linux-kernel - Re: [llvmlinux] [PATCH 3/4] x86: Allow building 16-bit code with -m16 with toolchains that support it

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Date:	Tue, 14 Jan 2014 20:14:53 +0100
From:	Jan-Simon Möller <dl9pf@....de>
To:	llvmlinux@...ts.linuxfoundation.org
Cc:	David Woodhouse <David.Woodhouse@...el.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [llvmlinux] [PATCH 3/4] x86: Allow building 16-bit code with -m16 with toolchains that support it

Hi David,

what version of clang did you use btw ?

-- 

Dipl.-Ing.
Jan-Simon Möller

jansimon.moeller@....de
Am Mittwoch, 8. Januar 2014, 11:21:22 schrieb David Woodhouse:
> I have this working with LLVM/Clang, and a PR is filed for GCC because
> the current hacks we have to do to *try* to ensure that .code16gcc is
> the first thing in the asm output are horrid.
> 
> Signed-off-by: David Woodhouse <David.Woodhouse@...el.com>
> ---
>  arch/x86/Makefile | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index ef19271..60fc67d 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -13,14 +13,21 @@ endif
> 
>  # How to compile the 16-bit code.  Note we always compile for -march=i386;
>  # that way we can complain to the user if the CPU is insufficient.
> -REALMODE_CFLAGS	:= -m32 -g -Os -D__KERNEL__ -DDISABLE_BRANCH_PROFILING \
> +#
> +# We really want GCC to have a -m16 option like clang, so that we don't
> +# have to play these evil and unreliable tricks to ensure that our
> +# asm(".code16gcc") is first in the asm output. http://gcc.gnu.org/PR59672
> +CODE16GCC_CFLAGS := -m32 -include $(srctree)/arch/x86/boot/code16gcc.h \
> +		    $(call cc-option, -fno-toplevel-reorder,\
> +		      $(call cc-option, -fno-unit-at-a-time))
> +M16_CFLAGS	 := -m16
> +
> +REALMODE_CFLAGS	:= -g -Os -D__KERNEL__ -DDISABLE_BRANCH_PROFILING \
> +		   $(call cc-option, $(M16_CFLAGS), $(CODE16GCC_CFLAGS)) \
>  		   -Wall -Wstrict-prototypes -march=i386 -mregparm=3 \
> -		   -include $(srctree)/arch/x86/boot/code16gcc.h \
>  		   -fno-strict-aliasing -fomit-frame-pointer -fno-pic \
>  		   -mno-mmx -mno-sse \
>  		   $(call cc-option, -ffreestanding) \
> -		   $(call cc-option, -fno-toplevel-reorder,\
> -		   $(call cc-option, -fno-unit-at-a-time)) \
>  		   $(call cc-option, -fno-stack-protector) \
>  		   $(call cc-option, -mpreferred-stack-boundary=2)
>  export REALMODE_CFLAGS

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