[PATCH] x86: Build 16-bit code with -m16 where possible

[David Woodhouse <dwmw2@infradead.org>]

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Both clang 3.5 and GCC 4.9 will support this (as of r207196 and r199754
respectively). Both have been tested to produce booting kernels when the
16-bit code is built with -m16. (Modulo LLVM PR3997, at least.)

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
---
Updated comments, and retested, now that GCC supports it too. I think
the two preparatory patches already made it into tip:x86/build?

Note that for clang -m16 to work you do need the patch I just posted to
fix has_eflag() too. And a workaround for PR3997 as before.

 arch/x86/Makefile | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 0f68712..0141cb0 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -13,14 +13,22 @@ 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 \
+#
+# The -m16 option is supported by GCC >= 4.9 and clang >= 3.5. For
+# older versions of GCC, we need to play evil and unreliable tricks to
+# attempt to ensure that our asm(".code16gcc") is first in the asm
+# output.
+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
-- 
1.8.5.3



-- 
dwmw2

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