Re: arch/powerpc/math-emu/fcmpu.c:13:2: warning: variable 'A_c' set but not used

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 14/06/2023 à 18:04, Nick Desaulniers a écrit :
> On Wed, Jun 14, 2023 at 10:11 AM Christophe Leroy
> <christophe.leroy@csgroup.eu> wrote:
>>
>> Hi,
>>
>> Le 22/05/2023 à 21:00, kernel test robot a écrit :
>>> Hi Christophe,
>>>
>>> FYI, the error/warning still remains.
>>>
>>> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
>>> head:   44c026a73be8038f03dbdeef028b642880cf1511
>>> commit: 7245fc5bb7a966852d5bd7779d1f5855530b461a powerpc/math-emu: Remove -w build flag and fix warnings
>>> date:   9 months ago
>>> config: powerpc-randconfig-r022-20230522 (https://download.01.org/0day-ci/archive/20230523/202305230231.3Wb7Q68G-lkp@intel.com/config)
>>> compiler: clang version 17.0.0 (https://github.com/llvm/llvm-project b0fb98227c90adf2536c9ad644a74d5e92961111)
>>> reproduce (this is a W=1 build):
>>>           wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
>>>           chmod +x ~/bin/make.cross
>>>           # install powerpc cross compiling tool for clang build
>>>           # apt-get install binutils-powerpc-linux-gnu
>>>           # https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=7245fc5bb7a966852d5bd7779d1f5855530b461a
>>>           git remote add linus https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
>>>           git fetch --no-tags linus master
>>>           git checkout 7245fc5bb7a966852d5bd7779d1f5855530b461a
>>>           # save the config file
>>>           mkdir build_dir && cp config build_dir/.config
>>>           COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross W=1 O=build_dir ARCH=powerpc olddefconfig
>>>           COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross W=1 O=build_dir ARCH=powerpc SHELL=/bin/bash arch/powerpc/math-emu/
>>>
>>> If you fix the issue, kindly add following tag where applicable
>>> | Reported-by: kernel test robot <lkp@intel.com>
>>> | Closes: https://lore.kernel.org/oe-kbuild-all/202305230231.3Wb7Q68G-lkp@intel.com/
>>>
>>> All warnings (new ones prefixed by >>):
>>>
>>>>> arch/powerpc/math-emu/fcmpu.c:13:2: warning: variable 'A_c' set but not used [-Wunused-but-set-variable]
>>
>> This code is imported, and there is no plan to make any change to it.
>> Can you desactivate that warning ?
> 
> If the code is imported, and we don't plan to push fixes upstream,
> does it make sense to just set -Wno-unusued-but-set-variable in
> arch/powerpc/math-emu/Makefile?

Yes, that's done already but clang ignores it apparently.

> 
> Wait, what?!
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/powerpc/math-emu/Makefile#n20
> arch/powerpc/math-emu/Makefile already has -Wunused-but-set-variable
> in ccflags-remove-y. But if KBUILD_EXTRA_WARN is set for W=1 builds,
> -Wunused-but-set-variable is added back, but only for math.o and
> math_efp.o.  The warning above comes from building fcmpu.o.  So how
> did  -Wunused-but-set-variable get set in the first place?

Indeed.

I looked once more and found out that:

   clang -Wp,-MMD,arch/powerpc/math-emu/.fcmpu.o.d -nostdinc 
-I./arch/powerpc/include -I./arch/powerpc/include/generated  -I./include 
-I./arch/powerpc/include/uapi -I./arch/powerpc/include/generated/uapi 
-I./include/uapi -I./include/generated/uapi -include 
./include/linux/compiler-version.h -include ./include/linux/kconfig.h 
-include ./include/linux/compiler_types.h -D__KERNEL__ -I ./arch/powerpc 
-DHAVE_AS_ATHIGH=1 -fmacro-prefix-map=./= -Wundef -DKBUILD_EXTRA_WARN1 
-Wall -Wundef -Werror=strict-prototypes -Wno-trigraphs 
-fno-strict-aliasing -fno-common -fshort-wchar -fno-PIE 
-Werror=implicit-function-declaration -Werror=implicit-int 
-Werror=return-type -Wno-format-security -funsigned-char -std=gnu11 
--target=powerpc64le-linux-gnu -fintegrated-as 
-Werror=unknown-warning-option -Werror=ignored-optimization-argument 
-Werror=option-ignored -Werror=unused-command-line-argument -mbig-endian 
-m32 -msoft-float -pipe -mcpu=powerpc -mno-prefixed -mno-pcrel 
-mno-altivec -mno-vsx -mno-mma -mno-spe -fno-asynchronous-unwind-tables 
-mbig-endian -fno-delete-null-pointer-checks -Wno-frame-address 
-Wno-address-of-packed-member -O2 -Wframe-larger-than=1024 
-fno-stack-protector -Wno-gnu -Wno-unused-but-set-variable 
-Wno-unused-const-variable -ftrivial-auto-var-init=zero 
-enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang 
-pg -Wdeclaration-after-statement -Wvla -Wno-pointer-sign 
-Wcast-function-type -Wimplicit-fallthrough -fno-strict-overflow 
-fno-stack-check -Werror=date-time -Werror=incompatible-pointer-types 
-Wextra -Wunused -Wno-unused-parameter -Wmissing-format-attribute 
-Wold-style-definition -Wmissing-include-dirs -Wunused-const-variable 
-Wno-missing-field-initializers -Wno-sign-compare -Wno-type-limits 
-Wno-shift-negative-value -g -Werror 
-DKBUILD_MODFILE='"arch/powerpc/math-emu/fcmpu"' 
-DKBUILD_BASENAME='"fcmpu"' -DKBUILD_MODNAME='"fcmpu"' 
-D__KBUILD_MODNAME=kmod_fcmpu -c -o arch/powerpc/math-emu/fcmpu.o 
arch/powerpc/math-emu/fcmpu.c   ; ./tools/objtool/objtool --mcount 
--uaccess --sec-address   arch/powerpc/math-emu/fcmpu.o


If you move -Wunused before -Wno-unused-but-set-variable, then you don't 
have the warning/error anymore. So what ? What the hell is clang 
behaving that way ?

GCC documentation is explicit, see 
https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html :

   The combined effect of positive and negative forms is that more 
specific options have priority over less specific ones, independently of 
their position in the command-line.

Can clang be fixed ?

Christophe