Updated DXR on dxr.mozilla.org

If you are a regular user of DXR, you may have noticed that the website today looks rather different from what you are used to. This is because it has finally been updated to a version dating back to mid-January, which means it also includes many of the changes developed over the course of the last summer, previously visible only on the development snapshot (which didn't update mozilla-central versions). In the coming months, I also plan to expand the repertoire of indexed repositories from one to two by including comm-central. Other changes that are currently being developed include a documentation output feature for DXR as well as an indexer that will grok our JS half of the codebase.

DXR's future potential

The original purpose of DXR, back when the "D" in its name wasn't a historical artifact, was to provide a replacement for MXR that grokked Mozilla's source code a lot better. In the intervening years, DXR has become a lot better at being an MXR replacement, so I think that perhaps it is worth thinking about ways that DXR can start going above and beyond MXR—beyond just letting you searched for things like "derived" and "calls" relationships.

The #ifdef problem

In discussing DXR at the 2011 LLVM Developers' Conference, perhaps the most common question I had was asking what it did about the #ifdef problem: how does it handle code present in the source files but excluded via conditional compilation constructs? The answer then, as it is now, was "nothing:" at present, it pretends that code not compiled doesn't really exist beyond some weak attempts to lex it for the purposes of syntax highlighting. One item that has been very low priority for several years was an idea to fix this issue by essentially building the code in all of its variations and merging the resulting database to produce a more complete picture of the code. I don't think it's a hard problem at all, but rather just an engineering concern that needs a lot of little details to be worked out, which makes it impractical to implement while the codebase is undergoing flux.

Documentation

Documentation is an intractable unsolved problem that makes me wonder why I bring it up here…oh wait, it's not. Still, from the poor quality of most documentation tools out there when it comes to grokking very large codebases (Doxygen, I'm looking at you), it's a wonder that no one has built a better one. Clang added a feature that lets it associate comments to AST elements, which means that DXR has all the information it needs to be able to build documentation from our in-tree documentation. With complete knowledge of the codebase and a C++ parser that won't get confused by macros, we have all the information we need to be able to make good documentation, and we also have a very good place to list all of this documentation.

Indexing dynamic languages

Here is where things get really hard. A language like Java or C# is very easy to index: every variable is statically typed and named, and fully-qualified names are generally sufficient for global uniqueness. C-based languages lose the last bit, since nothing enforces global uniqueness of type names. C++ templates are effectively another programming language that relies on duck-typing. However, that typing is still static and can probably be solved with some clever naming and UI; dynamic languages like JavaScript or Python make accurately finding the types of variables difficult to impossible.

Assigning static types to dynamic typing is a task I've given some thought to. The advantage in a tool like DXR is that we can afford to be marginally less accurate in typing in trade for precision. An example of such an inaccuracy would be ignoring what happens with JavaScript's eval function. Inaccuracies here could be thought of as inaccuracies resulting from a type-unsafe language (much like any C-based callgraph information is almost necessarily inaccurate due to problems inherent to pointer alias analysis). The actual underlying algorithms for recovering types appear known and documented in academic literature, so I don't think that actually doing this is theoretically hard. On the other hand, those are very famous last words…

2011 LLVM Developers' Meeting

This feels a bit late to be talking about the 2011 LLVM Developers' Meeting (seeing as how it happened almost exactly a month ago), but since the slides have been put up over the past week and the talks were only put up on Youtube this week, I suppose I can finally back notes up with links and not talk so abstractly.

Of the talks I went to, I think by far the most interesting one was Doug Gregor's talk on Extending Clang. It covered various extension points in Clang and some of their capabilities with simple examples. It also ended in what might be impolitely titled "Where Extending Clang Sucks" and what was politely titled "Help Wanted." This boils down to "plugins are hard to use" and "one-off rewriters are hard to write". Indeed, I think the refrain of problematic architecture for further tools was repeated in more than a few presentations: Clang has the information you want and need, but squeezing the information out is inordinately difficult.

What was probably the most popular talk was Chandler Carruth's talk on Clang MapReduce—Automatic C++ Refactoring at Google Scale (his slides appear to not yet be posted). From what I recall, the more interesting parts of the talk are closer to the end. He had a discussion on developing a language for semantic queries to identify code that needs to be replaced (the example query was essentially "find all calls to Foo::get()"); the previous things people have tried are regexes (C++ is not syntactically regular, it's recursively enumerable), XPath (unfortunately, ASTs, despite their name, aren't exactly tree structures), and pattern matching ASTs (not always sufficient textual clues). The team's idea was to use a matcher library on "AST" values as predicates. He also spoke a bit about some of the efforts they did on using Clang with Chromium.

