Commit-ID: 372cb9eef903d1e18961d2c1a34c75f027f0bd8f
Gitweb: http://git.kernel.org/tip/372cb9eef903d1e18961d2c1a34c75f027f0bd8f
Author: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
AuthorDate: Wed, 29 Jan 2014 12:16:47 +0000
Committer: H. Peter Anvin <hpa@xxxxxxxxxxxxxxx>
CommitDate: Wed, 29 Jan 2014 09:47:49 -0800
x86, build: Build 16-bit code with -m16 where possible
Both clang 3.5 and GCC 4.9 will support this (as of r199754 and r207196
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@xxxxxxxxx>
Link: http://lkml.kernel.org/r/1390997807.20153.133.camel@xxxxxxxxxxxxxxxx
Signed-off-by: H. Peter Anvin <hpa@xxxxxxxxxxxxxxx>
---
arch/x86/Makefile | 16 ++++++++++++----
1 file changed, 12 insertions(+), 4 deletions(-)
diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 9ecbc1c..dbc9b2b 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
--
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