Messages in this thread | | | Date | Wed, 13 May 2015 04:50:33 -0700 | Subject | Re: RFC: Add -mshared option to x86 ELF assembler | From | "H.J. Lu" <> |
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On Tue, May 12, 2015 at 5:14 PM, H.J. Lu <hjl.tools@gmail.com> wrote: > On Fri, May 8, 2015 at 1:16 PM, H.J. Lu <hjl.tools@gmail.com> wrote: >> On Fri, May 8, 2015 at 5:09 AM, H.J. Lu <hjl.tools@gmail.com> wrote: >>> On Thu, May 7, 2015 at 8:22 PM, Andy Lutomirski <luto@amacapital.net> wrote: >>>> On Thu, May 7, 2015 at 9:21 AM, H.J. Lu <hjl.tools@gmail.com> wrote: >>>>> On Thu, May 7, 2015 at 4:52 AM, Jan Beulich <JBeulich@suse.com> wrote: >>>>>>>>> On 07.05.15 at 08:02, <luto@amacapital.net> wrote: >>>>>>> AFAICT gas will produce relocations for jumps to global labels in the >>>>>>> same file. This doesn't seem directly harmful to me, except that, on >>>>>>> x86, it forces five-byte jumps instead of two-byte jumps. >>>>>>> >>>>>>> This seems especially unfortunate, since even hidden and protected >>>>>>> symbols have this problem. >>>>>>> >>>>>>> Given that many users don't want interposition support (especially the >>>>>>> kernel and anyone using .hidden or .protected), it would be nice to >>>>>>> have a command-line option to turn this off and probably also to turn >>>>>>> it off by default for hidden and protected symbols. Can gas do this? >>>>>> >>>>>> I've been running with the below changes (taken off of a bigger set >>>>>> of changes, so the line numbers may look a little odd) for the last >>>>>> couple of years. I never tried to submit this change because so far >>>>>> I couldn't find the time to check whether this would have any >>>>>> unwanted side effects on cases I don't normally use. >>>>>> >>>>> >>>>> This is the patch I checked in. >>>>> >>>>> Thanks. >>>>> >>>>> -- >>>>> H.J. >>>>> --- >>>>> Branches to global non-weak symbols defined in the same segment with >>>>> non-default visibility can be optimized the same way as branches to >>>>> local symbols. >>>> >>>> Would it make sense to also add a command line option along the lines >>>> of gcc's -fno-semantic-interposition or some way to override the >>>> default visibility? AFAICS this patch helps but only if asm code gets >>>> liberally sprinkled with .hidden or .protected directives. >>>> >>> >>> This is what I checked in. With >>> >>> diff --git a/arch/x86/Makefile b/arch/x86/Makefile >>> index 2fda005..186e6f7 100644 >>> --- a/arch/x86/Makefile >>> +++ b/arch/x86/Makefile >>> @@ -107,6 +107,10 @@ else >>> KBUILD_CFLAGS += $(call cc-option,-maccumulate-outgoing-args) >>> endif >>> >>> +NO_SHARED_CFLAGS = $(call as-option,-Wa$(comma)-mno-shared) >>> +KBUILD_CFLAGS += $(NO_SHARED_CFLAGS) >>> +KBUILD_AFLAGS += $(NO_SHARED_CFLAGS) >>> + >>> # Make sure compiler does not have buggy stack-protector support. >>> ifdef CONFIG_CC_STACKPROTECTOR >>> cc_has_sp := $(srctree)/scripts/gcc-x86_$(BITS)-has-stack-protector.sh >>> >>> On kernel master branch, I got >>> >>> text data bss dec hex filename >>> 10934167 2275232 1609728 14819127 e21f37 vmlinux.old >>> 10934119 2275232 1609728 14819079 e21f07 vmlinux >>> >>> It saves 48 bytes. >> >> This is before I fixed: >> >> /* This is global to keep gas from relaxing the jumps */ >> ENTRY(early_idt_handler) >> cld >> >> in arch/x86/kernel/head_64.S. With -mno-shared, we must >> make early_idt_handler weak to keep gas from relaxing the jumps. >> > > Here is a patch to change the assembler default to optimize out > relocations to defined non-weak global branch targets with default > visibility. It will generate slightly smaller object files. But Linux > kernel will be broken unless early_idt_handler is marked weak. > I am little uncomfortable with -mshare and I don't like -mno-shared > very much either. I may just simply remove -mno-shared. >
I reverted the -mno-shared change.
-- H.J.
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