[PATCH] x86/build: Specify stack alignment for clang

[Matthias Kaehlcke]

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 for gcc.

If the alignment is not specified clang assumes an alignment of 16 bytes,
as required by the standard ABI. However as mentioned in d9b0cde91c60
("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: Matthias Kaehlcke <mka@xxxxxxxxxxxx>
---
 arch/x86/Makefile | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 5851411e60fb..a32badbe87ad 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -27,7 +27,8 @@ REALMODE_CFLAGS	:= $(M16_CFLAGS) -g -Os -D__KERNEL__ \
 		   -mno-mmx -mno-sse \
 		   $(call cc-option, -ffreestanding) \
 		   $(call cc-option, -fno-stack-protector) \
-		   $(call cc-option, -mpreferred-stack-boundary=2)
+		   $(call cc-option, -mpreferred-stack-boundary=2) \
+		   $(call cc-option, -mstack-alignment=2)
 export REALMODE_CFLAGS
 
 # BITS is used as extension for files which are available in a 32 bit
@@ -64,8 +65,9 @@ ifeq ($(CONFIG_X86_32),y)
         # with nonstandard options
         KBUILD_CFLAGS += -fno-pic
 
-        # prevent gcc from keeping the stack 16 byte aligned
+        # prevent the compiler from keeping the stack 16 byte aligned
         KBUILD_CFLAGS += $(call cc-option,-mpreferred-stack-boundary=2)
+        KBUILD_CFLAGS += $(call cc-option,-mstack-alignment=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:
@@ -97,8 +99,9 @@ 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.
+	# Align the stack to 8 bytes if supported.
 	KBUILD_CFLAGS += $(call cc-option,-mpreferred-stack-boundary=3)
+	KBUILD_CFLAGS += $(call cc-option,-mstack-alignment=3)
 
 	# Use -mskip-rax-setup if supported.
 	KBUILD_CFLAGS += $(call cc-option,-mskip-rax-setup)
-- 
2.13.0.506.g27d5fe0cd-goog