Re: [PATCH v2 2/7] Revert "kbuild: disable clang's default use of -fmerge-all-constants"

[Fangrui Song]

On 2020-08-31, Nathan Chancellor wrote:
> On Mon, Aug 31, 2020 at 05:23:21PM -0700, Nick Desaulniers wrote:
> > This reverts commit 87e0d4f0f37fb0c8c4aeeac46fff5e957738df79.
> >
> > This was fixed in clang-6; the minimum supported version of clang in the
> > kernel is clang-10 (10.0.1).
> >
> > Link: https://reviews.llvm.org/rL329300.
> > Link: https://github.com/ClangBuiltLinux/linux/issues/9
> > Suggested-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
> > Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
>
> Reviewed-by: Nathan Chancellor <natechancellor@xxxxxxxxx>

How about expanding "This was fixed in clang-6" to be
-fno-merge-all-constants has been the default since clang-6?

(Both gcc|clang -fmerge-all-constants can cause an assertion failure for
the example on https://bugs.llvm.org/show_bug.cgi?id=18538 )

Reviewed-by: Fangrui Song <maskray@xxxxxxxxxx>

> > ---
> >  Makefile | 9 ---------
> >  1 file changed, 9 deletions(-)
> >
> > diff --git a/Makefile b/Makefile
> > index 37739ee53f27..144ac6a073ff 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -932,15 +932,6 @@ KBUILD_CFLAGS += $(call cc-disable-warning, maybe-uninitialized)
> >  # disable invalid "can't wrap" optimizations for signed / pointers
> >  KBUILD_CFLAGS	+= $(call cc-option,-fno-strict-overflow)
> >
> > -# clang sets -fmerge-all-constants by default as optimization, but this
> > -# is non-conforming behavior for C and in fact breaks the kernel, so we
> > -# need to disable it here generally.
> > -KBUILD_CFLAGS	+= $(call cc-option,-fno-merge-all-constants)
> > -
> > -# for gcc -fno-merge-all-constants disables everything, but it is fine
> > -# to have actual conforming behavior enabled.
> > -KBUILD_CFLAGS	+= $(call cc-option,-fmerge-constants)
> > -
> >  # Make sure -fstack-check isn't enabled (like gentoo apparently did)
> >  KBUILD_CFLAGS  += $(call cc-option,-fno-stack-check,)
> >
> > --
> > 2.28.0.402.g5ffc5be6b7-goog
>
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