* RFC: Add -mshared option to x86 ELF assembler
@ 2015-05-13 0:14 H.J. Lu
2015-05-13 11:50 ` H.J. Lu
0 siblings, 1 reply; 6+ messages in thread
From: H.J. Lu @ 2015-05-13 0:14 UTC (permalink / raw)
To: Andy Lutomirski, H. Peter Anvin; +Cc: Jan Beulich, Binutils, linux-kernel
[-- Attachment #1: Type: text/plain, Size: 4735 bytes --]
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.
>
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.
--
H.J.
---
This patch removes the newly added -mno-shared option from x86 ELF
assembler and adds -mshared option to x86 ELF assembler. By default,
assembler will optimize out relocations to defined non-weak global
branch targets with default visibility. The -mshared option tells
the assembler to generate code which may go into a shared library
where all non-weak global branch targets with default visibility can
be preempted. The resulting code is slightly bigger. This option
only affects the handling of branch instructions.
gas/
* config/tc-i386.c (no_shared): Renamed to ...
(shared): This.
(elf_symbol_resolved_in_segment_p): Add relocation argument.
Check PLT relocations and shared.
(md_estimate_size_before_relax): Pass fragP->fr_var to
elf_symbol_resolved_in_segment_p.
(OPTION_MNO_SHARED): Renamed to ...
(OPTION_MSHARED): This.
(md_longopts): Renamed -mno-shared to -mshared.
(md_show_usage): Likewise.
* doc/c-i386.texi: Likewise.
gas/testsuite/
* gas/i386/pcrel.d: Pass -mshared to assembler.
* gas/i386/relax-3.d: Likewise. Updated.
* gas/i386/x86-64-relax-2.d: Likewise.
* gas/i386/relax-3.s: Add test for PLT relocation.
* gas/i386/relax-4.d: Remove -mno-shared. Updated.
* gas/i386/x86-64-relax-3.d: Likewise.
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From 180c819ed8be897f02ef15a201ce4ea47899749f Mon Sep 17 00:00:00 2001
From: "H.J. Lu" <hjl.tools@gmail.com>
Date: Tue, 12 May 2015 16:52:11 -0700
Subject: [PATCH] Add -mshared option to x86 ELF assembler
This patch removes the newly added -mno-shared option from x86 ELF
assembler and adds -mshared option to x86 ELF assembler. By default,
assembler will optimize out relocations to defined non-weak global
branch targets with default visibility. The -mshared option tells
the assembler to generate code which may go into a shared library
where all non-weak global branch targets with default visibility can
be preempted. The resulting code is slightly bigger. This option
only affects the handling of branch instructions.
gas/
* config/tc-i386.c (no_shared): Renamed to ...
(shared): This.
(elf_symbol_resolved_in_segment_p): Add relocation argument.
Check PLT relocations and shared.
(md_estimate_size_before_relax): Pass fragP->fr_var to
elf_symbol_resolved_in_segment_p.
(OPTION_MNO_SHARED): Renamed to ...
(OPTION_MSHARED): This.
(md_longopts): Renamed -mno-shared to -mshared.
(md_show_usage): Likewise.
* doc/c-i386.texi: Likewise.
gas/testsuite/
* gas/i386/pcrel.d: Pass -mshared to assembler.
* gas/i386/relax-3.d: Likewise. Updated.
* gas/i386/x86-64-relax-2.d: Likewise.
* gas/i386/relax-3.s: Add test for PLT relocation.
* gas/i386/relax-4.d: Remove -mno-shared. Updated.
* gas/i386/x86-64-relax-3.d: Likewise.
---
gas/config/tc-i386.c | 36 ++++++++++++++++++++++-----------
gas/doc/c-i386.texi | 17 ++++++++--------
gas/testsuite/gas/i386/pcrel.d | 1 +
gas/testsuite/gas/i386/relax-3.d | 28 +++++++++++++------------
gas/testsuite/gas/i386/relax-3.s | 1 +
gas/testsuite/gas/i386/relax-4.d | 28 ++++++++++++-------------
gas/testsuite/gas/i386/x86-64-relax-2.d | 24 ++++++++++++----------
gas/testsuite/gas/i386/x86-64-relax-3.d | 28 ++++++++++++-------------
8 files changed, 91 insertions(+), 72 deletions(-)
diff --git a/gas/config/tc-i386.c b/gas/config/tc-i386.c
index db263ee..254548f 100644
--- a/gas/config/tc-i386.c
+++ b/gas/config/tc-i386.c
@@ -525,8 +525,8 @@ static int use_big_obj = 0;
#endif
#if defined (OBJ_ELF) || defined (OBJ_MAYBE_ELF)
-/* 1 if not generating code for a shared library. */
-static int no_shared = 0;
+/* 1 if generating code for a shared library. */
+static int shared = 0;
#endif
/* 1 for intel syntax,
@@ -8823,7 +8823,7 @@ i386_frag_max_var (fragS *frag)
