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Subject[PATCH v3] x86/build: Fix stack alignment for CLang

d77698df39a5 ("x86/build: Specify stack alignment for clang")

intended to use the same stack alignment for clang as with gcc.

The two compilers use different options to configure the stack alignment
(gcc: -mpreferred-stack-boundary=n, clang: -mstack-alignment=n).

The above commit assumes that the clang option uses the same parameter
type as gcc, i.e. that the alignment is specified as 2^n. However clang
interprets the value of this option literally to use an alignment of n,
in consequence the stack remains misaligned.

Change the values used with -mstack-alignment to be the actual alignment
instead of a power of two.

cc-option isn't used here with the typical pattern of KBUILD_CFLAGS +=
$(call cc-option ...). The reason is that older gcc versions don't
support the -mpreferred-stack-boundary option, since cc-option doesn't
verify whether the alternative option is valid it would incorrectly
select the clang option -mstack-alignment..

Signed-off-by: Matthias Kaehlcke <>
Changes in v3:
- Don't rely on cc-option to evaluate the alternative option. Instead
check once if the compiler supports the option and use variables
with the correct option for 4 and 8 byte alignment.

arch/x86/Makefile | 14 ++++++++------
1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 1e902f926be3..65ed359eb904 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -14,9 +14,11 @@ 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
+ cc_stack_align4 := -mpreferred-stack-boundary=2
+ cc_stack_align8 := -mpreferred-stack-boundary=3
+else ifneq ($(call cc-option, -mstack-alignment=16),)
+ cc_stack_align4 := -mstack-alignment=4
+ cc_stack_align8 := -mstack-alignment=8

# How to compile the 16-bit code. Note we always compile for -march=i386;
@@ -36,7 +38,7 @@ 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), $(cc_stack_align_opt)=2)
+REALMODE_CFLAGS += $(cc_stack_align4)

# BITS is used as extension for files which are available in a 32 bit
@@ -76,7 +78,7 @@ ifeq ($(CONFIG_X86_32),y)
# 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)
+ KBUILD_CFLAGS += $(cc_stack_align4)

# 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:
@@ -115,7 +117,7 @@ else
# 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)
+ KBUILD_CFLAGS += $(cc_stack_align8)

# Use -mskip-rax-setup if supported.
KBUILD_CFLAGS += $(call cc-option,-mskip-rax-setup)
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