cpp-cheat/gcc/source-tree.md

Source tree

Tested on version 5.1.

What GCC is coded with

Was C, then in 2012 at 4.8 started allowing C++.

There are however no .cpp or .hpp files: .c and .hpp extensions were kept.

Programs

The single source tree contains:

• C, C++, etc. compilers
• Binutils: as, ln, nm

Overview

The top 20 directories by size are:

1.6G    .git
360M    gcc
165M    libjava
78M     libstdc++-v3
15M     libgo
15M     libgcc
14M     gmp-4.3.2
8.0M    libgfortran
7.4M    mpfr-2.4.2
3.5M    zlib
3.5M    libcpp
3.1M    libgomp
3.1M    boehm-gc
3.0M    libffi
2.9M    mpc-0.8.1
2.5M    libiberty
1.9M    libsanitizer
1.7M    libquadmath
1.5M    libitm
1.5M    libdecnumber

The main source is under gcc:

175M    testsuite
49M     ada
32M     config
31M     po
7.8M    cp
6.4M    fortran
5.1M    doc
2.0M    go
1.9M    java
1.8M    ChangeLog-2004
1.5M    ChangeLog-2011
1.5M    ChangeLog-2010
1.4M    ChangeLog-2005
1.4M    ChangeLog-2003
1.3M    ChangeLog-2012
1.3M    ChangeLog-2009
1.3M    ChangeLog-2007
1.3M    ChangeLog-2002
1.2M    ChangeLog-2001
1.2M    ChangeLog-2000
1.1M    c-family
1.1M    c
1.1M    ChangeLog-2008
876K    ChangeLog-2006
872K    objc
836K    configure
784K    ChangeLog-1999
744K    ChangeLog.tree-ssa
716K    dwarf2out.c
640K    ChangeLog-1998
552K    FSFChangeLog.11
536K    fold-const.c

Entry points

Under gcc/:

• gcc-main.c: gcc. Immediately forwards to gcc.c driver::main.
• main.c: cc1. Immediately forwards to toplev.c toplev::main.
• collect2.c

Files

collect2

TODO vs ld?

https://gcc.gnu.org/onlinedocs/gcc-4.3.5/gccint/Collect2.html

cp

C++ specifics.

lto

Link Time Optimization.

C family

c-family

C-family includes: C, C++, Objective C.

gcc/doc/gccint.texi

Internals documentation, yay! https://gcc.gnu.org/onlinedocs/gccint/index.html

Parser location

http://stackoverflow.com/questions/2721071/c-grammar-in-gcc-source-code

In the past long ago C used an explicit grammar and Bison.

4.8 just parses manually with:

gcc/c/c-parser.c

and neighbours. A 10k LOC files.

Built files

Generated build files

gcc

xgcc

By default, xgcc is generated, and installed as gcc.

The x stands for Cross compiler.

gcc-ar

gcc-nm

gcc-ranlib

Wrappers for LTO support.

https://gcc.gnu.org/ml/gcc-help/2012-03/msg00100.html

libstdc++ location

Where did it go? Installation did install it however.

libstdc++

TODO is this the most popular C++ stdlib implementation?

Unlike glibc, it has no per-architecture optimizations. Likely glibc provides the hardcore layer, and C++ the boilerplate layer.

Register allocation

One of the hardest compiler problems, as it comes down to an NP complete graph coloring optimization.

Used to be done by the reload system, which was present from day 1, https://gcc.gnu.org/wiki/reload, but got replaced in 2012 by LRA by Red Hat: ftp://gcc.gnu.org/pub/gcc/summit/2003/Graph%20Coloring%20Register%20Allocation.pdf because reload was very unmaintainable.

On 5.1, lra.c is the main entry point, and has a nice block diagram documentation. The graph coloring algorithm is Chaitin-Briggs http://stackoverflow.com/questions/14399608/chaitin-briggs-algorithm-explanation