#if defined (OBJ_ELF) || defined (OBJ_MAYBE_ELF)
static int
-elf_symbol_resolved_in_segment_p (symbolS *fr_symbol)
+elf_symbol_resolved_in_segment_p (symbolS *fr_symbol, offsetT fr_var)
{
/* STT_GNU_IFUNC symbol must go through PLT. */
if ((symbol_get_bfdsym (fr_symbol)->flags
@@ -8834,13 +8834,24 @@ elf_symbol_resolved_in_segment_p (symbolS *fr_symbol)
/* Symbol may be weak or local. */
return !S_IS_WEAK (fr_symbol);
- /* Non-weak symbols won't be preempted. */
- if (no_shared)
+ /* Global symbols with non-default visibility can't be preempted. */
+ if (ELF_ST_VISIBILITY (S_GET_OTHER (fr_symbol)) != STV_DEFAULT)
return 1;
+ if (fr_var != NO_RELOC)
+ switch ((enum bfd_reloc_code_real) fr_var)
+ {
+ case BFD_RELOC_386_PLT32:
+ case BFD_RELOC_X86_64_PLT32:
+ /* Symbol with PLT relocatin may be preempted. */
+ return 0;
+ default:
+ abort ();
+ }
+
/* Global symbols with default visibility in a shared library may be
preempted by another definition. */
- return ELF_ST_VISIBILITY (S_GET_OTHER (fr_symbol)) != STV_DEFAULT;
+ return !shared;
}
#endif
@@ -8867,7 +8878,8 @@ md_estimate_size_before_relax (fragS *fragP, segT segment)
if (S_GET_SEGMENT (fragP->fr_symbol) != segment
#if defined (OBJ_ELF) || defined (OBJ_MAYBE_ELF)
|| (IS_ELF
- && !elf_symbol_resolved_in_segment_p (fragP->fr_symbol))
+ && !elf_symbol_resolved_in_segment_p (fragP->fr_symbol,
+ fragP->fr_var))
#endif
#if defined (OBJ_COFF) && defined (TE_PE)
|| (OUTPUT_FLAVOR == bfd_target_coff_flavour
@@ -9537,7 +9549,7 @@ const char *md_shortopts = "qn";
#define OPTION_MBIG_OBJ (OPTION_MD_BASE + 18)
#define OPTION_OMIT_LOCK_PREFIX (OPTION_MD_BASE + 19)
#define OPTION_MEVEXRCIG (OPTION_MD_BASE + 20)
-#define OPTION_MNO_SHARED (OPTION_MD_BASE + 21)
+#define OPTION_MSHARED (OPTION_MD_BASE + 21)
struct option md_longopts[] =
{
@@ -9548,7 +9560,7 @@ struct option md_longopts[] =
#endif
#if defined (OBJ_ELF) || defined (OBJ_MAYBE_ELF)
{"x32", no_argument, NULL, OPTION_X32},
- {"mno-shared", no_argument, NULL, OPTION_MNO_SHARED},
+ {"mshared", no_argument, NULL, OPTION_MSHARED},
#endif
{"divide", no_argument, NULL, OPTION_DIVIDE},
{"march", required_argument, NULL, OPTION_MARCH},
@@ -9610,8 +9622,8 @@ md_parse_option (int c, char *arg)
.stab instead of .stab.excl. We always use .stab anyhow. */
break;
- case OPTION_MNO_SHARED:
- no_shared = 1;
+ case OPTION_MSHARED:
+ shared = 1;
break;
#endif
#if (defined (OBJ_ELF) || defined (OBJ_MAYBE_ELF) \
@@ -10043,7 +10055,7 @@ md_show_usage (FILE *stream)
fprintf (stream, _("\
-madd-bnd-prefix add BND prefix for all valid branches\n"));
fprintf (stream, _("\
- -mno-shared enable branch optimization for non shared code\n"));
+ -mshared disable branch optimization for shared code\n"));
# if defined (TE_PE) || defined (TE_PEP)
fprintf (stream, _("\
-mbig-obj generate big object files\n"));
diff --git a/gas/doc/c-i386.texi b/gas/doc/c-i386.texi
index 47bcbbb..a1997f5 100644
--- a/gas/doc/c-i386.texi
+++ b/gas/doc/c-i386.texi
@@ -298,15 +298,16 @@ The @code{.att_syntax} and @code{.intel_syntax} directives will take precedent.
This option forces the assembler to add BND prefix to all branches, even
if such prefix was not explicitly specified in the source code.
-@cindex @samp{-mno-shared} option, i386
-@cindex @samp{-mno-shared} option, x86-64
+@cindex @samp{-mshared} option, i386
+@cindex @samp{-mshared} option, x86-64
@item -mno-shared
-On ELF target, the assembler normally generates code which can go into a
-shared library where non-weak symbols can be preempted. The
-@samp{-mno-shared} option tells the assembler to generate code not for
-a shared library, where non-weak symbols won't be preempted. The
-resulting code is slightly smaller. This option mainly affects the
-handling of branch instructions.
+On ELF target, the assembler normally optimizes out relocations to
+defined non-weak global branch targets with default visibility. The
+@samp{-mshared} option tells the assembler to generate code which
+may go into a shared library where all non-weak global branch targets
+with default visibility can be preempted. The resulting code is
+slightly bigger. This option only affects the handling of branch
+instructions.
