GCC Reducing Binary Bloat -- Strange Side Effect

Question

The Weirdness

I have compiled Google Protocol Buffers using no extra parameters for a "bloat" compile, and compile with the following command ./configure CXXFLAGS="-ffunction-sections -fdata-sections". a du-h reveals:

120K ./bloat/bin
124K ./bloat/include/google/protobuf/io
8.0K ./bloat/include/google/protobuf/compiler/java
12K ./bloat/include/google/protobuf/compiler/python
8.0K ./bloat/include/google/protobuf/compiler/cpp
128K ./bloat/include/google/protobuf/compiler
52K ./bloat/include/google/protobuf/stubs
848K ./bloat/include/google/protobuf
852K ./bloat/include/google
856K ./bloat/include
12K ./bloat/lib/pkgconfig
37M ./bloat/lib
38M ./bloat
20K ./unbloat/bin
124K ./unbloat/include/google/protobuf/io
8.0K ./unbloat/include/google/protobuf/compiler/java
12K ./unbloat/include/google/protobuf/compiler/python
8.0K ./unbloat/include/google/protobuf/compiler/cpp
128K ./unbloat/include/google/protobuf/compiler
52K ./unbloat/include/google/protobuf/stubs
848K ./unbloat/include/google/protobuf
852K ./unbloat/include/google
856K ./unbloat/include
12K ./unbloat/lib/pkgconfig
15M ./unbloat/lib
16M ./unbloat
53M .

Drill Down:

 ls -gGh bloat/lib/
    total 37M
    -rw-r--r-- 1  13M 2010-09-07 13:57 libprotobuf.a
    -rwxr-xr-x 1  986 2010-09-07 13:57 libprotobuf.la
    -rw-r--r-- 1 1.6M 2010-09-07 13:57 libprotobuf-lite.a
    -rwxr-xr-x 1 1021 2010-09-07 13:57 libprotobuf-lite.la
    lrwxrwxrwx 1   25 2010-09-07 13:57 libprotobuf-lite.so -> libprotobuf-lite.so.6.0.0
    lrwxrwxrwx 1   25 2010-09-07 13:57 libprotobuf-lite.so.6 -> libprotobuf-lite.so.6.0.0
    -rwxr-xr-x 1 771K 2010-09-07 13:57 libprotobuf-lite.so.6.0.0
    lrwxrwxrwx 1   20 2010-09-07 13:57 libprotobuf.so -> libprotobuf.so.6.0.0
    lrwxrwxrwx 1   20 2010-09-07 13:57 libprotobuf.so.6 -> libprotobuf.so.6.0.0
    -rwxr-xr-x 1 5.5M 2010-09-07 13:57 libprotobuf.so.6.0.0
    -rw-r--r-- 1  12M 2010-09-07 13:57 libprotoc.a
    -rwxr-xr-x 1 1.1K 2010-09-07 13:57 libprotoc.la
    lrwxrwxrwx 1   18 2010-09-07 13:57 libprotoc.so -> libprotoc.so.6.0.0
    lrwxrwxrwx 1   18 2010-09-07 13:57 libprotoc.so.6 -> libprotoc.so.6.0.0
    -rwxr-xr-x 1 4.6M 2010-09-07 13:57 libprotoc.so.6.0.0
    drwxr-xr-x 2 4.0K 2010-09-07 13:57 pkgconfig
    ls -gGh unbloat/lib/
    total 15M
    -rw-r--r-- 1 5.8M 2010-09-07 14:03 libprotobuf.a
    -rwxr-xr-x 1  988 2010-09-07 14:03 libprotobuf.la
    -rw-r--r-- 1 764K 2010-09-07 14:03 libprotobuf-lite.a
    -rwxr-xr-x 1 1023 2010-09-07 14:03 libprotobuf-lite.la
    lrwxrwxrwx 1   25 2010-09-07 14:03 libprotobuf-lite.so -> libprotobuf-lite.so.6.0.0
    lrwxrwxrwx 1   25 2010-09-07 14:03 libprotobuf-lite.so.6 -> libprotobuf-lite.so.6.0.0
    -rwxr-xr-x 1 393K 2010-09-07 14:03 libprotobuf-lite.so.6.0.0
    lrwxrwxrwx 1   20 2010-09-07 14:03 libprotobuf.so -> libprotobuf.so.6.0.0
    lrwxrwxrwx 1   20 2010-09-07 14:03 libprotobuf.so.6 -> libprotobuf.so.6.0.0
    -rwxr-xr-x 1 2.7M 2010-09-07 14:03 libprotobuf.so.6.0.0
    -rw-r--r-- 1 3.7M 2010-09-07 14:04 libprotoc.a
    -rwxr-xr-x 1 1.1K 2010-09-07 14:04 libprotoc.la
    lrwxrwxrwx 1   18 2010-09-07 14:04 libprotoc.so -> libprotoc.so.6.0.0
    lrwxrwxrwx 1   18 2010-09-07 14:04 libprotoc.so.6 -> libprotoc.so.6.0.0
    -rwxr-xr-x 1 1.3M 2010-09-07 14:04 libprotoc.so.6.0.0
    drwxr-xr-x 2 4.0K 2010-09-07 14:03 pkgconfig

The Question

I have not altered the build scripts to perform a "--gc-sections" during linking, therefore shouldn't the unbloat build be the same if not bigger ? What caused the reduction in size ?

Background

I am compiling a low-level library with gcc at the moment and the library is a ginormous 2.5MB unstriped and 970KB stripped. This is unacceptable, and I need to remove dead-code -- I depend on OpenSSL, Protocol Buffers and 3 Libraries from Boost, and I will static link the last 2 into my library. The two statically linked libraries will have to be compiled them with the "-ffunction-sections -fdata-sections" for me to remove dead code.

Related Question

My next question is about how to specify the root used to eliminate dead code.

Answer 1

I am afraid the gain has nothing to do with -ffunction-sections -fdata-sections: when you specified CXXFLAGS="-ffunction-sections -fdata-sections" on configure command line, you overwrote the default flags which were -O2 -g -DNDEBUG. As a consequence, your code was compiled with no optimizations.

You should redo your test with CXXFLAGS="-ffunction-sections -fdata-sections -O2 -g -DNDEBUG" and you will get the expected (i.e., identical) results.