On Tue, Oct 23, 2018 at 4:11 PM Nathan Chancellor <[email protected]> wrote: > > Commit 77b0bf55bc67 ("kbuild/Makefile: Prepare for using macros in > inline assembly code to work around asm() related GCC inlining bugs") > added -Wa,- to KBUILD_CFLAGS, which breaks compiling with Clang (hangs > indefinitely at compiling init/main.o). This happens because while Clang > accepts -pipe (and has it documented in its list of supported flags), it > silently ignores it after this 2010 commit (thanks to Nick Desaulniers > for tracking this down), meaning that gas just infinitely waits for > stdin and never receives it. > > https://github.com/llvm-mirror/clang/commit/c19a12dc3d441bec62eed55e312b76c12d6d9022 > > Initially, I had suggested just add -Wa,- to KBUILD_CFLAGS when GCC was > being used but that was before realizing it is because Clang doesn't do > anything with -pipe. H. Peter Anvin suggested checking to see if -pipe > gives us any gains out of GCC. Turns out it might actually be hurting: > > With -pipe: > > real 3m40.813s > real 3m44.449s > real 3m39.648s > > Without -pipe: > > real 3m38.492s > real 3m38.335s > real 3m38.975s > > The issue of -Wa,- being passed along to gas without -pipe being > supported should still probably be fixed on the LLVM side (open issue: > https://bugs.llvm.org/show_bug.cgi?id=39410) but this is not as much of > a workaround anymore since it helps both GCC and Clang. > > Link: https://github.com/ClangBuiltLinux/linux/issues/213 > Suggested-by: H. Peter Anvin <[email protected]> > Signed-off-by: Nathan Chancellor <[email protected]>
Thanks Nathan for this patch and the timings and HPA for the suggestions. Tested-and-Reviewed-by: Nick Desaulniers <[email protected]> > --- > arch/x86/Makefile | 4 +--- > 1 file changed, 1 insertion(+), 3 deletions(-) > > diff --git a/arch/x86/Makefile b/arch/x86/Makefile > index 73f4831283ac..672c689c1faa 100644 > --- a/arch/x86/Makefile > +++ b/arch/x86/Makefile > @@ -213,8 +213,6 @@ ifdef CONFIG_X86_64 > KBUILD_LDFLAGS += $(call ld-option, -z max-page-size=0x200000) > endif > > -# Speed up the build > -KBUILD_CFLAGS += -pipe > # Workaround for a gcc prelease that unfortunately was shipped in a suse > release > KBUILD_CFLAGS += -Wno-sign-compare > # > @@ -239,7 +237,7 @@ archheaders: > archmacros: > $(Q)$(MAKE) $(build)=arch/x86/kernel arch/x86/kernel/macros.s > > -ASM_MACRO_FLAGS = -Wa,arch/x86/kernel/macros.s -Wa,- > +ASM_MACRO_FLAGS = -Wa,arch/x86/kernel/macros.s > export ASM_MACRO_FLAGS > KBUILD_CFLAGS += $(ASM_MACRO_FLAGS) > > -- > 2.19.1 > -- Thanks, ~Nick Desaulniers