There is a point brought up in Chandler's talk that I want to reiterate. One of the problems with writing refactoring tools (or static analysis tools in general) is in getting people to trust that they work. For any sufficiently large codebase—millions of lines of code or more—it is fairly certain that the project will run up against the nasty edge cases in parsing tools. If a tool is intended to run mostly automated analysis on such code, it is impossible for anyone to be able to look at the output and ensure complete correctness. However, most sufficiently large codebases come with massive testsuites to help people believe their code is correct; if the same parser is capable of producing code that passes the entire testsuite, then one only needs to trust that the analysis is done correctly, a much smaller task. Without such a capability, no one would be willing to trust the analysis; in other words, any parser which is not capable of then generating code is never going to be trusted as a basis for further work, at least, not if you are looking at million-line projects.

Speaking of Chromium and Clang, there was talk on this too. As I am sure most of my audience is well aware, Chromium uses Clang for several of its buildbots (and all of its recent Mac development); I wish Mozilla could get Clang to be a tier 2 or tier 1 platform for Mac and Linux. As for why they prefer it, the brief rundown is this: better diagnostics, faster, smaller object sizes, they can write a style checker, and they can build better tools off of it (like AddressSanitizer, which, unsurprisingly, itself had a talk). Again, there were the complaints: building the rewriter proved to be difficult a task (notice a trend here?). Incidentally, I also attended the AddressSanitizer talk, but I'm getting tired of copying down notes of talks, so I'll let those slides and the video speak for themselves.

Finally, I did give a talk on DXR. It seemed to be well-received; I had a few people coming up to me later in the day thanking me for the talk and asking more questions about DXR. Something I did discover when giving it, though, is just how difficult giving a talk really is. I had specific notes on the slides written in my notebook, only to discover that I wasn't able to gracefully retreat to the podium and read the notes long enough to figure out what to say next. If you're wondering what I forgot to mention in the talk, it's mostly a more coherent explanation during the undead demo.

Not-so-random mozilla-central factoids

Back when I was looking at reducing disk usage of running DXR, I used the sizes of the generated CSV files to make a rough estimate of how many times the average line of mozilla-central needs to be parsed and compiled (the answer is around 20). Now, having just respun a build of mozilla-central with the newest version of DXR, I have a larger database with some fairly accurate statistics of its size. This all started when I wondered what our most common function name was, so I decided to make a list of factoids (it's also a chance to refresh my SQL memory). All of the statistics come from a build of Firefox today on Linux x86-64 with debug and tests disabled.

Notes on accuracy

DXR tries as hard as possible to correlate data back to the original source code, and it is very likely that it can get itself confused in some weird circumstances. I don't yet have a good idea of where all of the buggy cases are, but I am aware of some broad strokes. Pretty much all information that deals with references is likely missing large swathes of the codebase. Information about callers only counts explicit calls (i.e, call expressions). The most accurate data I have is the macro information and the least accurate is information involving a templated class in almost any fashion. I also suspect that scope information is malformed in a non-empty set of cases, and I'm pretty sure that the number of global objects in any count is overcounted.

Type statistics

I count in mozilla-central just 33,555 distinct types. Of these, we have 13,648 typedefs, 6,523 structs, 6,470 classes, 5,147 enums, 1,410 interfaces, and 357 unions. Of all of these, 13,740 types are nested in another in some fashion, and another 4,524 types are templated. I'm not counting separate template instantiations as new types, but I am counting specializations individually.

Then there's the inheritance. I found 7,715 direct inheritance relations, all but a handful of which (around 200) are public. These relations account for 2,317 distinct base classes and 6,157 distinct subclasses. Naturally, some types are inherited much more than others. The winner, by a factor of 6, is nsISupports, having a whopping 3,156 implementations. Pickle and IPC::Message tie for second place with 501 implementations each; nsIRunnable takes 4th place at 282 implementations. The 5th place goes to nsXPCOMCycleParticipant (242). Rounding out the top 10 are nsIDOMElement (226), nsRunnable (224), nsScriptObjectTracer (209), nsSupportsWeakReference (154), and nsIObserver with 144. Subclass relationships involving templates are not counted, which may bump a few classes up into this list.

