Strange symbols in shared library

[Lascha Lagidse]

Hi all,

I've build a shared library using ANT, and found that its size was really large (3.6MB). I am trying to reduce that size, so I used readelf.exe from the ndk toolchain and found a few problems:

1. I encountered symbols like $d and $a, which have no size but are still exported. There are about 15000 symbols like that!! It seems really redundant, what are they?

Symbol table '.symtab' contains 23111 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
    ...
    13: 000a7830     0 NOTYPE  LOCAL  DEFAULT    8 $d
    14: 000a7838     0 NOTYPE  LOCAL  DEFAULT    8 $a
    15: 000a7844     0 NOTYPE  LOCAL  DEFAULT    8 $d
    16: 000a7848     0 NOTYPE  LOCAL  DEFAULT    8 $a
    17: 000a786c     0 NOTYPE  LOCAL  DEFAULT    8 $d
    18: 000a7874     0 NOTYPE  LOCAL  DEFAULT    8 $a
    19: 000a7890     0 NOTYPE  LOCAL  DEFAULT    8 $d
    20: 000a7898     0 NOTYPE  LOCAL  DEFAULT    8 $a
    ...

2. I make use of templated objects, and they also seem to generate a very large amount of symbols, some of which seem like duplicates, like so:

  562: 000ac0c0    52 FUNC    WEAK   DEFAULT    8 _ZN9TSmartPtrI11TCallable1PIvRK22TMouseButtonPressEventEED2Ev
  563: 000ac0c0    52 FUNC    WEAK   DEFAULT    8 _ZN9TSmartPtrI11TCallable1PIvRK22TMouseButtonPressEventEED1Ev
  564: 000ac0f4    60 FUNC    WEAK   DEFAULT    8 _ZN9TSmartPtrI11TCallable1PIvRK22TMouseButtonPressEventEED0Ev
  ...
  16256: 00172c60     8 OBJECT  WEAK   DEFAULT   15 _ZTI9TSmartPtrI22TMouseButtonPressEventE
  16257: 001650e8    37 OBJECT  WEAK   DEFAULT   11 _ZTS9TSmartPtrI22TMouseButtonPressEventE

Note that the symbols are almost identical... What does this mean? It seems very redundant to me.

Not sure if this will help, but wouldn't hurt to add:
Compiler command line:
-Wno-psabi -funswitch-loops -D"ANDROID_NDK" -D"ANDROID" -D"__ANDROID__" -D"__ARM_EABI__" -D"__ARM_ARCH_5__" -D"__ARM_ARCH_5T__" -D"__ARM_ARCH_5E__" -D"__ARM_ARCH_5TE__" -fexceptions -fomit-frame-pointer -fno-strict-aliasing -I"D:\Development\Source\Engine\\Core" -I"../Middleware/glew/include" -I"../Middleware/zlib/" -I"../Middleware/libpng" -I"../Middleware/box2d/" -I"../Middleware/lua/src/" -I"../Middleware/luabridge/Source/LuaBridge/" -finline-limit=100 -fstack-protector -fno-short-enums -D"NDEBUG" -D"_MBCS" -D"GLEW_NO_GLU" -O3 -frtti -I"C:\android-ndk-r8e\platforms\android-8\arch-arm\usr\include" -I"C:\android-ndk-r8e\sources\cxx-stl\gnu-libstdc++\4.7\include" -I"C:\android-ndk-r8e\sources\cxx-stl\gnu-libstdc++\4.7\libs\armeabi\include" -marm -o "Android\Release\%(FileName).o"
Linker command line:
-o "D:\Development\Source\Android\Release\libEngine.so" -Wl,--no-undefined "-lGLESv2" "-lpng" "-llua" "-lbox2d" "-lstdc++" "-lsupc++" "C:\android-ndk-r8e\sources\cxx-stl\gnu-libstdc++\4.7\libs\armeabi\libgnustl_shared.so" "-lgcc" --sysroot="C:\android-ndk-r8e\platforms\android-8\arch-arm\usr\lib\..\.." -L"C:\android-ndk-r8e\platforms\android-8\arch-arm\usr\lib" -L"C:\android-ndk-r8e\sources\cxx-stl\gnu-libstdc++\4.7\libs\armeabi" -l"c" -l"m" -l"log" -l"gcc" -l"z" -shared -Wl,-z,noexecstack -Wl,-rpath-link="C:\android-ndk-r8e\platforms\android-8\arch-arm\usr\lib" -L"D:\Development\Source\/Android/Release/"

[David Turner]

On Sun, Dec 29, 2013 at 3:07 PM, Lascha Lagidse <lolly...@gmail.com> wrote:

Hi all,

I've build a shared library using ANT, and found that its size was really large (3.6MB). I am trying to reduce that size, so I used readelf.exe from the ndk toolchain and found a few problems:

1. I encountered symbols like $d and $a, which have no size but are still exported. There are about 15000 symbols like that!! It seems really redundant, what are they?

