| Message ID | 20220330234528.1426991-2-nathan@kernel.org |
|---|---|
| State | Handled Elsewhere |
| Headers | show |
| Series | Remove '-mno-global-merge' from KBUILD_CFLAGS | expand |
On Thu, Mar 31, 2022 at 7:46 AM Nathan Chancellor <nathan@kernel.org> wrote: > > 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 61163efae020 ("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> > --- This seems to work fine here under the KUnit tooling, with both x86_64 and arm64. That admittedly doesn't do anything with modules, so wouldn't reveal any of those issues, but at least the warnings about --mno-global-merge existing remain gone. Tested-by: David Gow <davidgow@google.com> Cheers, -- David
On Thu, Mar 31, 2022 at 5:27 AM Nathan Chancellor <nathan@kernel.org> wrote: > > 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 61163efae020 ("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> > --- > Makefile | 4 ---- > 1 file changed, 4 deletions(-) > > diff --git a/Makefile b/Makefile > index daeb5c88b50b..f2723d9bfca4 100644 > --- a/Makefile > +++ b/Makefile > @@ -784,10 +784,6 @@ ifdef CONFIG_CC_IS_CLANG > KBUILD_CPPFLAGS += -Qunused-arguments > # The kernel builds with '-std=gnu89' so use of GNU extensions is acceptable. > KBUILD_CFLAGS += -Wno-gnu > -# 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 > else > > # gcc inanely warns about local variables called 'main' > -- > 2.35.1 > I have tested this several times and was able to boot into bar metal - no problems with building and/or booting my kernel-modules. Tested-by: Sedat Dilek <sedat.dilek@gmail.com> Reviewed-by: Sedat Dilek <sedat.dilek@gmail.com> Just as a side-note: As with Linux v5.18-rc1 and -std=gnu11 we change the above comment ...? # The kernel builds with '-std=gnu89' so use of GNU extensions is acceptable. KBUILD_CFLAGS += -Wno-gnu - Sedat -
On Thu, Mar 31, 2022 at 09:11:12AM +0200, Sedat Dilek wrote: > On Thu, Mar 31, 2022 at 5:27 AM Nathan Chancellor <nathan@kernel.org> wrote: > > > > 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 61163efae020 ("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> > > --- > > Makefile | 4 ---- > > 1 file changed, 4 deletions(-) > > > > diff --git a/Makefile b/Makefile > > index daeb5c88b50b..f2723d9bfca4 100644 > > --- a/Makefile > > +++ b/Makefile > > @@ -784,10 +784,6 @@ ifdef CONFIG_CC_IS_CLANG > > KBUILD_CPPFLAGS += -Qunused-arguments > > # The kernel builds with '-std=gnu89' so use of GNU extensions is acceptable. > > KBUILD_CFLAGS += -Wno-gnu > > -# 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 > > else > > > > # gcc inanely warns about local variables called 'main' > > -- > > 2.35.1 > > > > I have tested this several times and was able to boot into bar metal - > no problems with building and/or booting my kernel-modules. > > Tested-by: Sedat Dilek <sedat.dilek@gmail.com> > Reviewed-by: Sedat Dilek <sedat.dilek@gmail.com> I would be very concerned if you did see any impact, given this flag is ARM specific :) thanks as always for verifying! > Just as a side-note: > As with Linux v5.18-rc1 and -std=gnu11 we change the above comment ...? > > # The kernel builds with '-std=gnu89' so use of GNU extensions is acceptable. > KBUILD_CFLAGS += -Wno-gnu It was updated as part of the shift to '-std=gnu11': https://git.kernel.org/linus/e8c07082a810fbb9db303a2b66b66b8d7e588b53 The UML tree is based on 5.17-rc6, which does not have that change. There should not be a merge conflict though. Cheers, Nathan
On Thu, Mar 31, 2022 at 5:37 PM Nathan Chancellor <nathan@kernel.org> wrote: > > On Thu, Mar 31, 2022 at 09:11:12AM +0200, Sedat Dilek wrote: > > On Thu, Mar 31, 2022 at 5:27 AM Nathan Chancellor <nathan@kernel.org> wrote: > > > > > > 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 61163efae020 ("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> > > > --- > > > Makefile | 4 ---- > > > 1 file changed, 4 deletions(-) > > > > > > diff --git a/Makefile b/Makefile > > > index daeb5c88b50b..f2723d9bfca4 100644 > > > --- a/Makefile > > > +++ b/Makefile > > > @@ -784,10 +784,6 @@ ifdef CONFIG_CC_IS_CLANG > > > KBUILD_CPPFLAGS += -Qunused-arguments > > > # The kernel builds with '-std=gnu89' so use of GNU extensions is acceptable. > > > KBUILD_CFLAGS += -Wno-gnu > > > -# 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 > > > else > > > > > > # gcc inanely warns about local variables called 'main' > > > -- > > > 2.35.1 > > > > > > > I have tested this several times and was able to boot into bar metal - > > no problems with building and/or booting my kernel-modules. > > > > Tested-by: Sedat Dilek <sedat.dilek@gmail.com> > > Reviewed-by: Sedat Dilek <sedat.dilek@gmail.com> > > I would be very concerned if you did see any impact, given this flag is > ARM specific :) thanks as always for verifying! > > > Just as a side-note: > > As with Linux v5.18-rc1 and -std=gnu11 we change the above comment ...? > > > > # The kernel builds with '-std=gnu89' so use of GNU extensions is acceptable. > > KBUILD_CFLAGS += -Wno-gnu > > It was updated as part of the shift to '-std=gnu11': > > https://git.kernel.org/linus/e8c07082a810fbb9db303a2b66b66b8d7e588b53 > > The UML tree is based on 5.17-rc6, which does not have that change. > There should not be a merge conflict though. > Ah OK, a different base for UML. - Sedat -
diff --git a/Makefile b/Makefile index daeb5c88b50b..f2723d9bfca4 100644 --- a/Makefile +++ b/Makefile @@ -784,10 +784,6 @@ ifdef CONFIG_CC_IS_CLANG KBUILD_CPPFLAGS += -Qunused-arguments # The kernel builds with '-std=gnu89' so use of GNU extensions is acceptable. KBUILD_CFLAGS += -Wno-gnu -# 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 else # gcc inanely warns about local variables called 'main'
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 61163efae020 ("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> --- Makefile | 4 ---- 1 file changed, 4 deletions(-)