How can I dump an abstract syntax tree generated by gcc into a .dot file?

There are two methods, both including two steps

1. Using GCC internal vcg support

   1. Compile your code (say test.c) with vcg dumps

      gcc -fdump-tree-vcg -g test.c

   2. Use any third party tool to get dot output from vcg

      graph-easy test.c.006t.vcg --as_dot

2. Compile with raw dumps and then preprocess them with some scripts to form dot files (like in this useful article)

Both methods have their own good and bad sides -- with first you can really get only one dump of AST before gimple translation, but it is easy. With second you may convert any raw dump to dot-format, but you must support scripts, that is overhead.

What to prefer -- is on your own choice.

---

UPD: times are changing. Brand new option for gcc 4.8.2 makes it possible to generate dot files immediately. Just supply:

gcc test.c -fdump-tree-all-graph

and you will get a plenty of already formatted for you dot files:

test.c.008t.lower.dot
test.c.012t.cfg.dot
test.c.016t.ssa.dot
... etc ...

Please be sure to use new versions of GCC with this option.

According to the man page, you can get this information via -fdump- command.

Let's look at a dummy example:

// main.c

int sum(int a, int b) {
    return a + b;
}

int main(void) {
    if (sum(8, 10) < 20) {
        return -1;
    }
    return 1;
}

For gcc 7.3.0:

gcc -fdump-tree-all-graph main.c -o main

There are a lot of options to get the necessary information. Check out the manual for this info.

After that, you'll get many files. Some of them with .dot respresentation(graph option is used):

main.c.003t.original
main.c.004t.gimple
main.c.006t.omplower
...
main.c.011t.cfg
main.c.011t.cfg.dot
...

With GraphViz we can retrieve a pretty-printed graph for each function:

dot -Tpng main.c.011t.cfg.dot -o main.png

You'll get something like this: main.png

There are a lot of developer options which can help you understand how compiler process your file at a low level: GCC Developer Options