Macros

There are 42,457 distinct definitions of macros to produce 38,045 distinct names. Of these, 30,475 look like variables and 7,647 look like functions, which implies that 77 macros take on both depending on how they got defined.

How about calling them? I count just 482,142 macro invocations, so each macro is being invoked about 12 times on average. But... 20,628 of our macros are never used (or almost 48.6% of them), so the average is closer to 25 times. Of course, some macros really get used. Here are the top 5:

Count	Macro name
23,109	nsnull
22,941	PR_FALSE
20,139	NS_OK
13,154	NS_IMETHOD
13,138	NS_IMETHODIMP

Functions

There are 137,903 functions in mozilla-central. Of these, there are 68,867 having a distinct name. Of these, I found 33,693 in the global scope, and 8,943 that were a member of a class. Directly templated functions comprise about 2,291 functions. Just for fun, I found that there are 774 functions named exactly "Init" and a further 1,681 that begin with "Init" (case-insensitively in the last case).

In terms of calling these functions, I found 291,598 distinct edges in the callgraph (this is definitely an underestimate, since I am missing a large number of cases). For my usage, a callgraph is not a traditional directed graph but rather a hypergraph, where each edge goes from a single head to a set of nodes in the tail. These comprise 85,144 distinct callers and 67,098 distinct targets. Of the targets, I found 51,590 distinct functions being called statically, 13,274 distinct virtual functions invoked, and 2,234 distinct function pointers or pointers-to-member-functions being called. If I break it up by calls, 246,086 of the calls are static function calls, 41,770 virtual function calls, and 3,742 function pointer calls. I want to emphasize here that information pertaining to templates, in particular nsCOMPtr is completely missing, so a lot of calls to nsISupport's methods are missing, which is going to horribly skew the statistics.

Counting the function pointers, we have 65 pointers-to-member-functions and around 1700-1800 function pointers (those numbers do not add up to what I should get above, but I'm not sure who's in error here). I count about 19,721 virtual functions that I generated target information for (a subset of all virtual functions), and 65,761 implementations of those virtual functions, so the average virtual function has about 3.4 implementations. In addition, I found about 981 of these virtual functions were also called statically.

Now I'm sure, having mentioned it earlier, that you too are now wondering what the most common function name in mozilla-central is. The answer should be pretty obvious when you consider the most heavily-implemented class. And the winners are...

Count	Function name
1,687	Release
1,680	AddRef
1,600	GetIID
1,587	QueryInterface
774	Init
659	operator=
549	Log
517	Read
506	Write
396	GetType

The top 4 methods are related to XPCOM; the famed Init method is a mere 5th place. Of interesting note is that there are 659 assignment operators; I'm guessing some of these may be default copy constructors implemented for non-POD classes.

Variables

There's not much to say here. We have some 623,237 variables of some kind. Most of these, naturally, are local variables or parameters: 516,627, to be precise. We additionally have 82,008 members of some compound type, and 24,602 global variables. Some interesting statistics would be to compute the number of variables whose names defy our naming convention or the number of static constructors that need to be run before startup, but that data is harder to compute.

Warnings

I count 5,283 reported warnings for mozilla-central. Of these, 1,608 are warning about the use of non-virtual destructors with virtual functions. Another 940 warn about our use of mismatched enumeration types. There are 1,348 warnings of unused things. Finally, there are 1,551 warnings about use of extensions, leaving 466 "miscellaneous warnings".

Build statistics

The final set of statistics I have is mere size statistics for the build information. There are about 8,581,746 non-empty lines of text comprising some 320MiB of data. These are organized into about 51,469 files, including 1,469 files of generated files in the build directory. My output SQLite file was 464MiB, easily comprising around 3.5 million rows of data. We also have about 38MiB of binary files (like PNGs) in the source tree. Almost 15MiB of the generated included files were produced, comprising around 354,893 non-empty lines of text.

I think the best way to summarize this data is "mozilla-central is a massive codebase." It also goes to show you why just looking at source code without compiling is a bad idea: around 3-5% of our source code actually isn't in the source tree to begin with but is instead automatically created at compile time.

