Messages in this thread | | | From | Andy Lutomirski <> | Date | Mon, 18 May 2015 12:36:44 -0700 | Subject | Re: Avoiding unnecessary jump relocations in gas? |
| |
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. >
I wonder if it would make sense to have explicit mnemonics for the one-byte offset and four-byte offset jump variants. Sometimes users want a jump with a 32-bit offset for reasons that have nothing to do with link-time or load-time relocations.
--Andy
| |