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Re: Binutils gc-sections

From: "Ivan Petrov" <petrov at mail dot mobyline dot com>
To: "Alan Modra" <amodra at bigpond dot net dot au>
Cc: <binutils at sources dot redhat dot com>
Date: Sun, 11 Dec 2005 02:06:11 +0500
Subject: Re: Binutils gc-sections
References: <000601c5fd0f$1bf70f00$6401a8c0@HARMMOB> <20051210033620.GO10488@bubble.grove.modra.org>

Hi Alan!

Thanks, your testcase showed me the real problem.  BTW, it is usually
better to ask questions or discuss patches on public mailing lists.  See
http://www.eyrie.org/~eagle/faqs/questions.html

Ok, i see you.

* elflink.c (elf_gc_mark_dynamic_ref_symbol): Use !info-executable
to test for linking shared libs, not info->shared.
Index: bfd/elflink.c
===================================================================
RCS file: /cvs/src/src/bfd/elflink.c,v
retrieving revision 1.198
diff -u -p -r1.198 elflink.c
--- bfd/elflink.c 7 Dec 2005 14:43:53 -0000 1.198
+++ bfd/elflink.c 10 Dec 2005 03:25:48 -0000
@@ -9068,7 +9075,7 @@ elf_gc_mark_dynamic_ref_symbol (struct e
  if ((h->root.type == bfd_link_hash_defined
       || h->root.type == bfd_link_hash_defweak)
      && (h->ref_dynamic
-   || (info->shared
+   || (!info->executable
       && h->def_regular
       && ELF_ST_VISIBILITY (h->other) != STV_INTERNAL
       && ELF_ST_VISIBILITY (h->other) != STV_HIDDEN)))

About you patch. As I show in my previous atached samples problem was on --pic-executable flag. After you remove restriction in "elflink.c":

[code]
 if (!get_elf_backend_data (abfd)->can_gc_sections
     || info->relocatable
     || info->emitrelocations
     || info->shared

[/code]

ld begin allow it with --gc-sections key (see CMD line parser for detail).

I make other patch now and test again on my file, results followed later. While all look like good. But please see on string where i remove "h->ref_dynamic", with out it, "--gc-sections" not work correctly. I not sure on 100% about it, but on my test it work correctly.

[ ------- patch ------- ]
diff -u elflink.c elflink.c
--- elflink.c_ Wed Dec  7 14:43:53 2005
+++ elflink.c Sat Dec 10 20:39:27 2005
@@ -9067,11 +9067,10 @@

  if ((h->root.type == bfd_link_hash_defined
       || h->root.type == bfd_link_hash_defweak)
-      && (h->ref_dynamic
-   || (!info->executable
+      && (info->shared && !info->pie
       && h->def_regular
       && ELF_ST_VISIBILITY (h->other) != STV_INTERNAL
-       && ELF_ST_VISIBILITY (h->other) != STV_HIDDEN)))
+       && ELF_ST_VISIBILITY (h->other) != STV_HIDDEN))
    h->root.u.def.section->flags |= SEC_KEEP;

  return TRUE;
[ ------- end patch ------- ]

[ ------- sample ------- ]
/* simple internal program function */
void exec_if() {}

/* function ptr declarated in data */
void (*fptr)() = exec_if;

/* exectable entry */
void exec_entry() { fptr(); }

/* shared lib internal function */
void lib_if() {}

/* shared lib exproted function */
__attribute__((visibility("default")))
void lib_ef() {}

/* shared lib exported data */
__attribute__((visibility("default")))
const char lib_ed[] = { "test_exp_str" };
[ ------- end sample ------- ]

[ ------- build ------- ] arm-elf-gcc -Os -c -fvisibility=internal -ffunction-sections -fdata-sections sample.c arm-elf-ld -s --gc-sections -shared -o sample.so sample.o arm-elf-ld -s --gc-sections --pic-executable --entry=exec_entry -o sample sample.o [ ------- end build ------- ]

