| Message ID | 20170613005531.77656-4-mka@chromium.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
* 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 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@chromium.org> > --- > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kbuild" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
El Tue, Jun 13, 2017 at 08:38:00AM +0200 Ingo Molnar ha dit: > > * 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 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@chromium.org> > > --- > > 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. Sounds good > > @@ -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? I don't know for sure what is the reason for the 4 byte alignment, since there is no history for the option being added (apparently it was added in 1999 by Artur Skawina). I suppose it is an optimization to save stack space and reduce code size by reducing the need for extra alignment instructions. Please correct me if I'm wrong. > > # 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? Probably the same as for 32-bit, plus the stack being aligned in any case at an 8-byte boundary by the kernel entry. If you have more wisdom to add please let me know. -- To unsubscribe from this list: send the line "unsubscribe linux-kbuild" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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 + # 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-no-kbuild, $(REALMODE_CFLAGS), -ffreestanding) REALMODE_CFLAGS += $(call cc-option-no-kbuild, $(REALMODE_CFLAGS), -fno-stack-protector) -REALMODE_CFLAGS += $(call cc-option-no-kbuild, $(REALMODE_CFLAGS), -mpreferred-stack-boundary=2) +REALMODE_CFLAGS += $(call cc-option-no-kbuild, $(REALMODE_CFLAGS), $(stack_align_opt)=2) export REALMODE_CFLAGS # BITS is used as extension for files which are available in a 32 bit @@ -65,8 +73,8 @@ ifeq ($(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) # 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) # Use -mskip-rax-setup if supported. KBUILD_CFLAGS += $(call cc-option,-mskip-rax-setup)
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@chromium.org> --- arch/x86/Makefile | 18 +++++++++++++----- 1 file changed, 13 insertions(+), 5 deletions(-)