Bug 12818 - gold linker failed to remove all dead code dependency properly
Summary: gold linker failed to remove all dead code dependency properly
Status: RESOLVED FIXED
Alias: None
Product: binutils
Classification: Unclassified
Component: gold (show other bugs)
Version: 2.22
: P2 normal
Target Milestone: ---
Assignee: Ian Lance Taylor
URL:
Keywords:
Depends on:
Blocks:
 
Reported: 2011-05-27 11:04 UTC by lion
Modified: 2011-06-29 14:45 UTC (History)
1 user (show)

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Last reconfirmed:


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Description lion 2011-05-27 11:04:32 UTC
Initially reported as
https://bugzilla.novell.com/show_bug.cgi?id=694266
http://sourceware.org/bugzilla/show_bug.cgi?id=12772

I`ve found, that this bug applied to gold linker too:
----- test.cpp
#include <string.h>
#include <stdlib.h>
#include <unistd.h>

extern "C" int __attribute__((visibility("default"))) func(char *ptr)
{
    static const char g_const[] = { 1, 1, 0 };
    memcpy(ptr, g_const, 3);

    return 0;
}

extern "C" char *func2(char *ptr)
{
    static char buf[4096];
    if (!getcwd(buf, sizeof(buf)))
        buf[0] = 0;
#define STR "test_string"
    memcpy(ptr, STR, sizeof(STR));
    return buf;
}
-----
gcc -O3 -c -fpic -shared -fdata-sections -ffunction-sections \
-fvisibility-inlines-hidden -fvisibility=hidden -o test.o test.cpp
gold -m elf_x86_64 -gc-sections -s -shared -o test.so test.o
readelf -s 5540450000000000.so

Symbol table '.dynsym' contains 7 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 0000000000000270    23 FUNC    GLOBAL DEFAULT    4 func
     2: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND _GLOBAL_OFFSET_TABLE_
     3: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND getcwd
     4: 0000000000001380     0 NOTYPE  GLOBAL DEFAULT  ABS _edata
     5: 0000000000001380     0 NOTYPE  GLOBAL DEFAULT  ABS __bss_start
     6: 0000000000001380     0 NOTYPE  GLOBAL DEFAULT  ABS _end

As we see - getcwd dependency not discarded properly.
Also I`ve note that this way I can get smallest shared object size - 1888 bytes,
while using 
"gcc -O3 -Wl,-x,-gc-sections,-s -fpic -flto -shared -fdata-sections -ffunction-sections -fvisibility-inlines-hidden -fvisibility=hidden -o test.o test.cpp"
size is 4552, and without -flto:
"gcc -O3 -Wl,-x,-gc-sections,-s -fpic -shared -fdata-sections -ffunction-sections -fvisibility-inlines-hidden -fvisibility=hidden -o test.o test.cpp"
size is 6280
I assume something going not ringht.
Comment 1 cvs-commit@gcc.gnu.org 2011-06-29 14:43:11 UTC
CVSROOT:	/cvs/src
Module name:	src
Changes by:	ian@sourceware.org	2011-06-29 14:43:08

Modified files:
	gold           : ChangeLog symtab.cc 

Log message:
	PR gold/12818
	* symtab.cc (Symbol::should_add_dynsym_entry): Don't add undefined
	symbols which are not used in a relocation.

Patches:
http://sourceware.org/cgi-bin/cvsweb.cgi/src/gold/ChangeLog.diff?cvsroot=src&r1=1.773&r2=1.774
http://sourceware.org/cgi-bin/cvsweb.cgi/src/gold/symtab.cc.diff?cvsroot=src&r1=1.155&r2=1.156
Comment 2 Ian Lance Taylor 2011-06-29 14:45:15 UTC
Fixed in the development sources.  Thanks for noticing it.