[ ------- sample.so disasm ------- ] .text:0000025C ; Processor : ARM .text:0000025C ; Target assembler: Generic assembler for ARM .text:0000025C ; Byte sex : Little endian .text:0000025C .text:0000025C ; =========================================================================== .text:0000025C .text:0000025C ; Segment type: Pure code .text:0000025C AREA .text, CODE, READWRITE .text:0000025C ; ORG 0x25C .text:0000025C CODE32 .text:0000025C .text:0000025C ; --------------- S U B R O U T I N E --------------------------------------- .text:0000025C .text:0000025C .text:0000025C EXPORT lib_ef .text:0000025C lib_ef .text:0000025C BX LR .text:0000025C ; End of function lib_ef .text:0000025C .text:0000025C ; _text ends .text:0000025C .rodata:00000260 ; =========================================================================== .rodata:00000260 .rodata:00000260 ; Segment type: Pure data .rodata:00000260 AREA .rodata, DATA, READONLY .rodata:00000260 ; ORG 0x260 .rodata:00000260 EXPORT lib_ed .rodata:00000260 lib_ed DCB "test_exp_str",0 .rodata:0000026D ALIGN 0x10 .rodata:0000026D ; _rodata ends .rodata:0000026D .got:000003C8 ; =========================================================================== .got:000003C8 .got:000003C8 ; Segment type: Pure data .got:000003C8 AREA .got, DATA .got:000003C8 ; ORG 0x3C8 .got:000003C8 DCD 0x370, 0, 0 .got:000003C8 ; _got ends .got:000003C8 abs:000003D4 ; =========================================================================== abs:000003D4 abs:000003D4 ; Segment type: Absolute symbols abs:000003D4 __exidx_end = 0x270 abs:000003D8 _bss_end__ = 0x3D4 abs:000003DC __bss_start__ = 0x3D4 abs:000003E0 __exidx_start = 0x270 abs:000003E4 __bss_end__ = 0x3D4 abs:000003E8 __bss_start = 0x3D4 abs:000003EC __end__ = 0x3D4 abs:000003F0 _edata = 0x3D4 abs:000003F4 _end = 0x3D4 abs:000003F8 _stack = 0x80000 abs:000003FC __data_start = 0x3D4 abs:000003FC abs:000003FC END [ ------- end sample.so disasm ------- ]

[ ------- sample disasm ------- ] .text:00000288 ; Processor : ARM .text:00000288 ; Target assembler: Generic assembler for ARM .text:00000288 ; Byte sex : Little endian .text:00000288 .text:00000288 ; =========================================================================== .text:00000288 .text:00000288 ; Segment type: Pure code .text:00000288 AREA .text, CODE, READWRITE .text:00000288 ; ORG 0x288 .text:00000288 CODE32 .text:00000288 .text:00000288 ; --------------- S U B R O U T I N E --------------------------------------- .text:00000288 .text:00000288 .text:00000288 nullsub_1 ; DATA XREF: start:__data_starto .text:00000288 BX LR .text:00000288 ; End of function nullsub_1 .text:00000288 .text:0000028C .text:0000028C ; --------------- S U B R O U T I N E --------------------------------------- .text:0000028C .text:0000028C .text:0000028C EXPORT start .text:0000028C start .text:0000028C .text:0000028C var_4 = -4 .text:0000028C arg_4 = 4 .text:0000028C .text:0000028C ; FUNCTION CHUNK AT .data:00000444 SIZE 00000004 BYTES .text:0000028C .text:0000028C STR LR, [SP,#var_4]! .text:00000290 LDR R3, =__data_start .text:00000294 MOV LR, PC .text:00000298 LDR PC, [R3] .text:0000029C LDR PC, [SP],#arg_4 .text:0000029C ; End of function start .text:0000029C .text:0000029C ; --------------------------------------------------------------------------- .text:000002A0 off_2A0 DCD __data_start ; DATA XREF: start+4r .text:000002A0 ; _text ends .text:000002A0 .got:00000438 ; =========================================================================== .got:00000438 .got:00000438 ; Segment type: Pure data .got:00000438 AREA .got, DATA .got:00000438 ; ORG 0x438 .got:00000438 DCD 0x3B8, 0, 0 .got:00000438 ; _got ends .got:00000438 .data:00000444 ; =========================================================================== .data:00000444 .data:00000444 ; Segment type: Pure data .data:00000444 AREA .data, DATA .data:00000444 ; ORG 0x444 .data:00000444 ; START OF FUNCTION CHUNK FOR start .data:00000444 EXPORT __data_start .data:00000444 __data_start DCD nullsub_1 ; CODE XREF: start+10j .data:00000444 ; DATA XREF: .text:off_2A0o .data:00000444 ; END OF FUNCTION CHUNK FOR start .data:00000444 ; _data ends .data:00000444 abs:00000448 ; =========================================================================== abs:00000448 abs:00000448 ; Segment type: Absolute symbols abs:00000448 __exidx_end = 0x2B5 abs:0000044C _bss_end__ = 0x448 abs:00000450 __bss_start__ = 0x448 abs:00000454 __exidx_start = 0x2B5 abs:00000458 __bss_end__ = 0x448 abs:0000045C __bss_start = 0x448 abs:00000460 __end__ = 0x448 abs:00000464 _edata = 0x448 abs:00000468 _end = 0x448 abs:0000046C _stack = 0x80000 abs:0000046C abs:0000046C END start [ ------- end sample disasm ------- ]

-- Regards Ivan Petrov

References:
- Re: Binutils gc-sections
  - From: Alan Modra

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