Callgraph

There remains, as of this blog post, one major regression from the original dehydra-based DXR, and that is the lack of support for callgraph. The main problem I have here is trying to pin down the scope of the feature. To give an idea of why this hard, I pose the following challenge: how many different ways are there in C++ of invoking a function, as would be viewed from the generated assembly code? Here is my list:

Global function

Invocation of non-virtual member

Nonvirtual invocation of virtual member

Constructor, copy constructor invocations

Implicit destructor invocation

Functors (objects that implement operator())

C++0x lambdas (which appears to be a functor)

Count all of these as "one," if you like, since they're all still more or less boil down to a call addr instruction, so it really boils down to how pedantic you get about the differences. Most of these look fairly distinct in source code level as well too. But, on the plus side, being a statically-known function call, all of these are easy to handle.

Virtual member function invoked virtually

This is clearly different at an assembly level, since it requires a vtable lookup with the added possibility of thunking. From a handling perspective, though, this is more or less statically known, since we know the possible targets of a virtual method call are limited to subclasses of the method, if we assume that people aren't sneaky.

Function pointers

Easy to dip into, impossible to think about off the top of your head (how many tries does it take you to write the type of a function pointer that returns a function pointer without using typedefs?), and it requires data-flow analysis to solve. The main question is if it's even worth trying to at least plan to support function pointers, or if they should just be left out entirely.

Pointer-to-member function pointers

Theoretically easier than function pointers (because we again have a more limited state space), but they still suffer from the same need to data flow analysis. Not to mention that they are much harder to use, and that their implementations are much different from function pointers.

Template dependent name resolution

Templates, as far as C++ is concerned, is essentially a macro system that is more powerful and more inane than the C preprocessor. So if the expression that is to be called is dependent on the type parameter, then the function call can be any (or all) of the above depending on what was passed in.

throw

The entire exception handling semantics in C++ essentially gets handled by the ABI as a combination of magic function calls and magic data sections; undoubtedly a throw itself is a function call which gets to do stack unwinding. Most of this stuff isn't exposed in the code, it just leaks out in semantics.

One goal I've had in mind is to make sure I can do some reasonable support for dynamic languages as well as static languages. To that end, I've looked up other callgraph implementations to see what they do in the realm of indirect function calls. The answer is a mixture of either "do runtime instrumentation to build the callgraph" or "ignore them altogether." The closest I've seen is one implementation that indicated when the address of a function was taken.

Viewsource up and running

This morning, I have made available a public version of the viewsource code. Viewsource was originally a web tool to output a pretty-printed version of the Dehydra objects for simple snippets of source code. Since I found it a useful idea for debugging, I also added dumps for the now-defunct Pork rewriting toolset, JSHydra, and now, finally Clang.

The odd tool out here is clang, as it has neither a debug-useful XML output nor a simple JS representation that can report the output, which requires me to make the tool a two-stage process: the first stage is a compiler plugin that figures out from clang::RecursiveASTVisitor which functions and classes it needs to worry about dumping out, and then writes out the code for the second stage, another compiler plugin that actually dumps the generated AST information to the console. In other words, I have a plugin to write a plugin that is used to dump information to write plugins.

Eventually, I hope to be more complete in the information I can dump out (particularly type locations and type information), but this is complete enough to be at least somewhat useful for finding interesting things, such as understanding what the location information actually refers to. As I work on this tool more and more, this should enable me to find and fix many bugs in DXR.

DXR alpha 3 release

I am pleased to announce the third release of DXR. Compared to the previous release, the UI has been tweaked and several bugs have been noticed and fixed. In particular, I have improved the support of links, and fixed the bug that prevented the type hierarchy from being properly realized. I have also introduced support for indexing the IDL code in Mozilla's codebase, as well as indexing generated code in general. I have also added yet another tree, comm-central.

Compared to the original implementation of DXR by Dave Humphrey, there remains one unimplemented feature, namely support for the callgraph. My plan for the next release will focus on implementing this feature as well as any other bugs I uncover via real-world testing. The other major features for the next release will be a JSON query API for data and improved support for multiple trees.

In a more general viewpoint, the plugin architecture has been much improved. There is now support for viewing code-coverage data in the source tree (I do not have a visible demo of this yet), and the support for IDL is entirely a separate plugin. Implementing these plugins has caused me to realize that there is need for better hooks in the current architecture (especially with regards to the generated HTML), and these will also be forthcoming in the next release.

