How to edit and re-build the GCC libstdc++ C++ sta

Question

I am working on some research and would like to edit some of the source code in the libstdc++ library for experimentation. I am, specifically, interested in experimenting with the parallel sorting algorithms. Is there a place I can find documentation to easily edit and build the source code?

I have tried, unsuccessfully, to build various versions of the libstdc++ library. It seems like most new versions require building the entire gcc package, which is a much more lengthy process, especially if I am going to be editing and experimenting with a few files in libstdc++.

I have also been unable to find the source files that contain the parallel sorting algorithms. I can only seem to find the header files that define the functions, and not the source code itself. Any advice or links to documentation would be greatly appreciated.

Answer 1

Minimal step-by-step example

Compile GCC from source. Condensed commands:

sudo apt-get build-dep gcc
git clone git://gcc.gnu.org/git/gcc.git
cd gcc
git checkout gcc-6_4_0-release
./contrib/download_prerequisites
mkdir build
cd build
../configure --enable-languages=c,c++ --prefix="$(pwd)/install"
make -j`nproc`

Wait from 30-minutes to two hours. Now let's use this test program a.cpp:

#include <cassert>
#include <queue>

int main() {
    std::priority_queue<int> q;
    q.emplace(2);
    q.emplace(1);
    q.emplace(3);
    assert(q.top() == 3);
    q.pop();
    assert(q.top() == 2);
    q.pop();
    assert(q.top() == 1);
    q.pop();
}

First compile and run it to ensure that the initial compilation worked:

gcc/build/install/bin/g++ -g -std=c++11 -O0 -o a.out ./a.cpp
./a.out

Now let's hack up the priority_queue constructor.

First, you can find the actual constructor easily with GDB as explained at: When should I use make_heap vs. Priority Queue?

So we hack it up with this patch:

diff --git a/libstdc++-v3/include/bits/stl_queue.h b/libstdc++-v3/include/bits/stl_queue.h
index 5d255e7300b..deec7bc4d99 100644
--- a/libstdc++-v3/include/bits/stl_queue.h
+++ b/libstdc++-v3/include/bits/stl_queue.h
@@ -61,6 +61,7 @@
 #if __cplusplus >= 201103L
 # include <bits/uses_allocator.h>
 #endif
+#include <iostream>

 namespace std _GLIBCXX_VISIBILITY(default)
 {
@@ -444,7 +445,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       priority_queue(const _Compare& __x = _Compare(),
             _Sequence&& __s = _Sequence())
       : c(std::move(__s)), comp(__x)
-      { std::make_heap(c.begin(), c.end(), comp); }
+      {
+        std::cout << "hacked" << std::endl;
+        std::make_heap(c.begin(), c.end(), comp);
+      }

       template<typename _Alloc, typename _Requires = _Uses<_Alloc>>
    explicit

Then rebuild and re-install just libstdc++ to save a lot of time:

cd gcc/build/x86_64-pc-linux-gnu/libstdc++-v3
make -j`nproc`
make install

and now the next build and run:

gcc/build/install/bin/g++ -g -std=c++11 -O0 -o a.out ./a.cpp
./a.out

outputs:

hacked

Tested on Ubuntu 16.04.

glibc

As a bonus, if you are also interested in C: Multiple glibc libraries on a single host

Answer 2

Yes, you have to build the whole of GCC, but once you've done that you only need to rebuild the libstdc++ part.

Building GCC is described at http://gcc.gnu.org/wiki/InstallingGCC

The libstdc++ sources are in the libstdc++-v3 directory. The parallel algorithms are in libstdc++-v3/include/parallel, they are templates so all the code is in headers. The small amount of non-header code is in libstdc++-v3/src/c++98/parallel-settings.cc

To rebuild libstdc++ from the top-level build dir go into the $TARGET/libstdc++-v3 directory (where $TARGET is something like x86_64-unknown-linux-gnu) and run make.