@cindex @samp{-mbig-obj} option, x86-64
@item -mbig-obj
diff --git a/gas/testsuite/gas/i386/pcrel.d b/gas/testsuite/gas/i386/pcrel.d
index 5b61c23..8a91a1a 100644
--- a/gas/testsuite/gas/i386/pcrel.d
+++ b/gas/testsuite/gas/i386/pcrel.d
@@ -1,4 +1,5 @@
#objdump: -drw
+#as: -mshared
#name: i386 pcrel reloc
.*: +file format .*i386.*
diff --git a/gas/testsuite/gas/i386/relax-3.d b/gas/testsuite/gas/i386/relax-3.d
index 8aa94e9..4610553 100644
--- a/gas/testsuite/gas/i386/relax-3.d
+++ b/gas/testsuite/gas/i386/relax-3.d
@@ -1,3 +1,4 @@
+#as: -mshared
#objdump: -dwr
.*: +file format .*
@@ -5,26 +6,27 @@
Disassembly of section .text:
0+ <foo>:
-[ ]*[a-f0-9]+: eb 1f jmp 21 <local>
-[ ]*[a-f0-9]+: eb 19 jmp 1d <hidden_def>
-[ ]*[a-f0-9]+: e9 fc ff ff ff jmp 5 <foo\+0x5> 5: (R_386_PC)?(DISP)?32 global_def
-[ ]*[a-f0-9]+: e9 fc ff ff ff jmp a <foo\+0xa> a: (R_386_PC)?(DISP)?32 weak_def
-[ ]*[a-f0-9]+: e9 fc ff ff ff jmp f <foo\+0xf> f: (R_386_PC)?(DISP)?32 weak_hidden_undef
-[ ]*[a-f0-9]+: e9 fc ff ff ff jmp 14 <foo\+0x14> 14: (R_386_PC)?(DISP)?32 weak_hidden_def
-[ ]*[a-f0-9]+: e9 fc ff ff ff jmp 19 <foo\+0x19> 19: (R_386_PC)?(DISP)?32 hidden_undef
-
-0+1d <hidden_def>:
+[ ]*[a-f0-9]+: eb 24 jmp 26 <local>
+[ ]*[a-f0-9]+: eb 1e jmp 22 <hidden_def>
+[ ]*[a-f0-9]+: e9 fc ff ff ff jmp 5 <foo\+0x5> 5: R_386_PC32 global_def
+[ ]*[a-f0-9]+: e9 fc ff ff ff jmp a <foo\+0xa> a: R_386_PLT32 global_def
+[ ]*[a-f0-9]+: e9 fc ff ff ff jmp f <foo\+0xf> f: R_386_PC32 weak_def
+[ ]*[a-f0-9]+: e9 fc ff ff ff jmp 14 <foo\+0x14> 14: R_386_PC32 weak_hidden_undef
+[ ]*[a-f0-9]+: e9 fc ff ff ff jmp 19 <foo\+0x19> 19: R_386_PC32 weak_hidden_def
+[ ]*[a-f0-9]+: e9 fc ff ff ff jmp 1e <foo\+0x1e> 1e: R_386_PC32 hidden_undef
+
+0+22 <hidden_def>:
[ ]*[a-f0-9]+: c3 ret
-0+1e <weak_hidden_def>:
+0+23 <weak_hidden_def>:
[ ]*[a-f0-9]+: c3 ret
-0+1f <global_def>:
+0+24 <global_def>:
[ ]*[a-f0-9]+: c3 ret
-0+20 <weak_def>:
+0+25 <weak_def>:
[ ]*[a-f0-9]+: c3 ret
-0+21 <local>:
+0+26 <local>:
[ ]*[a-f0-9]+: c3 ret
#pass
diff --git a/gas/testsuite/gas/i386/relax-3.s b/gas/testsuite/gas/i386/relax-3.s
index ab52185..48ea917 100644
--- a/gas/testsuite/gas/i386/relax-3.s
+++ b/gas/testsuite/gas/i386/relax-3.s
@@ -4,6 +4,7 @@ foo:
jmp local
jmp hidden_def
jmp global_def
+ jmp global_def@PLT
jmp weak_def
jmp weak_hidden_undef
jmp weak_hidden_def
diff --git a/gas/testsuite/gas/i386/relax-4.d b/gas/testsuite/gas/i386/relax-4.d
index b188841..2039251 100644
--- a/gas/testsuite/gas/i386/relax-4.d
+++ b/gas/testsuite/gas/i386/relax-4.d
@@ -1,5 +1,4 @@
#source: relax-3.s
-#as: -mno-shared
#objdump: -dwr
.*: +file format .*
@@ -7,26 +6,27 @@
Disassembly of section .text:
0+ <foo>:
-[ ]*[a-f0-9]+: eb 1c jmp 1e <local>
-[ ]*[a-f0-9]+: eb 16 jmp 1a <hidden_def>
-[ ]*[a-f0-9]+: eb 16 jmp 1c <global_def>
-[ ]*[a-f0-9]+: e9 fc ff ff ff jmp 7 <foo\+0x7> 7: (R_386_PC)?(DISP)?32 weak_def
-[ ]*[a-f0-9]+: e9 fc ff ff ff jmp c <foo\+0xc> c: (R_386_PC)?(DISP)?32 weak_hidden_undef
-[ ]*[a-f0-9]+: e9 fc ff ff ff jmp 11 <foo\+0x11> 11: (R_386_PC)?(DISP)?32 weak_hidden_def
-[ ]*[a-f0-9]+: e9 fc ff ff ff jmp 16 <foo\+0x16> 16: (R_386_PC)?(DISP)?32 hidden_undef
-
-0+1a <hidden_def>:
+[ ]*[a-f0-9]+: eb 21 jmp 23 <local>
+[ ]*[a-f0-9]+: eb 1b jmp 1f <hidden_def>
+[ ]*[a-f0-9]+: eb 1b jmp 21 <global_def>
+[ ]*[a-f0-9]+: e9 fc ff ff ff jmp 7 <foo\+0x7> 7: R_386_PLT32 global_def
+[ ]*[a-f0-9]+: e9 fc ff ff ff jmp c <foo\+0xc> c: R_386_PC32 weak_def
+[ ]*[a-f0-9]+: e9 fc ff ff ff jmp 11 <foo\+0x11> 11: R_386_PC32 weak_hidden_undef
+[ ]*[a-f0-9]+: e9 fc ff ff ff jmp 16 <foo\+0x16> 16: R_386_PC32 weak_hidden_def
+[ ]*[a-f0-9]+: e9 fc ff ff ff jmp 1b <foo\+0x1b> 1b: R_386_PC32 hidden_undef
+
+0+1f <hidden_def>:
[ ]*[a-f0-9]+: c3 ret
-0+1b <weak_hidden_def>:
+0+20 <weak_hidden_def>:
[ ]*[a-f0-9]+: c3 ret
-0+1c <global_def>:
+0+21 <global_def>:
[ ]*[a-f0-9]+: c3 ret
-0+1d <weak_def>:
+0+22 <weak_def>:
[ ]*[a-f0-9]+: c3 ret
-0+1e <local>:
+0+23 <local>:
[ ]*[a-f0-9]+: c3 ret
#pass
diff --git a/gas/testsuite/gas/i386/x86-64-relax-2.d b/gas/testsuite/gas/i386/x86-64-relax-2.d
index 7b0bd56..c124102 100644
--- a/gas/testsuite/gas/i386/x86-64-relax-2.d
+++ b/gas/testsuite/gas/i386/x86-64-relax-2.d
@@ -1,4 +1,5 @@
#source: relax-3.s
+#as: -mshared
#objdump: -dwr
.*: +file format .*
@@ -7,26 +8,27 @@
Disassembly of section .text:
0+ <foo>:
-[ ]*[a-f0-9]+: eb 1f jmp 21 <local>
-[ ]*[a-f0-9]+: eb 19 jmp 1d <hidden_def>
+[ ]*[a-f0-9]+: eb 24 jmp 26 <local>
+[ ]*[a-f0-9]+: eb 1e jmp 22 <hidden_def>
