* Matthias Kaehlcke <[email protected]> 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 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 <[email protected]>
> ---
> arch/x86/Makefile | 18 +++++++++++++-----
> 1 file changed, 13 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 86b725d69423..7f6c33f4d428 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)
> + stack_align_opt := -mstack-alignment
> +else
> + stack_align_opt := -mpreferred-stack-boundary
> +endif
Nit: I'd name it cc_stack_align_opt or so, to make it clear this is a C
compiler
option.
> @@ -65,8 +73,8 @@ 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)
> + # prevent the compiler from keeping the stack 16 byte aligned
> + KBUILD_CFLAGS += $(call cc-option,$(stack_align_opt)=2)
So the comment appears inaccurate: the point isn't really to 'keep' the
compiler
from 16-byte alignment (there's nothing wrong with that, functionally), the
point
is to use a more optimal alignment, right?
>
> # 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 +106,8 @@ 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)
> + # Align the stack to 8 bytes if supported.
> + KBUILD_CFLAGS += $(call cc-option,$(stack_align_opt)=3)
Here too the reason should be outlined: performance, features or correctness?
Thanks,
Ingo
