linux-kernel - Re: Avoiding unnecessary jump relocations in gas?

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Message-ID: <CAMe9rOrA2QGEagix2PK=QwwapBWMptr4gxW8t06-HhkodBTVwA@mail.gmail.com>
Date:	Fri, 8 May 2015 13:16:34 -0700
From:	"H.J. Lu" <hjl.tools@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Jan Beulich <JBeulich@...e.com>,
	Binutils <binutils@...rceware.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Avoiding unnecessary jump relocations in gas?

On Fri, May 8, 2015 at 5:09 AM, H.J. Lu <hjl.tools@...il.com> wrote:
> On Thu, May 7, 2015 at 8:22 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> On Thu, May 7, 2015 at 9:21 AM, H.J. Lu <hjl.tools@...il.com> wrote:
>>> On Thu, May 7, 2015 at 4:52 AM, Jan Beulich <JBeulich@...e.com> wrote:
>>>>>>> On 07.05.15 at 08:02, <luto@...capital.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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