[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq 9 <foo\+0x9> 5: R_X86_64_PC32 global_def-0x4
-[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq e <foo\+0xe> a: R_X86_64_PC32 weak_def-0x4
-[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq 13 <foo\+0x13> f: R_X86_64_PC32 weak_hidden_undef-0x4
-[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq 18 <foo\+0x18> 14: R_X86_64_PC32 weak_hidden_def-0x4
-[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq 1d <hidden_def> 19: R_X86_64_PC32 hidden_undef-0x4
+[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq e <foo\+0xe> a: R_X86_64_PLT32 global_def-0x4
+[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq 13 <foo\+0x13> f: R_X86_64_PC32 weak_def-0x4
+[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq 18 <foo\+0x18> 14: R_X86_64_PC32 weak_hidden_undef-0x4
+[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq 1d <foo\+0x1d> 19: R_X86_64_PC32 weak_hidden_def-0x4
+[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq 22 <hidden_def> 1e: R_X86_64_PC32 hidden_undef-0x4
-0+1d <hidden_def>:
+0+22 <hidden_def>:
[ ]*[a-f0-9]+: c3 retq
-0+1e <weak_hidden_def>:
+0+23 <weak_hidden_def>:
[ ]*[a-f0-9]+: c3 retq
-0+1f <global_def>:
+0+24 <global_def>:
[ ]*[a-f0-9]+: c3 retq
-0+20 <weak_def>:
+0+25 <weak_def>:
[ ]*[a-f0-9]+: c3 retq
-0+21 <local>:
+0+26 <local>:
[ ]*[a-f0-9]+: c3 retq
#pass
diff --git a/gas/testsuite/gas/i386/x86-64-relax-3.d b/gas/testsuite/gas/i386/x86-64-relax-3.d
index d0c7ee4..98fd28d 100644
--- a/gas/testsuite/gas/i386/x86-64-relax-3.d
+++ b/gas/testsuite/gas/i386/x86-64-relax-3.d
@@ -1,5 +1,4 @@
#source: relax-3.s
-#as: -mno-shared
#objdump: -dwr
.*: +file format .*
@@ -8,26 +7,27 @@
Disassembly of section .text:
0+ <foo>:
-[ ]*[a-f0-9]+: eb 1c jmp 1e <local>
-[ ]*[a-f0-9]+: eb 16 jmp 1a <hidden_def>
-[ ]*[a-f0-9]+: eb 16 jmp 1c <global_def>
-[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq b <foo\+0xb> 7: R_X86_64_PC32 weak_def-0x4
-[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq 10 <foo\+0x10> c: R_X86_64_PC32 weak_hidden_undef-0x4
-[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq 15 <foo\+0x15> 11: R_X86_64_PC32 weak_hidden_def-0x4
-[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq 1a <hidden_def> 16: R_X86_64_PC32 hidden_undef-0x4
-
-0+1a <hidden_def>:
+[ ]*[a-f0-9]+: eb 21 jmp 23 <local>
+[ ]*[a-f0-9]+: eb 1b jmp 1f <hidden_def>
+[ ]*[a-f0-9]+: eb 1b jmp 21 <global_def>
+[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq b <foo\+0xb> 7: R_X86_64_PLT32 global_def-0x4
+[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq 10 <foo\+0x10> c: R_X86_64_PC32 weak_def-0x4
+[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq 15 <foo\+0x15> 11: R_X86_64_PC32 weak_hidden_undef-0x4
+[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq 1a <foo\+0x1a> 16: R_X86_64_PC32 weak_hidden_def-0x4
+[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq 1f <hidden_def> 1b: R_X86_64_PC32 hidden_undef-0x4
+
+0+1f <hidden_def>:
[ ]*[a-f0-9]+: c3 retq
-0+1b <weak_hidden_def>:
+0+20 <weak_hidden_def>:
[ ]*[a-f0-9]+: c3 retq
-0+1c <global_def>:
+0+21 <global_def>:
[ ]*[a-f0-9]+: c3 retq
-0+1d <weak_def>:
+0+22 <weak_def>:
[ ]*[a-f0-9]+: c3 retq
-0+1e <local>:
+0+23 <local>:
[ ]*[a-f0-9]+: c3 retq
#pass
--
1.9.3
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: RFC: Add -mshared option to x86 ELF assembler
2015-05-13 0:14 RFC: Add -mshared option to x86 ELF assembler H.J. Lu
@ 2015-05-13 11:50 ` H.J. Lu
2015-05-13 12:59 ` H.J. Lu
0 siblings, 1 reply; 6+ messages in thread
From: H.J. Lu @ 2015-05-13 11:50 UTC (permalink / raw)
To: Andy Lutomirski, H. Peter Anvin; +Cc: Jan Beulich, Binutils, linux-kernel
On Tue, May 12, 2015 at 5:14 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> 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.
>>
>
> 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.
--
H.J.
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: RFC: Add -mshared option to x86 ELF assembler
2015-05-13 11:50 ` H.J. Lu
@ 2015-05-13 12:59 ` H.J. Lu
2015-05-20 20:02 ` Andy Lutomirski
0 siblings, 1 reply; 6+ messages in thread
From: H.J. Lu @ 2015-05-13 12:59 UTC (permalink / raw)
To: Andy Lutomirski, H. Peter Anvin; +Cc: Jan Beulich, Binutils, linux-kernel
[-- Attachment #1: Type: text/plain, Size: 5822 bytes --]
On Wed, May 13, 2015 at 4:50 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Tue, May 12, 2015 at 5:14 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> 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.
