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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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