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Message-ID: <CAMe9rOrMjibLi_nSBBPCeHcQNUnv277oEMRPrrj98AenVixCjQ@mail.gmail.com>
Date:	Wed, 13 May 2015 05:59:49 -0700
From:	"H.J. Lu" <hjl.tools@...il.com>
To:	Andy Lutomirski <luto@...capital.net>,
	"H. Peter Anvin" <hpa@...or.com>
Cc:	Jan Beulich <JBeulich@...e.com>,
	Binutils <binutils@...rceware.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: RFC: Add -mshared option to x86 ELF assembler

On Wed, May 13, 2015 at 4:50 AM, H.J. Lu <hjl.tools@...il.com> wrote:
> On Tue, May 12, 2015 at 5:14 PM, H.J. Lu <hjl.tools@...il.com> wrote:
>> On Fri, May 8, 2015 at 1:16 PM, H.J. Lu <hjl.tools@...il.com> wrote:
>>> 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.
>>>
>>
>> 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.
>

Here is a patch to add -mshared, which is off by default.  On Linux kernel
with this change:

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index a468c0a..9a10e05 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -339,8 +339,8 @@ early_idt_handlers:
  i = i + 1
  .endr

-/* This is global to keep gas from relaxing the jumps */
-ENTRY(early_idt_handler)
+/* This is weak to keep gas from relaxing the jumps */
+WEAK(early_idt_handler)
  cld

  cmpl $2,(%rsp) # X86_TRAP_NMI
-- 
2.1.0

I got

[hjl@...-tools-1 kernel.org]$ readelf -r old/vmlinux.o | head -5

Relocation section '.rela.text' at offset 0xafea2f0 contains 205717 entries:
  Offset          Info           Type           Sym. Value    Sym. Name + Addend
000000000001  1253100000002 R_X86_64_PC32     0000000000001e70 __fentry__ - 4
000000000009  1c8c00000002 R_X86_64_PC32     0000000000000000 .data + 51bc
[hjl@...-tools-1 kernel.org]$ readelf -r new/vmlinux.o | head -5

Relocation section '.rela.text' at offset 0xafea280 contains 205711 entries:
  Offset          Info           Type           Sym. Value    Sym. Name + Addend
000000000001  1253100000002 R_X86_64_PC32     0000000000001e70 __fentry__ - 4
000000000009  1c8c00000002 R_X86_64_PC32     0000000000000000 .data + 51bc
[hjl@...-tools-1 kernel.org]$

It removes 6 relocations.  On gcc master branch,

[hjl@...-tools-1 gcc-misc]$ size build-x86_64-linux*/gcc/cc1
   text   data    bss    dec    hex filename
21529621  62256 1348312 22940189 15e0a1d build-x86_64-linux.branch/gcc/cc1
21529749  62256 1348312 22940317 15e0a9d build-x86_64-linux/gcc/cc1
[hjl@...-tools-1 gcc-misc]$ size build-x86_64-linux*/gcc/cc1plus
   text   data    bss    dec    hex filename
23713509  62400 1372760 25148669 17fbcfd build-x86_64-linux.branch/gcc/cc1plus
23713669  62400 1372760 25148829 17fbd9d build-x86_64-linux/gcc/cc1plus
[hjl@...-tools-1 gcc-misc]$

It is more effective.  I will run more tests.


-- 
H.J.

View attachment "binutils-mshared.patch" of type "text/x-patch" (14613 bytes)

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