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kbuild: Remove '-mno-global-merge'

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This flag is specific to clang, where it is only used by the 32-bit and
64-bit ARM backends. In certain situations, the presence of this flag
will cause a warning, as shown by commit 6580c5c18fb3 ("um: clang: Strip
out -mno-global-merge from USER_CFLAGS").

Since commit 61163efae0 ("kbuild: LLVMLinux: Add Kbuild support for
building kernel with Clang") that added this flag back in 2014, there
have been quite a few changes to the GlobalMerge pass in LLVM. Building
several different ARCH=arm and ARCH=arm64 configurations with LLVM 11
(minimum) and 15 (current main version) with this flag removed (i.e.,
with the default of '-mglobal-merge') reveals no modpost warnings, so it
is likely that the issue noted in the comment is no longer relevant due
to changes in LLVM or modpost, meaning this flag can be removed.

If any new warnings show up that are a result of the removal of this
flag, it can be added back under arch/arm{,64}/Makefile to avoid
warnings on other architectures.

Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: David Gow <davidgow@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Reviewed-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Change-Id: Ice39a960619319828b83c8091798fe383395a2b0
Signed-off-by: Alexander Winkowski <dereference23@outlook.com>
This commit is contained in:
Nathan Chancellor 2022-03-30 16:45:27 -07:00 committed by Giovanni Ricca
parent 2f25fcac4c
commit e83c037c23
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1 changed files with 0 additions and 4 deletions
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Makefile
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@ -797,10 +797,6 @@ KBUILD_CFLAGS += -Wno-format-invalid-specifier
KBUILD_CFLAGS += -Wno-gnu
# Quiet clang warning: comparison of unsigned expression < 0 is always false
KBUILD_CFLAGS += -Wno-tautological-compare
# CLANG uses a _MergedGlobals as optimization, but this breaks modpost, as the
# source of a reference will be _MergedGlobals and not on of the whitelisted names.
# See modpost pattern 2
KBUILD_CFLAGS += -mno-global-merge
KBUILD_CFLAGS += $(call cc-disable-warning, undefined-optimized)
else