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

[Alexey Kardashevskiy]

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@xxxxxxxxx>
> > ---
> > 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