From: Yonghong Song <yhs@fb.com>
To: <linux-kbuild@vger.kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>,
Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>,
<bpf@vger.kernel.org>, <kernel-team@fb.com>,
Masahiro Yamada <masahiroy@kernel.org>,
Michal Marek <michal.lkml@markovi.net>,
Nick Desaulniers <ndesaulniers@google.com>
Subject: [PATCH kbuild v3 0/2] add an elfnote with type BUILD_COMPILER_LTO_INFO
Date: Wed, 31 Mar 2021 18:24:06 -0700 [thread overview]
Message-ID: <20210401012406.1800957-1-yhs@fb.com> (raw)
Currently, clang LTO built vmlinux won't work with pahole.
LTO introduced cross-cu dwarf tag references and broke
current pahole model which handles one cu as a time.
The solution is to merge all cu's as one pahole cu as in [1].
We would like to do this merging only if cross-cu dwarf
references happens. The LTO build mode is a pretty good
indication for that.
In earlier version of this patch ([2]), clang flag
-grecord-gcc-switches is proposed to add to compilation flags
so pahole could detect "-flto" and then merging cu's.
This will increate the binary size of 1% without LTO though.
Arnaldo suggested to use a note to indicate the vmlinux
is built with LTO. Such a cheap way to get whether the vmlinux
is built with LTO or not helps pahole but is also useful
for tracing as LTO may inline/delete/demote global functions,
promote static functions, etc.
This patch set added an elfnote with type BUILD_COMPILER_LTO_INFO.
The owner of the note is "Linux". Patch #1 did some refactoring
and Patch #2 added the elfnote with BUILD_COMPILER_LTO_INFO to
indicate whether vmlinux is built with LTO or not.
[1] https://lore.kernel.org/bpf/20210325065316.3121287-1-yhs@fb.com/
[2] https://lore.kernel.org/bpf/20210331001623.2778934-1-yhs@fb.com/
Yonghong Song (2):
kbuild: move LINUX_ELFNOTE_BUILD_SALT to elfnote.h
kbuild: add an elfnote with type BUILD_COMPILER_LTO_INFO
include/linux/build-salt.h | 2 --
include/linux/compiler.h | 8 ++++++++
include/linux/elfnote.h | 6 ++++++
init/version.c | 2 ++
scripts/mod/modpost.c | 1 +
5 files changed, 17 insertions(+), 2 deletions(-)
--
2.30.2
next reply other threads:[~2021-04-01 1:26 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-01 1:24 Yonghong Song [this message]
2021-04-01 1:24 ` [PATCH kbuild v3 1/2] kbuild: move LINUX_ELFNOTE_BUILD_SALT to elfnote.h Yonghong Song
2021-04-01 1:24 ` [PATCH kbuild v3 2/2] kbuild: add an elfnote with type BUILD_COMPILER_LTO_INFO Yonghong Song
2021-04-01 18:28 ` Nick Desaulniers
2021-04-01 20:50 ` Yonghong Song
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