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

[David Woodhouse <David.Woodhouse@intel.com>]

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@intel.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
-- 
1.8.4.2