linux-kernel - Re: [PATCH kernel v3] powerpc/makefile: Do not redefine $(CPP) for preprocessor

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Message-ID: <af1e3d74-a373-09ae-ba61-8db2a906d71a@ozlabs.ru>
Date:   Fri, 14 May 2021 11:56:01 +1000
From:   Alexey Kardashevskiy <aik@...abs.ru>
To:     Nathan Chancellor <nathan@...nel.org>,
        linuxppc-dev@...ts.ozlabs.org
Cc:     linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org,
        clang-built-linux@...glegroups.com,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Michael Ellerman <mpe@...erman.id.au>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Segher Boessenkool <segher@...nel.crashing.org>
Subject: Re: [PATCH kernel v3] powerpc/makefile: Do not redefine $(CPP) for
 preprocessor



On 14/05/2021 04:59, Nathan Chancellor wrote:
> On 5/13/2021 4:59 AM, Alexey Kardashevskiy wrote:
>> The $(CPP) (do only preprocessing) macro is already defined in Makefile.
>> However POWERPC redefines it and adds $(KBUILD_CFLAGS) which results
>> in flags duplication. Which is not a big deal by itself except for
>> the flags which depend on other flags and the compiler checks them
>> as it parses the command line.
>>
>> Specifically, scripts/Makefile.build:304 generates ksyms for .S files.
>> If clang+llvm+sanitizer are enabled, this results in
>>
>> -emit-llvm-bc -fno-lto -flto -fvisibility=hidden \
>>   -fsanitize=cfi-mfcall -fno-lto  ...
>>
>> in the clang command line and triggers error:
>>
>> clang-13: error: invalid argument '-fsanitize=cfi-mfcall' only allowed 
>> with '-flto'
>>
>> This removes unnecessary CPP redefinition. Which works fine as in most
>> place KBUILD_CFLAGS is passed to $CPP except
>> arch/powerpc/kernel/vdso64/vdso(32|64).lds. To fix vdso, this does:
>> 1. add -m(big|little)-endian to $CPP
>> 2. add target to $KBUILD_CPPFLAGS as otherwise clang ignores 
>> -m(big|little)-endian if
>> the building platform does not support big endian (such as x86).
>>
>> Signed-off-by: Alexey Kardashevskiy <aik@...abs.ru>
>> ---
>> Changes:
>> v3:
>> * moved vdso cleanup in a separate patch
>> * only add target to KBUILD_CPPFLAGS for CLANG
>>
>> v2:
>> * fix KBUILD_CPPFLAGS
>> * add CLANG_FLAGS to CPPFLAGS
>> ---
>>   Makefile              | 1 +
>>   arch/powerpc/Makefile | 3 ++-
>>   2 files changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/Makefile b/Makefile
>> index 15b6476d0f89..5b545bef7653 100644
>> --- a/Makefile
>> +++ b/Makefile
>> @@ -576,6 +576,7 @@ CC_VERSION_TEXT = $(subst $(pound),,$(shell $(CC) 
>> --version 2>/dev/null | head -
>>   ifneq ($(findstring clang,$(CC_VERSION_TEXT)),)
>>   ifneq ($(CROSS_COMPILE),)
>>   CLANG_FLAGS    += --target=$(notdir $(CROSS_COMPILE:%-=%))
>> +KBUILD_CPPFLAGS    += --target=$(notdir $(CROSS_COMPILE:%-=%))
> 
> You can avoid the duplication here by just doing:
> 
> KBUILD_CPPFLAGS    += $(CLANG_FLAGS)

This has potential of duplicating even more flags which is exactly what 
I am trying to avoid here.


> I am still not super happy about the flag duplication but I am not sure 
> I can think of a better solution. If KBUILD_CPPFLAGS are always included 
> when building .o files,


My understanding is that KBUILD_CPPFLAGS should not be added for .o. Who 
does know or decide for sure about what CPPFLAGS are for? :)


> maybe we should just add $(CLANG_FLAGS) to 
> KBUILD_CPPFLAGS instead of KBUILD_CFLAGS?
> 
>>   endif
>>   ifeq ($(LLVM_IAS),1)
>>   CLANG_FLAGS    += -integrated-as
>> diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
>> index 3212d076ac6a..306bfd2797ad 100644
>> --- a/arch/powerpc/Makefile
>> +++ b/arch/powerpc/Makefile
>> @@ -76,6 +76,7 @@ endif
>>   ifdef CONFIG_CPU_LITTLE_ENDIAN
>>   KBUILD_CFLAGS    += -mlittle-endian
>> +KBUILD_CPPFLAGS    += -mlittle-endian
>>   KBUILD_LDFLAGS    += -EL
>>   LDEMULATION    := lppc
>>   GNUTARGET    := powerpcle
>> @@ -83,6 +84,7 @@ MULTIPLEWORD    := -mno-multiple
>>   KBUILD_CFLAGS_MODULE += $(call cc-option,-mno-save-toc-indirect)
>>   else
>>   KBUILD_CFLAGS += $(call cc-option,-mbig-endian)
>> +KBUILD_CPPFLAGS += $(call cc-option,-mbig-endian)
>>   KBUILD_LDFLAGS    += -EB
>>   LDEMULATION    := ppc
>>   GNUTARGET    := powerpc
>> @@ -208,7 +210,6 @@ KBUILD_CPPFLAGS    += -I $(srctree)/arch/$(ARCH) 
>> $(asinstr)
>>   KBUILD_AFLAGS    += $(AFLAGS-y)
>>   KBUILD_CFLAGS    += $(call cc-option,-msoft-float)
>>   KBUILD_CFLAGS    += -pipe $(CFLAGS-y)
>> -CPP        = $(CC) -E $(KBUILD_CFLAGS)
>>   CHECKFLAGS    += -m$(BITS) -D__powerpc__ -D__powerpc$(BITS)__
>>   ifdef CONFIG_CPU_BIG_ENDIAN
>>
> 

-- 
Alexey