How to edit and re-build the GCC libstdc++ C++ standard library source?

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

After configuring and building the whole of GCC as normal, you can rebuild libstdc++ by running make in the $TARGET/libstdc++-v3 directory (where $TARGET is something like x86_64-pc-linux-gnu).

By default the makefiles don't have proper dependencies that cause objects to be rebuilt after headers change, so you might need to do make clean then make again to get your changes to be picked up.