Re: [PATCH] x86: Remove -pipe from KBUILD_CFLAGS

[Nadav Amit <namit@vmware.com>]

From: Nathan Chancellor <natechancellor@gmail.com>
Sent: October 23, 2018 at 11:11:25 PM GMT
> To: Thomas Gleixner; Ingo Molnar; Borislav Petkov
> Cc: H. Peter Anvin; x86@kernel.org, linux-kernel@vger.kernel.org, Nadav Amit; Kees Cook; Masahiro Yamada; Nick Desaulniers; Nathan Chancellor
> Subject: [PATCH] x86: Remove -pipe from KBUILD_CFLAGS
> 
> 
> 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://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fllvm-mirror%2Fclang%2Fcommit%2Fc19a12dc3d441bec62eed55e312b76c12d6d9022&amp;data=02%7C01%7Cnamit%40vmware.com%7C56678e0018894955601908d6393ce6d2%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C1%7C0%7C636759331072072215&amp;sdata=AjoP%2BerS26K58QQINE0UDJ00ftyGMa5ovlaKvZViZ4w%3D&amp;reserved=0
> 
> 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://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugs.llvm.org%2Fshow_bug.cgi%3Fid%3D39410&amp;data=02%7C01%7Cnamit%40vmware.com%7C56678e0018894955601908d6393ce6d2%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C1%7C0%7C636759331072082215&amp;sdata=MlOeR1p8%2BRROFxhjw3SQi0Uai9zuUeEesBxSMaM8wsg%3D&amp;reserved=0) but this is not as much of
> a workaround anymore since it helps both GCC and Clang.
> 
> Link: https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2FClangBuiltLinux%2Flinux%2Fissues%2F213&amp;data=02%7C01%7Cnamit%40vmware.com%7C56678e0018894955601908d6393ce6d2%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C1%7C0%7C636759331072082215&amp;sdata=q9iCdYa80V49o0BnZqFf3WYcMwRMl%2BtuS1MN8RZCPFI%3D&amp;reserved=0
> Suggested-by: H. Peter Anvin <hpa@zytor.com>
> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
> ---
> 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

This pipe was a pain in the … I have actually already removed it in one
version of the versions of my patch-set.

Reviewed-by: Nadav Amit <namit@vmware.com>