>>>
>>
>> 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 $2,(%rsp) # X86_TRAP_NMI
--
2.1.0
I got
[hjl@gnu-tools-1 kernel.org]$ 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@gnu-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@gnu-tools-1 kernel.org]$
It removes 6 relocations. On gcc master branch,
[hjl@gnu-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@gnu-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@gnu-tools-1 gcc-misc]$
It is more effective. I will run more tests.
--
H.J.
[-- Attachment #2: binutils-mshared.patch --]
[-- Type: text/x-patch, Size: 14613 bytes --]
From cf1509c67f6c2ca0919c60220489e8d10bb4963a Mon Sep 17 00:00:00 2001
From: "H.J. Lu" <hjl.tools@gmail.com>
Date: Tue, 12 May 2015 16:52:11 -0700
Subject: [PATCH] Add -mshared option to x86 ELF assembler
This patch adds -mshared option to x86 ELF assembler. By default,
assembler will optimize out relocations to defined non-weak global
branch targets with default visibility. The -mshared option tells
the assembler to generate code which may go into a shared library
where all non-weak global branch targets with default visibility can
be preempted. The resulting code is slightly bigger. This option
only affects the handling of branch instructions.
gas/
* config/tc-i386.c (shared): New.
(OPTION_MSHARED): Likewise.
(elf_symbol_resolved_in_segment_p): Add relocation argument.
Check PLT relocations and shared.
(md_estimate_size_before_relax): Pass fragP->fr_var to
elf_symbol_resolved_in_segment_p.
(md_longopts): Add -mshared.
(md_show_usage): Likewise.
(md_parse_option): Handle OPTION_MSHARED.
* doc/c-i386.texi: Document -mshared.
gas/testsuite/
* gas/i386/i386.exp: Run relax-4 and x86-64-relax-3.
* gas/i386/pcrel.d: Pass -mshared to assembler.
* gas/i386/relax-3.d: Likewise. Updated.
* gas/i386/x86-64-relax-2.d: Likewise.
* gas/i386/relax-3.s: Add test for PLT relocation.
* gas/i386/relax-4.d: New file.
* gas/i386/x86-64-relax-3.d: Likewise.
---
gas/config/tc-i386.c | 35 ++++++++++++++++++++++++++++++---
gas/doc/c-i386.texi | 11 +++++++++++
gas/testsuite/gas/i386/i386.exp | 2 ++
gas/testsuite/gas/i386/pcrel.d | 1 +
gas/testsuite/gas/i386/relax-3.d | 28 ++++++++++++++------------
gas/testsuite/gas/i386/relax-3.s | 1 +
gas/testsuite/gas/i386/relax-4.d | 32 ++++++++++++++++++++++++++++++
gas/testsuite/gas/i386/x86-64-relax-2.d | 24 +++++++++++-----------
gas/testsuite/gas/i386/x86-64-relax-3.d | 33 +++++++++++++++++++++++++++++++
9 files changed, 140 insertions(+), 27 deletions(-)
create mode 100644 gas/testsuite/gas/i386/relax-4.d
create mode 100644 gas/testsuite/gas/i386/x86-64-relax-3.d
diff --git a/gas/config/tc-i386.c b/gas/config/tc-i386.c
index 75f268f..254548f 100644
--- a/gas/config/tc-i386.c
+++ b/gas/config/tc-i386.c
@@ -524,6 +524,11 @@ static enum x86_elf_abi x86_elf_abi = I386_ABI;
static int use_big_obj = 0;
#endif
+#if defined (OBJ_ELF) || defined (OBJ_MAYBE_ELF)
+/* 1 if generating code for a shared library. */
+static int shared = 0;
+#endif
+
/* 1 for intel syntax,
0 if att syntax. */
static int intel_syntax = 0;
@@ -8818,7 +8823,7 @@ i386_frag_max_var (fragS *frag)
#if defined (OBJ_ELF) || defined (OBJ_MAYBE_ELF)
static int
-elf_symbol_resolved_in_segment_p (symbolS *fr_symbol)
+elf_symbol_resolved_in_segment_p (symbolS *fr_symbol, offsetT fr_var)
{
/* STT_GNU_IFUNC symbol must go through PLT. */
if ((symbol_get_bfdsym (fr_symbol)->flags
@@ -8829,9 +8834,24 @@ elf_symbol_resolved_in_segment_p (symbolS *fr_symbol)
/* Symbol may be weak or local. */
return !S_IS_WEAK (fr_symbol);
+ /* Global symbols with non-default visibility can't be preempted. */
+ if (ELF_ST_VISIBILITY (S_GET_OTHER (fr_symbol)) != STV_DEFAULT)
+ return 1;
+
+ if (fr_var != NO_RELOC)
+ switch ((enum bfd_reloc_code_real) fr_var)
+ {
+ case BFD_RELOC_386_PLT32:
+ case BFD_RELOC_X86_64_PLT32:
+ /* Symbol with PLT relocatin may be preempted. */
+ return 0;
+ default:
+ abort ();
+ }
+
/* Global symbols with default visibility in a shared library may be
preempted by another definition. */
- return ELF_ST_VISIBILITY (S_GET_OTHER (fr_symbol)) != STV_DEFAULT;
+ return !shared;
}
#endif
@@ -8858,7 +8878,8 @@ md_estimate_size_before_relax (fragS *fragP, segT segment)
if (S_GET_SEGMENT (fragP->fr_symbol) != segment
#if defined (OBJ_ELF) || defined (OBJ_MAYBE_ELF)
|| (IS_ELF
- && !elf_symbol_resolved_in_segment_p (fragP->fr_symbol))
+ && !elf_symbol_resolved_in_segment_p (fragP->fr_symbol,
+ fragP->fr_var))
#endif
#if defined (OBJ_COFF) && defined (TE_PE)