Symbol table '.symtab' contains 23111 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
    ...
    13: 000a7830     0 NOTYPE  LOCAL  DEFAULT    8 $d
    14: 000a7838     0 NOTYPE  LOCAL  DEFAULT    8 $a
    15: 000a7844     0 NOTYPE  LOCAL  DEFAULT    8 $d
    16: 000a7848     0 NOTYPE  LOCAL  DEFAULT    8 $a
    17: 000a786c     0 NOTYPE  LOCAL  DEFAULT    8 $d
    18: 000a7874     0 NOTYPE  LOCAL  DEFAULT    8 $a
    19: 000a7890     0 NOTYPE  LOCAL  DEFAULT    8 $d
    20: 000a7898     0 NOTYPE  LOCAL  DEFAULT    8 $a
    ...

These are ARM-specific symbols uses to indicate transitions between 32-bit ARM and 16-bit Thumb sections of code.

See https://sourceware.org/binutils/docs/as/ARM-Mapping-Symbols.html

 

2. I make use of templated objects, and they also seem to generate a very large amount of symbols, some of which seem like duplicates, like so:

  562: 000ac0c0    52 FUNC    WEAK   DEFAULT    8 _ZN9TSmartPtrI11TCallable1PIvRK22TMouseButtonPressEventEED2Ev
  563: 000ac0c0    52 FUNC    WEAK   DEFAULT    8 _ZN9TSmartPtrI11TCallable1PIvRK22TMouseButtonPressEventEED1Ev
  564: 000ac0f4    60 FUNC    WEAK   DEFAULT    8 _ZN9TSmartPtrI11TCallable1PIvRK22TMouseButtonPressEventEED0Ev
  ...
  16256: 00172c60     8 OBJECT  WEAK   DEFAULT   15 _ZTI9TSmartPtrI22TMouseButtonPressEventE
  16257: 001650e8    37 OBJECT  WEAK   DEFAULT   11 _ZTS9TSmartPtrI22TMouseButtonPressEventE

Note that the symbols are almost identical... What does this mean? It seems very redundant to me.

Welcome to the wonderful world of C++ :-)

D2Ev, D1Ev, D0Ev are suffixes uses to denote several types of destructors, as required by the C++ specification.

From http://refspecs.linuxbase.org/cxxabi-1.75.html#mangling

Constructors and destructors are simply special cases of <unqualified-name>, where the final <unqualified-name> of a nested name is replaced by one of the following:

  <ctor-dtor-name> ::= C1	# complete object constructor
		   ::= C2	# base object constructor
		   ::= C3	# complete object allocating constructor
		   ::= D0	# deleting destructor
		   ::= D1	# complete object destructor
		   ::= D2	# base object destructor

These are auto-generated by the compiler if you don't specify them. Most of them should be purged from your final binaries if you use the right compiler and linker flags. 

Not sure if this will help, but wouldn't hurt to add:
Compiler command line:
-Wno-psabi -funswitch-loops -D"ANDROID_NDK" -D"ANDROID" -D"__ANDROID__" -D"__ARM_EABI__" -D"__ARM_ARCH_5__" -D"__ARM_ARCH_5T__" -D"__ARM_ARCH_5E__" -D"__ARM_ARCH_5TE__" -fexceptions -fomit-frame-pointer -fno-strict-aliasing -I"D:\Development\Source\Engine\\Core" -I"../Middleware/glew/include" -I"../Middleware/zlib/" -I"../Middleware/libpng" -I"../Middleware/box2d/" -I"../Middleware/lua/src/" -I"../Middleware/luabridge/Source/LuaBridge/" -finline-limit=100 -fstack-protector -fno-short-enums -D"NDEBUG" -D"_MBCS" -D"GLEW_NO_GLU" -O3 -frtti -I"C:\android-ndk-r8e\platforms\android-8\arch-arm\usr\include" -I"C:\android-ndk-r8e\sources\cxx-stl\gnu-libstdc++\4.7\include" -I"C:\android-ndk-r8e\sources\cxx-stl\gnu-libstdc++\4.7\libs\armeabi\include" -marm -o "Android\Release\%(FileName).o"
Linker command line:
-o "D:\Development\Source\Android\Release\libEngine.so" -Wl,--no-undefined "-lGLESv2" "-lpng" "-llua" "-lbox2d" "-lstdc++" "-lsupc++" "C:\android-ndk-r8e\sources\cxx-stl\gnu-libstdc++\4.7\libs\armeabi\libgnustl_shared.so" "-lgcc" --sysroot="C:\android-ndk-r8e\platforms\android-8\arch-arm\usr\lib\..\.." -L"C:\android-ndk-r8e\platforms\android-8\arch-arm\usr\lib" -L"C:\android-ndk-r8e\sources\cxx-stl\gnu-libstdc++\4.7\libs\armeabi" -l"c" -l"m" -l"log" -l"gcc" -l"z" -shared -Wl,-z,noexecstack -Wl,-rpath-link="C:\android-ndk-r8e\platforms\android-8\arch-arm\usr\lib" -L"D:\Development\Source\/Android/Release/"

Try adding "-ffunction-section -fdata-section" to your compiler flags, and "-Wl,--gc-sections" to your linker flags to get rid of a lot of C++ template cruft at link time. This may slow down your build though.

Hope this helps.

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