Messages in this thread | | | Date | Fri, 8 May 2015 13:16:34 -0700 | Subject | Re: Avoiding unnecessary jump relocations in gas? | From | "H.J. Lu" <> |
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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.
-- H.J.
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