Commit d77698df authored by Matthias Kaehlcke's avatar Matthias Kaehlcke Committed by Masahiro Yamada
Browse files

x86/build: Specify stack alignment for clang 



For gcc stack alignment is configured with -mpreferred-stack-boundary=N,
clang has the option -mstack-alignment=N for that purpose. Use the same
alignment as with gcc.

If the alignment is not specified clang assumes an alignment of
16 bytes, as required by the standard ABI. However as mentioned in
d9b0cde9 ("x86-64, gcc: Use -mpreferred-stack-boundary=3 if
supported") the standard kernel entry on x86-64 leaves the stack
on an 8-byte boundary, as a consequence clang will keep the stack
misaligned.

Signed-off-by: default avatarMatthias Kaehlcke <mka@chromium.org>
Acked-by: default avatarIngo Molnar <mingo@kernel.org>
Signed-off-by: default avatarMasahiro Yamada <yamada.masahiro@socionext.com>
parent 032a2c4f

arch/x86/Makefile

+21 −5
Original line number Diff line number Diff line
@@ -11,6 +11,14 @@ else 
        KBUILD_DEFCONFIG := $(ARCH)_defconfig

endif



# For gcc stack alignment is specified with -mpreferred-stack-boundary,

# clang has the option -mstack-alignment for that purpose.

ifneq ($(call cc-option, -mpreferred-stack-boundary=4),)

        cc_stack_align_opt := -mpreferred-stack-boundary

else ifneq ($(call cc-option, -mstack-alignment=4),)

        cc_stack_align_opt := -mstack-alignment

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.

#
@@ -28,7 +36,7 @@ REALMODE_CFLAGS := $(M16_CFLAGS) -g -Os -D__KERNEL__ \ 


REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), -ffreestanding)

REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), -fno-stack-protector)

REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), -mpreferred-stack-boundary=2)

REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), $(cc_stack_align_opt)=2)

export REALMODE_CFLAGS



# BITS is used as extension for files which are available in a 32 bit
@@ -65,8 +73,10 @@ ifeq ($(CONFIG_X86_32),y) 
        # with nonstandard options

        KBUILD_CFLAGS += -fno-pic



        # prevent gcc from keeping the stack 16 byte aligned

        KBUILD_CFLAGS += $(call cc-option,-mpreferred-stack-boundary=2)

        # Align the stack to the register width instead of using the default

        # alignment of 16 bytes. This reduces stack usage and the number of

        # alignment instructions.

        KBUILD_CFLAGS += $(call cc-option,$(cc_stack_align_opt)=2)



        # Disable unit-at-a-time mode on pre-gcc-4.0 compilers, it makes gcc use

        # a lot more stack due to the lack of sharing of stacklots:
@@ -98,8 +108,14 @@ else 
        KBUILD_CFLAGS += $(call cc-option,-mno-80387)

        KBUILD_CFLAGS += $(call cc-option,-mno-fp-ret-in-387)



	# Use -mpreferred-stack-boundary=3 if supported.

	KBUILD_CFLAGS += $(call cc-option,-mpreferred-stack-boundary=3)

        # By default gcc and clang use a stack alignment of 16 bytes for x86.

        # However the standard kernel entry on x86-64 leaves the stack on an

        # 8-byte boundary. If the compiler isn't informed about the actual

        # alignment it will generate extra alignment instructions for the

        # default alignment which keep the stack *mis*aligned.

        # Furthermore an alignment to the register width reduces stack usage

        # and the number of alignment instructions.

        KBUILD_CFLAGS += $(call cc-option,$(cc_stack_align_opt)=3)



	# Use -mskip-rax-setup if supported.

	KBUILD_CFLAGS += $(call cc-option,-mskip-rax-setup)

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