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

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Date:	Fri, 8 May 2015 05:09:07 -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 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.

-- 
H.J.

View attachment "0001-Add-mno-shared-to-x86-assembler.patch" of type "text/x-patch" (9130 bytes)