Re: [PATCH v4 3/3] x86/build: Specify stack alignment for clang

hpa
2017-06-19T20:34:42Z

On June 19, 2017 11:37:57 AM PDT, Matthias Kaehlcke <mka@chromium.org> wrote:
>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
>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@chromium.org>
>---
> arch/x86/Makefile | 26 +++++++++++++++++++++-----
> 1 file changed, 21 insertions(+), 5 deletions(-)
>
>diff --git a/arch/x86/Makefile b/arch/x86/Makefile
>index b2dae639f778..9406d3670452 100644
>--- a/arch/x86/Makefile
>+++ b/arch/x86/Makefile
>@@ -11,6 +11,14 @@ else
> KBUILD_DEFCONFIG := $(ARCH)_defconfig
> endif
>
>+# Handle different option names for specifying stack alignment with
>gcc and
>+# clang.
>+ifeq ($(cc-name),clang)
>+ cc_stack_align_opt := -mstack-alignment
>+else
>+ cc_stack_align_opt := -mpreferred-stack-boundary
>+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)

Goddammit.

How many times do I have to say NAK to

>+ifeq ($(cc-name),clang)

... before it sinks in?
--
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