The final point to make is that I have moved the viewsource code into its own repository, as well as adding support for a limited subset of the clang AST. The website does not properly support clang at this time owing to problems in the server's current toolchain configuration; when these issues are resolved, I will be demonstating the tool better.

DXR alpha 2 release

I am pleased to announce a second alpha release of DXR. Several improvements have been made, including changes to the UI as well as better cooperation with macros. A complete list of changes can be found at the end. As this is the second release, I have expanded the live demo to include not one but two separate indexing trees: clang and mozilla-central, both of which are current as of last night.

The advantage of DXR is that it sees the source code by instrumenting the compiler at build time, so it is able to understand classes most of whose methods are defined by macros, like Clang's RecursiveASTVisitor. Like any source code browser, it is possible to click on an identifier and then go see information about the identifier, such as its points of declaration, or its definition. Unlike LXR and MXR, however, it knows what type of identifier it's working with, so you won't be offered the definition of BuiltinType::getKind if you look up the getKind method of an Expr object.

Another new feature is support for plugins, to allow people to use different compilers, add in support for more languages, or even augment the UI with new features like capturing line-coverage information. The current support is rudimentary, but there is already active work on a dehydra-based indexer.

A list of new features:

• Improved search speed at runtime, and dropped glimpse as a prerequisite
• The web UI works in newer versions of Firefox
• Support for plugins has been added
• Disk space requirements have been radically reduced
• Setup configuration is easier
• Link extents have been fixed in many cases
• Declarations can now be seen along with definitions
• Macros can now be browsed like source code

Mozilla-central and DXR

Some of you may be wondering why it is taking me so long to build a DXR index of mozilla-central. Part of the answer is that I'm waiting to bring DXR on clang back up to the original instantiation in terms of feature support. The other part of the answer is that mozilla-central is rather big to index. How big? Well, some statistics await…

I first noticed a problem when I was compiling and found myself at 4 GiB of space remaining and dwindling fast. I'm used to fairly tight space restrictions (a result of malportioning the partitions on my last laptop), so that's not normally a source of concern. Except I had been at 10 GiB of free space an hour before that, before I started the run to index mozilla-central.

The original indexing algorithm is simple: for each translation unit (i.e., C++ file), I produce a corresponding CSV file of the "interesting" things I find in that translation unit—which includes all of the header files included. As a result, I emit the data for every header file each time it is included by some compiled file. The total size of all data files produced by the mozilla-central run turned out to be around 12GiB. When I collected all of the data and removed duplicate lines, the total size turned out to be about 573MiB.

Step back and think about what this means for a moment. Since "interesting" things to DXR basically boil down to all warnings, declarations, definitions, and references (macros and templates underrepresented), this implies that every declaration, definition, and reference is parsed by the compiler, on average, about 20-25 times. Or, if you take this as a proxy for the "interesting" lines of code, the compiler must read every line of code in mozilla-central about 20-25 times.

The solution for indexing is obviously not to output that much data in the first place. Since everyone who includes, say, nscore.h does so in the same way, there's no reason to reoutput its data in all of the thousands of files that end up including it. However, a file like nsTString.h (for the uninitiated, this is part of the standard internal string code interfaces, implemented using C's versions of templates, more commonly known as "the preprocessor") can be included multiple times but produce different results: one for nsString and one for nsCString in the same place. Also, there is the question of making it work even when people compile with -jN, since I have 4 cores that are begging for the work.

It was Taras who thought up the solution. What we do is we separate out all of the CSV data by the file that it comes in. Then, we store each of the CSV data in a separate file whose name is a function of both the file it comes in and its contents (actually, it's <file>.<sha1(contents)>.csv, for the curious). This also solves the problem of multiple compilers trying to write the same file at the same time: if we open with O_CREAT | O_EXCL and the open fails because someone else created the file… we don't need to do anything because the person who opens the file will write the same data we wanted to write! Applying this technique brings the total generated CSV file data down to around 1GiB (declaration/definition mappings account for the need for duplicates), or down to about 2 times the real data size instead of 20 times. Hence why the commit message for fixing this is titled Generate much less data. MUCH LESS.