|| (OUTPUT_FLAVOR == bfd_target_coff_flavour
@@ -9528,6 +9549,7 @@ const char *md_shortopts = "qn";
#define OPTION_MBIG_OBJ (OPTION_MD_BASE + 18)
#define OPTION_OMIT_LOCK_PREFIX (OPTION_MD_BASE + 19)
#define OPTION_MEVEXRCIG (OPTION_MD_BASE + 20)
+#define OPTION_MSHARED (OPTION_MD_BASE + 21)
struct option md_longopts[] =
{
@@ -9538,6 +9560,7 @@ struct option md_longopts[] =
#endif
#if defined (OBJ_ELF) || defined (OBJ_MAYBE_ELF)
{"x32", no_argument, NULL, OPTION_X32},
+ {"mshared", no_argument, NULL, OPTION_MSHARED},
#endif
{"divide", no_argument, NULL, OPTION_DIVIDE},
{"march", required_argument, NULL, OPTION_MARCH},
@@ -9598,6 +9621,10 @@ md_parse_option (int c, char *arg)
/* -s: On i386 Solaris, this tells the native assembler to use
.stab instead of .stab.excl. We always use .stab anyhow. */
break;
+
+ case OPTION_MSHARED:
+ shared = 1;
+ break;
#endif
#if (defined (OBJ_ELF) || defined (OBJ_MAYBE_ELF) \
|| defined (TE_PE) || defined (TE_PEP) || defined (OBJ_MACH_O))
@@ -10027,6 +10054,8 @@ md_show_usage (FILE *stream)
-mold-gcc support old (<= 2.8.1) versions of gcc\n"));
fprintf (stream, _("\
-madd-bnd-prefix add BND prefix for all valid branches\n"));
+ fprintf (stream, _("\
+ -mshared disable branch optimization for shared code\n"));
# if defined (TE_PE) || defined (TE_PEP)
fprintf (stream, _("\
-mbig-obj generate big object files\n"));
diff --git a/gas/doc/c-i386.texi b/gas/doc/c-i386.texi
index 1645c8c..a1997f5 100644
--- a/gas/doc/c-i386.texi
+++ b/gas/doc/c-i386.texi
@@ -298,6 +298,17 @@ The @code{.att_syntax} and @code{.intel_syntax} directives will take precedent.
This option forces the assembler to add BND prefix to all branches, even
if such prefix was not explicitly specified in the source code.
+@cindex @samp{-mshared} option, i386
+@cindex @samp{-mshared} option, x86-64
+@item -mno-shared
+On ELF target, the assembler normally optimizes out relocations to
+defined non-weak global branch targets with default visibility. The
+@samp{-mshared} option tells the assembler to generate code which
+may go into a shared library where all non-weak global branch targets
+with default visibility can be preempted. The resulting code is
+slightly bigger. This option only affects the handling of branch
+instructions.
+
@cindex @samp{-mbig-obj} option, x86-64
@item -mbig-obj
On x86-64 PE/COFF target this option forces the use of big object file
diff --git a/gas/testsuite/gas/i386/i386.exp b/gas/testsuite/gas/i386/i386.exp
index b6f2810..c66dbc5 100644
--- a/gas/testsuite/gas/i386/i386.exp
+++ b/gas/testsuite/gas/i386/i386.exp
@@ -396,6 +396,7 @@ if [expr ([istarget "i*86-*-*"] || [istarget "x86_64-*-*"]) && [gas_32_check]]
run_dump_test "note"
run_dump_test "relax-3"
+ run_dump_test "relax-4"
if {![istarget "*-*-nacl*"]} then {
run_dump_test "iamcu-1"
@@ -763,6 +764,7 @@ if [expr ([istarget "i*86-*-*"] || [istarget "x86_64-*-*"]) && [gas_64_check]] t
run_list_test "x86-64-size-inval-1" "-al"
run_dump_test "x86-64-relax-2"
+ run_dump_test "x86-64-relax-3"
run_dump_test "x86-64-jump"
}
diff --git a/gas/testsuite/gas/i386/pcrel.d b/gas/testsuite/gas/i386/pcrel.d
index 5b61c23..8a91a1a 100644
--- a/gas/testsuite/gas/i386/pcrel.d
+++ b/gas/testsuite/gas/i386/pcrel.d
@@ -1,4 +1,5 @@
#objdump: -drw
+#as: -mshared
#name: i386 pcrel reloc
.*: +file format .*i386.*
diff --git a/gas/testsuite/gas/i386/relax-3.d b/gas/testsuite/gas/i386/relax-3.d
index 8aa94e9..4610553 100644
--- a/gas/testsuite/gas/i386/relax-3.d
+++ b/gas/testsuite/gas/i386/relax-3.d
@@ -1,3 +1,4 @@
+#as: -mshared
#objdump: -dwr
.*: +file format .*
@@ -5,26 +6,27 @@
Disassembly of section .text:
0+ <foo>:
-[ ]*[a-f0-9]+: eb 1f jmp 21 <local>
-[ ]*[a-f0-9]+: eb 19 jmp 1d <hidden_def>
-[ ]*[a-f0-9]+: e9 fc ff ff ff jmp 5 <foo\+0x5> 5: (R_386_PC)?(DISP)?32 global_def
-[ ]*[a-f0-9]+: e9 fc ff ff ff jmp a <foo\+0xa> a: (R_386_PC)?(DISP)?32 weak_def
-[ ]*[a-f0-9]+: e9 fc ff ff ff jmp f <foo\+0xf> f: (R_386_PC)?(DISP)?32 weak_hidden_undef
-[ ]*[a-f0-9]+: e9 fc ff ff ff jmp 14 <foo\+0x14> 14: (R_386_PC)?(DISP)?32 weak_hidden_def
-[ ]*[a-f0-9]+: e9 fc ff ff ff jmp 19 <foo\+0x19> 19: (R_386_PC)?(DISP)?32 hidden_undef
-
-0+1d <hidden_def>:
+[ ]*[a-f0-9]+: eb 24 jmp 26 <local>
+[ ]*[a-f0-9]+: eb 1e jmp 22 <hidden_def>
+[ ]*[a-f0-9]+: e9 fc ff ff ff jmp 5 <foo\+0x5> 5: R_386_PC32 global_def
+[ ]*[a-f0-9]+: e9 fc ff ff ff jmp a <foo\+0xa> a: R_386_PLT32 global_def
+[ ]*[a-f0-9]+: e9 fc ff ff ff jmp f <foo\+0xf> f: R_386_PC32 weak_def
