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. 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. 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
