [PATCH bpf-next 3/5] selftests/bpf: fix test_cpp compilation failure with clang
Date: Mon, 12 Apr 2021 07:15:40 -0700 [thread overview]
Message-ID: <1b52c9f6-cf2e-f7f4-3ce6-e7acd449884e@fb.com> (raw)
In-Reply-To: <CA+icZUWYNFpL+ueU3i2+1N=C1s51BgRv0D1kusfhzZsYscMTUA@mail.gmail.com>
On 4/11/21 11:06 PM, Sedat Dilek wrote:
> On Mon, Apr 12, 2021 at 7:42 AM Yonghong Song <yhs@fb.com> wrote:
>>
>>
>>
>> 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.
>>
>
> ( My previous comment was from my samrtphone - so I started into my desktop. )
>
> Thanks for your analysis and hint about inlining.
>
> My inbox is full with that different handling of inlining "GCC vs. LLVM/Clang".
>
> When I recall correctly and I have not to care about the inlining
> optimization of clang++, then we can drop
> "$(OUTPUT)/test_core_extern.skel.h" from the BPF selftests Makefile:
>
> [ tools/testing/selftests/bpf/Makefile ]
>
> # Make sure we are able to include and link libbpf against c++.
> $(OUTPUT)/test_cpp: test_cpp.cpp $(OUTPUT)/test_core_extern.skel.h $(BPFOBJ)
> $(call msg,CXX,,$@)
> $(Q)$(CXX) $(CFLAGS) test_cpp.cpp $(BPFOBJ) $(LDLIBS) -o $@
>
> Note: This is with your patchset applied against Linus Git
>
> As we have the include here:
>
> [ tools/testing/selftests/bpf/test_cpp.cpp ]
>
> /* SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) */
> #include <iostream>
> #include <bpf/libbpf.h>
> #include <bpf/bpf.h>
> #include <bpf/btf.h>
> #include "test_core_extern.skel.h"
> ...
>
> With and without keeping "test_core_extern.skel.h" I got the same
> output with g++ and llvm-objdump.
> Compiling with clang++ did not show that "CPP-file and C-header"
> build-error when dropping "test_core_extern.skel.h" from the Makefile.
Thanks for testing. Great to see all g++/clang++ compilations passed!
>
> As said here all my testings with Linus Git not bpf-next.
>
> Thanks for your precious time!
>
> - Sedat -
>
next prev parent reply other threads:[~2021-04-12 14:15 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-10 16:49 [PATCH bpf-next 0/5] support build selftests/bpf with clang Yonghong Song
2021-04-10 16:49 ` [PATCH bpf-next 1/5] selftests: set CC to clang in lib.mk if LLVM is set Yonghong Song
2021-04-11 10:22 ` Sedat Dilek
2021-04-11 16:51 ` Yonghong Song
2021-04-11 17:10 ` Sedat Dilek
2021-04-10 16:49 ` [PATCH bpf-next 2/5] tools: allow proper CC/CXX/... override with LLVM=1 in Makefile.include Yonghong Song
2021-04-11 10:24 ` Sedat Dilek
2021-04-11 16:52 ` Yonghong Song
2021-04-10 16:49 ` [PATCH bpf-next 3/5] selftests/bpf: fix test_cpp compilation failure with clang Yonghong Song
2021-04-11 10:47 ` Sedat Dilek
2021-04-11 17:20 ` Yonghong Song
2021-04-11 17:31 ` Sedat Dilek
2021-04-11 19:08 ` Yonghong Song
2021-04-12 4:47 ` Sedat Dilek
2021-04-12 5:42 ` Yonghong Song
2021-04-12 6:06 ` Sedat Dilek
2021-04-12 14:15 ` Yonghong Song [this message]
2021-04-13 4:32 ` Andrii Nakryiko
2021-04-13 6:12 ` Yonghong Song
2021-04-13 17:50 ` Andrii Nakryiko
2021-04-10 16:49 ` [PATCH bpf-next 4/5] selftests/bpf: silence clang compilation warnings Yonghong Song
2021-04-11 11:12 ` Sedat Dilek
2021-04-11 17:40 ` Yonghong Song
2021-04-10 16:49 ` [PATCH bpf-next 5/5] bpftool: fix a clang compilation warning Yonghong Song
2021-04-11 11:05 ` Sedat Dilek
2021-04-11 17:24 ` Yonghong Song
2021-04-10 17:23 ` [PATCH bpf-next 0/5] support build selftests/bpf with clang Alexei Starovoitov
2021-04-10 17:38 ` Yonghong Song
2021-04-10 19:19 ` Sedat Dilek
2021-04-11 16:46 ` Yonghong Song
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