+[ ]*[a-f0-9]+: e9 fc ff ff ff jmp 14 <foo\+0x14> 14: R_386_PC32 weak_hidden_undef
+[ ]*[a-f0-9]+: e9 fc ff ff ff jmp 19 <foo\+0x19> 19: R_386_PC32 weak_hidden_def
+[ ]*[a-f0-9]+: e9 fc ff ff ff jmp 1e <foo\+0x1e> 1e: R_386_PC32 hidden_undef
+
+0+22 <hidden_def>:
[ ]*[a-f0-9]+: c3 ret
-0+1e <weak_hidden_def>:
+0+23 <weak_hidden_def>:
[ ]*[a-f0-9]+: c3 ret
-0+1f <global_def>:
+0+24 <global_def>:
[ ]*[a-f0-9]+: c3 ret
-0+20 <weak_def>:
+0+25 <weak_def>:
[ ]*[a-f0-9]+: c3 ret
-0+21 <local>:
+0+26 <local>:
[ ]*[a-f0-9]+: c3 ret
#pass
diff --git a/gas/testsuite/gas/i386/relax-3.s b/gas/testsuite/gas/i386/relax-3.s
index ab52185..48ea917 100644
--- a/gas/testsuite/gas/i386/relax-3.s
+++ b/gas/testsuite/gas/i386/relax-3.s
@@ -4,6 +4,7 @@ foo:
jmp local
jmp hidden_def
jmp global_def
+ jmp global_def@PLT
jmp weak_def
jmp weak_hidden_undef
jmp weak_hidden_def
diff --git a/gas/testsuite/gas/i386/relax-4.d b/gas/testsuite/gas/i386/relax-4.d
new file mode 100644
index 0000000..2039251
--- /dev/null
+++ b/gas/testsuite/gas/i386/relax-4.d
@@ -0,0 +1,32 @@
+#source: relax-3.s
+#objdump: -dwr
+
+.*: +file format .*
+
+Disassembly of section .text:
+
+0+ <foo>:
+[ ]*[a-f0-9]+: eb 21 jmp 23 <local>
+[ ]*[a-f0-9]+: eb 1b jmp 1f <hidden_def>
+[ ]*[a-f0-9]+: eb 1b jmp 21 <global_def>
+[ ]*[a-f0-9]+: e9 fc ff ff ff jmp 7 <foo\+0x7> 7: R_386_PLT32 global_def
+[ ]*[a-f0-9]+: e9 fc ff ff ff jmp c <foo\+0xc> c: R_386_PC32 weak_def
+[ ]*[a-f0-9]+: e9 fc ff ff ff jmp 11 <foo\+0x11> 11: R_386_PC32 weak_hidden_undef
+[ ]*[a-f0-9]+: e9 fc ff ff ff jmp 16 <foo\+0x16> 16: R_386_PC32 weak_hidden_def
+[ ]*[a-f0-9]+: e9 fc ff ff ff jmp 1b <foo\+0x1b> 1b: R_386_PC32 hidden_undef
+
+0+1f <hidden_def>:
+[ ]*[a-f0-9]+: c3 ret
+
+0+20 <weak_hidden_def>:
+[ ]*[a-f0-9]+: c3 ret
+
+0+21 <global_def>:
+[ ]*[a-f0-9]+: c3 ret
+
+0+22 <weak_def>:
+[ ]*[a-f0-9]+: c3 ret
+
+0+23 <local>:
+[ ]*[a-f0-9]+: c3 ret
+#pass
diff --git a/gas/testsuite/gas/i386/x86-64-relax-2.d b/gas/testsuite/gas/i386/x86-64-relax-2.d
index 7b0bd56..c124102 100644
--- a/gas/testsuite/gas/i386/x86-64-relax-2.d
+++ b/gas/testsuite/gas/i386/x86-64-relax-2.d
@@ -1,4 +1,5 @@
#source: relax-3.s
+#as: -mshared
#objdump: -dwr
.*: +file format .*
@@ -7,26 +8,27 @@
Disassembly of section .text:
0+ <foo>:
-[ ]*[a-f0-9]+: eb 1f jmp 21 <local>
-[ ]*[a-f0-9]+: eb 19 jmp 1d <hidden_def>
+[ ]*[a-f0-9]+: eb 24 jmp 26 <local>
+[ ]*[a-f0-9]+: eb 1e jmp 22 <hidden_def>
[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq 9 <foo\+0x9> 5: R_X86_64_PC32 global_def-0x4
-[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq e <foo\+0xe> a: R_X86_64_PC32 weak_def-0x4
-[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq 13 <foo\+0x13> f: R_X86_64_PC32 weak_hidden_undef-0x4
-[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq 18 <foo\+0x18> 14: R_X86_64_PC32 weak_hidden_def-0x4
-[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq 1d <hidden_def> 19: R_X86_64_PC32 hidden_undef-0x4
+[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq e <foo\+0xe> a: R_X86_64_PLT32 global_def-0x4
+[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq 13 <foo\+0x13> f: R_X86_64_PC32 weak_def-0x4
+[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq 18 <foo\+0x18> 14: R_X86_64_PC32 weak_hidden_undef-0x4
+[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq 1d <foo\+0x1d> 19: R_X86_64_PC32 weak_hidden_def-0x4
+[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq 22 <hidden_def> 1e: R_X86_64_PC32 hidden_undef-0x4
-0+1d <hidden_def>:
+0+22 <hidden_def>:
[ ]*[a-f0-9]+: c3 retq
-0+1e <weak_hidden_def>:
+0+23 <weak_hidden_def>:
[ ]*[a-f0-9]+: c3 retq
-0+1f <global_def>:
+0+24 <global_def>:
[ ]*[a-f0-9]+: c3 retq
-0+20 <weak_def>:
+0+25 <weak_def>:
[ ]*[a-f0-9]+: c3 retq
-0+21 <local>:
+0+26 <local>:
[ ]*[a-f0-9]+: c3 retq
#pass
diff --git a/gas/testsuite/gas/i386/x86-64-relax-3.d b/gas/testsuite/gas/i386/x86-64-relax-3.d
new file mode 100644
index 0000000..98fd28d
--- /dev/null
+++ b/gas/testsuite/gas/i386/x86-64-relax-3.d
@@ -0,0 +1,33 @@
+#source: relax-3.s
+#objdump: -dwr
+
+.*: +file format .*
+
+
+Disassembly of section .text:
+
+0+ <foo>:
+[ ]*[a-f0-9]+: eb 21 jmp 23 <local>
+[ ]*[a-f0-9]+: eb 1b jmp 1f <hidden_def>
+[ ]*[a-f0-9]+: eb 1b jmp 21 <global_def>
+[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq b <foo\+0xb> 7: R_X86_64_PLT32 global_def-0x4
+[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq 10 <foo\+0x10> c: R_X86_64_PC32 weak_def-0x4
