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. > > 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 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
