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.) [ hpa: folded test for -m16 into M16_CFLAGS ] Signed-off-by: David Woodhouse <David.Woodhouse@intel.com> Link: http://lkml.kernel.org/r/1390997807.20153.133.camel@i7.infradead.org Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
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@ -13,14 +13,22 @@ endif
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# How to compile the 16-bit code. Note we always compile for -march=i386;
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# that way we can complain to the user if the CPU is insufficient.
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REALMODE_CFLAGS := -m32 -g -Os -D__KERNEL__ -DDISABLE_BRANCH_PROFILING \
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#
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# The -m16 option is supported by GCC >= 4.9 and clang >= 3.5. For
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# older versions of GCC, we need to play evil and unreliable tricks to
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# attempt to ensure that our asm(".code16gcc") is first in the asm
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# output.
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CODE16GCC_CFLAGS := -m32 -include $(srctree)/arch/x86/boot/code16gcc.h \
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$(call cc-option, -fno-toplevel-reorder,\
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$(call cc-option, -fno-unit-at-a-time))
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M16_CFLAGS := $(call cc-option, -m16, $(CODE16GCC_CFLAGS))
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REALMODE_CFLAGS := $(M16_CFLAGS) -g -Os -D__KERNEL__ \
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-DDISABLE_BRANCH_PROFILING \
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-Wall -Wstrict-prototypes -march=i386 -mregparm=3 \
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-include $(srctree)/arch/x86/boot/code16gcc.h \
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-fno-strict-aliasing -fomit-frame-pointer -fno-pic \
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-mno-mmx -mno-sse \
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$(call cc-option, -ffreestanding) \
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$(call cc-option, -fno-toplevel-reorder,\
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$(call cc-option, -fno-unit-at-a-time)) \
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$(call cc-option, -fno-stack-protector) \
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$(call cc-option, -mpreferred-stack-boundary=2)
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export REALMODE_CFLAGS
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