Re: [PATCH] kbuild: Allow Clang global merging if !MODULES

[Masahiro Yamada <masahiroy@kernel.org>]

On Sun, Jul 5, 2020 at 11:43 AM Nathan Chancellor
<natechancellor@gmail.com> wrote:
>
> Hi Danny,
>
> On Thu, Jul 02, 2020 at 04:39:29PM -0700, Danny Lin wrote:
> > The old reasoning for disabling Clang's global merging optimization is
> > that it breaks modpost by coalescing many symbols into _MergedGlobals.
> > However, modpost is only used in builds with dynamic modules;
> > vmlinux.symvers is still created during standalone builds, but it's
> > effectively just an empty dummy file.

I am not convinced with this statement.

modpost is used even if CONFIG_MODULES=n

You can see the following log for allnoconfig.

  MODPOST vmlinux.symvers


Yes, vmlinux.symver is empty.
This is because EXPORT_SYMBOL is no-op when CONFIG_MODULES=n.



> >
> > Enabling the optimization whenever possible allows us to reap the
> > benefits of reduced register pressure when many global variables are
> > used in the same function.
>
> Have you run into any place within the kernel that this is the case or
> this more of a "could help if this ever happens" type of deal?
>
> > An x86 defconfig kernel built with this optimization boots fine in qemu,
> > and a downstream 4.14 kernel has been used on arm64 for nearly a year
> > without any issues caused by this optimization.
>
> If I am reading LLVM's source correctly, this option only seems relevant
> for ARM and AArch64?
>
> $ rg --no-heading createGlobalMergePass
> llvm/lib/CodeGen/GlobalMerge.cpp:679:Pass *llvm::createGlobalMergePass(const TargetMachine *TM, unsigned Offset,
> llvm/lib/Target/AArch64/AArch64TargetMachine.cpp:524:    addPass(createGlobalMergePass(TM, 4095, OnlyOptimizeForSize,
> llvm/lib/Target/ARM/ARMTargetMachine.cpp:456:    addPass(createGlobalMergePass(TM, 127, OnlyOptimizeForSize,
> llvm/include/llvm/CodeGen/Passes.h:419:  Pass *createGlobalMergePass(const TargetMachine *TM, unsigned MaximalOffset,
>
> Otherwise, I think this is probably okay. According to [1], when the
> optimization level is less than -O3, we get a less aggressive version of
> this optimization level, which is good for code size:
>
> https://github.com/llvm/llvm-project/commit/8207641251706ea808df6d2a1ea8f87b8ee04c6d


I do not understand how -mglobal-merge works.
Do you have test code to generate _MergedGlobals ?


Thanks.





> However, we do potentially get merging of extern globals if we do not
> specify -mglobal-merge (if I am reading the source correctly), which
> this commit claims might hurt performance? Not sure if there is any way
> to test or verify that?
>
> https://github.com/llvm/llvm-project/commit/de73404b8c4332190750537eb93ce0d5b6451300
>
> > Signed-off-by: Danny Lin <danny@kdrag0n.dev>
> > ---
> >  Makefile | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/Makefile b/Makefile
> > index a60c98519c37..f04c3639cf61 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -772,10 +772,13 @@ ifdef CONFIG_CC_IS_CLANG
> >  KBUILD_CPPFLAGS += -Qunused-arguments
> >  KBUILD_CFLAGS += -Wno-format-invalid-specifier
> >  KBUILD_CFLAGS += -Wno-gnu
> > +
> > +ifdef CONFIG_MODULES
> >  # CLANG uses a _MergedGlobals as optimization, but this breaks modpost, as the
> >  # source of a reference will be _MergedGlobals and not on of the whitelisted names.
> >  # See modpost pattern 2
> >  KBUILD_CFLAGS += -mno-global-merge
> > +endif
> >  else
> >
> >  # These warnings generated too much noise in a regular build.
> > --
> > 2.27.0
> >
>
> Cheers,
> Nathan



--
Best Regards
Masahiro Yamada