On Sun, Jul 5, 2020 at 11:43 AM Nathan Chancellor <natechancellor@xxxxxxxxx> 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@xxxxxxxxxxx> > > --- > > 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
