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Re: [PATCH] kbuild: clang: remove crufty HOSTCFLAGS

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  Re: [PATCH] kbuild: clang: remove crufty HOSTCFLAGS Masahiro Yamada <yamada.masahiro@socionext.com> - 2017-09-28 13:00 +0200
    Re: [PATCH] kbuild: clang: remove crufty HOSTCFLAGS Nick Desaulniers <nick.desaulniers@gmail.com> - 2017-10-01 01:20 +0200
      Re: [PATCH] kbuild: clang: remove crufty HOSTCFLAGS Segher Boessenkool <segher@kernel.crashing.org> - 2017-10-01 02:40 +0200

#1741423 — Re: [PATCH] kbuild: clang: remove crufty HOSTCFLAGS

Hi Nick,

2017-09-26 11:28 GMT+09:00 Nick Desaulniers <nick.desaulniers@gmail.com>:
> When compiling with `make CC=clang HOSTCC=clang`, I was seeing warnings
> that clang did not recognize -fno-delete-null-pointer-checks for HOSTCC
> targets.  These were added in commit 61163efae020 ("kbuild: LLVMLinux:
> Add Kbuild support for building kernel with Clang").  That patch wraps
> that flag in cc-option for KBUILD_CFLAGS, but not hostcc-option for
> HOSTCFLAGS. Either hostcc-option did not exist, or the author was not
> setting HOSTCC to clang as well as CC when authored.
>
> It's not clear why the other warnings were disabled, and just for
> HOSTCFLAGS, but I can remove them, add -Werror to HOSTCFLAGS and compile
> with clang just fine.
>
> Signed-off-by: Nick Desaulniers <nick.desaulniers@gmail.com>
> ---
> * It may also be worthwhile keep the old flags, and simply wrap
>   everything in hostcc-option.
>
>  Makefile | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index d1119941261c..2e908969e0d8 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -301,16 +301,12 @@ HOST_LFS_LIBS := $(shell getconf LFS_LIBS)
>  HOSTCC       = gcc
>  HOSTCXX      = g++
>  HOSTCFLAGS   := -Wall -Wmissing-prototypes -Wstrict-prototypes -O2 \
> +               $(call hostcc-option,-fno-delete-null-pointer-checks) \
>                 -fomit-frame-pointer -std=gnu89 $(HOST_LFS_CFLAGS)


You call hostcc-option
before Kbuild.include is included around line 341.

So, $(call hostcc-option, ...) returns always an empty string here
whether the compiler supports the option or not.



>  HOSTCXXFLAGS := -O2 $(HOST_LFS_CFLAGS)
>  HOSTLDFLAGS  := $(HOST_LFS_LDFLAGS)
>  HOST_LOADLIBES := $(HOST_LFS_LIBS)
>
> -ifeq ($(shell $(HOSTCC) -v 2>&1 | grep -c "clang version"), 1)
> -HOSTCFLAGS  += -Wno-unused-value -Wno-unused-parameter \
> -               -Wno-missing-field-initializers -fno-delete-null-pointer-checks
> -endif
> -


The logic is very strange in the first place.

Even very old GCC supports -fno-delete-null-pointer-checks,
but clang does not.

Here, -fno-delete-null-pointer-checks is added only when
we are using clang for HOSTCC.  This is opposite.

I guess we can remove all of them
unless somebody can explain the rationale.



>  # Decide whether to build built-in, modular, or both.
>  # Normally, just do built-in.
>




-- 
Best Regards
Masahiro Yamada

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#1742703

On Thu, Sep 28, 2017 at 07:52:35PM +0900, Masahiro Yamada wrote:
> 2017-09-26 11:28 GMT+09:00 Nick Desaulniers <nick.desaulniers@gmail.com>:
> >  HOSTCFLAGS   := -Wall -Wmissing-prototypes -Wstrict-prototypes -O2 \
> > +               $(call hostcc-option,-fno-delete-null-pointer-checks) \
> >                 -fomit-frame-pointer -std=gnu89 $(HOST_LFS_CFLAGS)
> 
> You call hostcc-option
> before Kbuild.include is included around line 341.
> 
> So, $(call hostcc-option, ...) returns always an empty string here
> whether the compiler supports the option or not.

So calling a yet-to-be defined variable results in an empty string
rather than a loud failure?  Chalk that up there with language features
no one ever asked for.  That kind of implicit conversion gets languages
like JavaScript (with its loose type system, not that C is without its
own implicit type conversions/promotions) in a lot of hot water.

If that's the case, why are includes not at the top of Makefiles, if
silent failure is a possibility?  Is there a reason the include is so
far into the Makefile?

Is your sugguestion to raise the include or lower the HOSTCFLAGS
definition?

> > -ifeq ($(shell $(HOSTCC) -v 2>&1 | grep -c "clang version"), 1)
> > -HOSTCFLAGS  += -Wno-unused-value -Wno-unused-parameter \
> > -               -Wno-missing-field-initializers -fno-delete-null-pointer-checks
> > -endif
>
> The logic is very strange in the first place.
>
> Even very old GCC supports -fno-delete-null-pointer-checks,
> but clang does not.
>
> Here, -fno-delete-null-pointer-checks is added only when
> we are using clang for HOSTCC.  This is opposite.
>
> I guess we can remove all of them
> unless somebody can explain the rationale.

+llvm-linux

I suppose maybe different ARCH's have different host binaries made
during the build?  I tested x86_64 and arm64.  The commit message that
added them missed any context or justification.

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#1742715

On Sat, Sep 30, 2017 at 04:14:50PM -0700, Nick Desaulniers wrote:
> On Thu, Sep 28, 2017 at 07:52:35PM +0900, Masahiro Yamada wrote:
> > 2017-09-26 11:28 GMT+09:00 Nick Desaulniers <nick.desaulniers@gmail.com>:
> > >  HOSTCFLAGS   := -Wall -Wmissing-prototypes -Wstrict-prototypes -O2 \
> > > +               $(call hostcc-option,-fno-delete-null-pointer-checks) \
> > >                 -fomit-frame-pointer -std=gnu89 $(HOST_LFS_CFLAGS)
> > 
> > You call hostcc-option
> > before Kbuild.include is included around line 341.
> > 
> > So, $(call hostcc-option, ...) returns always an empty string here
> > whether the compiler supports the option or not.
> 
> So calling a yet-to-be defined variable results in an empty string
> rather than a loud failure?  Chalk that up there with language features
> no one ever asked for.  That kind of implicit conversion gets languages
> like JavaScript (with its loose type system, not that C is without its
> own implicit type conversions/promotions) in a lot of hot water.

make --warn-undefined-variables

(and it warns all over the place during a kernel build -- having undefined
variables expand to the empty string is a useful feature, too, not just a
trap for the unwary).


Segher

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