+[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq 15 <foo\+0x15> 11: R_X86_64_PC32 weak_hidden_undef-0x4
+[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq 1a <foo\+0x1a> 16: R_X86_64_PC32 weak_hidden_def-0x4
+[ ]*[a-f0-9]+: e9 00 00 00 00 jmpq 1f <hidden_def> 1b: R_X86_64_PC32 hidden_undef-0x4
+
+0+1f <hidden_def>:
+[ ]*[a-f0-9]+: c3 retq
+
+0+20 <weak_hidden_def>:
+[ ]*[a-f0-9]+: c3 retq
+
+0+21 <global_def>:
+[ ]*[a-f0-9]+: c3 retq
+
+0+22 <weak_def>:
+[ ]*[a-f0-9]+: c3 retq
+
+0+23 <local>:
+[ ]*[a-f0-9]+: c3 retq
+#pass
--
2.1.0
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: RFC: Add -mshared option to x86 ELF assembler
2015-05-13 12:59 ` H.J. Lu
@ 2015-05-20 20:02 ` Andy Lutomirski
2015-05-20 20:32 ` H. Peter Anvin
0 siblings, 1 reply; 6+ messages in thread
From: Andy Lutomirski @ 2015-05-20 20:02 UTC (permalink / raw)
To: H.J. Lu; +Cc: H. Peter Anvin, Jan Beulich, Binutils, linux-kernel
On Wed, May 13, 2015 at 5:59 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Wed, May 13, 2015 at 4:50 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> On Tue, May 12, 2015 at 5:14 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>> 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.
>>>>
>>>
>>> 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 $2,(%rsp) # X86_TRAP_NMI
> --
> 2.1.0
>
> I got
>
> [hjl@gnu-tools-1 kernel.org]$ 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@gnu-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@gnu-tools-1 kernel.org]$
>
> It removes 6 relocations. On gcc master branch,
>
> [hjl@gnu-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@gnu-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@gnu-tools-1 gcc-misc]$
>
> It is more effective. I will run more tests.
This seems like a sensible idea, but I can imagine it breaking some
weird use cases (like that one Linux thing). Is that okay?
--Andy
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: RFC: Add -mshared option to x86 ELF assembler
2015-05-20 20:02 ` Andy Lutomirski
@ 2015-05-20 20:32 ` H. Peter Anvin
2015-05-20 20:54 ` Andy Lutomirski
0 siblings, 1 reply; 6+ messages in thread
From: H. Peter Anvin @ 2015-05-20 20:32 UTC (permalink / raw)
To: Andy Lutomirski, H.J. Lu; +Cc: Jan Beulich, Binutils, linux-kernel
On 05/20/2015 01:02 PM, Andy Lutomirski wrote:
>>
>> I got
>>
>> [hjl@gnu-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@gnu-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@gnu-tools-1 kernel.org]$
>>
>> It removes 6 relocations. On gcc master branch,
>>
>> [hjl@gnu-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@gnu-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@gnu-tools-1 gcc-misc]$
>>
>> It is more effective. I will run more tests.
>
> This seems like a sensible idea, but I can imagine it breaking some
> weird use cases (like that one Linux thing). Is that okay?
>
What about the patch I posted recently?
-hpa
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: RFC: Add -mshared option to x86 ELF assembler
2015-05-20 20:32 ` H. Peter Anvin
@ 2015-05-20 20:54 ` Andy Lutomirski
0 siblings, 0 replies; 6+ messages in thread
From: Andy Lutomirski @ 2015-05-20 20:54 UTC (permalink / raw)
To: H. Peter Anvin; +Cc: H.J. Lu, Jan Beulich, Binutils, linux-kernel
On Wed, May 20, 2015 at 1:32 PM, H. Peter Anvin <hpa@zytor.com> wrote:
> On 05/20/2015 01:02 PM, Andy Lutomirski wrote:
>>>
>>> I got
>>>
>>> [hjl@gnu-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@gnu-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@gnu-tools-1 kernel.org]$
>>>
>>> It removes 6 relocations. On gcc master branch,
>>>
>>> [hjl@gnu-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@gnu-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@gnu-tools-1 gcc-misc]$
>>>
>>> It is more effective. I will run more tests.
>>
>> This seems like a sensible idea, but I can imagine it breaking some
>> weird use cases (like that one Linux thing). Is that okay?
>>
>
> What about the patch I posted recently?
>
I replied in that thread.
--Andy
> -hpa
>
>
--
Andy Lutomirski
AMA Capital Management, LLC
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2015-05-20 20:54 UTC | newest]
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2015-05-13 0:14 RFC: Add -mshared option to x86 ELF assembler H.J. Lu
2015-05-13 11:50 ` H.J. Lu
2015-05-13 12:59 ` H.J. Lu
2015-05-20 20:02 ` Andy Lutomirski
2015-05-20 20:32 ` H. Peter Anvin
2015-05-20 20:54 ` Andy Lutomirski
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