API Changes in Clang

I’ve started contributing to Clang, in the hope that I can improve the API for tooling. This will eventually mean changes to the C++ API of Clang, the CMake buildsystem, and new features in the tooling. Hopefully I’ll remember to blog about changes I make.

The Department of Redundancy Department

I’ve been implementing custom clang-tidy checks and have become quite familiar with the AST Node API. Because of my background in Qt, I was immediately disoriented by some API inconsistency. Certain API classes had both getStartLoc and getLocStart methods, as well as both getEndLoc and getLocEnd etc. The pairs of methods return the same content, so at least one set of them is redundant.

I’m used to working on stable library APIs, but Clang is different in that it offers no API stability guarantees at all. As an experiment, we staggered the introduction of new API and removal of old API. I ended up replacing the getStartLoc and getLocStart methods with getBeginLoc for consistency with other classes, and replaced getLocEnd with getEndLoc. Both old and new APIs are in the Clang 7.0.0 release, but the old APIs are already removed from Clang master. Users of the old APIs should port to the new ones at the next opportunity as described here.

Wait a minute, Where’s me dump()er?

Clang AST classes have a dump() method which is very useful for debugging. Several tools shipped with Clang are based on dumping AST nodes.

The SourceLocation type also provides a dump() method which outputs the file, line and column corresponding to a location. The problem with it though has always been that it does not include a newline at the end of the output, so the output gets lost in noise. This 2013 video tutorial shows the typical developer experience using that dump method. I’ve finally fixed that in Clang, but it did not make it into Clang 7.0.0.

In the same vein, I also added a dump() method to the SourceRange class. This prints out locations in the an angle-bracket format which shows only what changed between the beginning and end of the range.

Let it bind

When writing clang-tidy checks using AST Matchers, it is common to factor out intermediate variables for re-use or for clarity in the code.

auto valueMethod = cxxMethodDecl(hasName("value"));
Finer->addMatcher(valueMethod.bind("methodDecl"));

clang-query has an analogous way to create intermediate matcher variables, but binding to them did not work. As of my recent commit, it is possible to create matcher variables and bind them later in a matcher:

let valueMethod cxxMethodDecl(hasName("value"))
match valueMethod.bind("methodDecl")
match callExpr(callee(valueMethod.bind("methodDecl"))).bind("methodCall")

Preload your Queries

Staying on the same topic, I extended clang-query with a --preload option. This allows starting clang-query with some commands already invoked, and then continue using it as a REPL:

bash$ cat cmds.txt
let valueMethod cxxMethodDecl(hasName("value"))

bash$ clang-query --preload cmds.txt somefile.cpp
clang-query> match valueMethod.bind("methodDecl")

Match #1:

somefile.cpp:4:2: note: "methodDecl" binds here
        void value();
        ^~~~~~~~~~~~

1 match.

Previously, it was only possible to run commands from a file without also creating a REPL using the -c option. The --preload option with the REPL is useful when experimenting with matchers and having to restart clang-query regularly. This happens a lot when modifying code to examine changes to AST nodes.

Enjoy!