Of course, that doesn't solve all of my problems. I still have several hundred MiBs of real data that need to be processed and turned into the SQL database and HTML files. Consuming the data in python requires a steady state of about 3-3.5 GiB of resident memory, which is a problem for me since I stupidly gave my VM only 4GiB of memory and no swap. Switching the blob serialization from using cPickle (which tries to keep track of duplicate references) to marshal (which doesn't) allowed me to postpone crashing until I generate the SQL database, where SQL allocates just enough memory to push me over the edge, despite generating all of the SQL statements using iterators (knowing that duplicate processing is handled more implicitly). I also switched from using multiple processes to using multiple threads to avoid Linux failing to fork Python due to not enough memory (shouldn't it only need to copy-on-write?).

Obviously, I need to do more work on shrinking the memory usage. I stripped out the use of the SQL for HTML generation and achieved phenomenal speedups (I swore that I had to have broken something since it went from a minute to near-instantaneous). I probably need to move to a light-weight database solution, for example, LevelDB, but I don't quite have a simple (key, value) situation but a (file, plugin, table, key, value) one. Still not hard to do, but more than I want to test for a first pass.

In short, DXR indexing for mozilla-central is being recreated as I write this. Enjoy.

Alpha release of dxr

I am pleased to announce an alpha release of DXR built on top of Clang. A live demo of DXR can be found at http://dxr.mozilla.org/clang/, which is an index of a relatively recent copy of the Clang source code. Since this is merely an alpha release, expect to find bugs and inconsistencies in the output. For more information, you can go to #static on irc.mozilla.org or contact the static analysis mailing list. A list of most of the bugs I am aware of is at the end of this blog post.

So what is DXR? DXR is a smart code browser that works by using instrumented compilers to use what the compiler knows about the code to provide a database of the code. For C and C++ in particular, using an instrumented compiler is necessary, since it is the only reliable way to fix the issue of macros. Take, for instance, RecursiveASTVistor in the Clang codebase. Most of the almost 1200 functions are defined via macros as opposed to in raw code; as a consequence, the doxygen output for this class is useless: as far as I can tell, there are only five methods I can override to visit AST nodes. On the other hand, DXR neatly tells me all of the methods that are defined, and can point me to the place where that function is defined (within the macro, of course).

Where can you get the code? DXR is available both as a github repository (use the dxr-clang branch) and as a Mercurial repository. Instructions on how to use can be found on the wiki page.

The following is a list of known problems:

• Links occur at odd boundaries
• Some lines have id="l234"/> prepended
• Non-root installs (i.e., installing to http://dxr.mozilla.org/clang/) cause issues. Interestingly, refreshing the page often causes things to work.
• There is a long list of scrolling text when compiling code. Ignore it.
• HTML generation produces IndexErrors
• .csv files are created in the source directory and HTML code is generated.
• Inheritance searches don't match the full hierarchy, only one or two levels.

DXR updates

It's been almost a week since I last discussed DXR, but, if you look at the commit log, you can see that I've not exactly been laying back and doing nothing. No, the main reason is because most of the changes I've been doing so far aren't exactly ground-breaking.

In terms of UI, the only thing that's changed is that I know actually link the variable declarations correctly, a result of discovering the typo that was causing it not to work properly in the midst of the other changes. From a user's perspective, I've also radically altered the invocation of DXR. Whereas before it involved a long series of steps consisting of "build here, export these variables, run this script, build there, run that script, then run this file, and I hope you did everything correctly otherwise you'll wait half an hour to find you didn't" (most of which takes place in different directories), it's now down to . dxr/setup-env.sh (if you invoke that incorrectly, you get a loud error message telling you how to do it correctly. Unfortunately, the nature of shells prohibits me from just doing the right thing in the first place), build your code, and then python dxr/dxr-index.py in the directory with the proper dxr.config. Vastly improved, then. But most of my changes are merely in cleaning up the code.

The first major change I made was replacing the libclang API with a real clang plugin. There's only one thing I've found harder (figuring out inheritance, not that I'm sure I've had it right in the first place), and there are a lot of things I've found easier. Given how crazy people get with build systems, latching onto a compiler makes things go a lot smoother. The biggest downside I've seen with clang is that it's documentation is lacking. RecursiveASTVisitor is the main visitor API I'm using, but the documentation for that class is complete and utter crap, a result of doxygen failing to handle the hurdle of comprehending macros. I ended up using the libindex-based dxr to dump a database of all of the functions in the class, of which there are something like 1289, or around 60 times the functions listed in the documentation. Another example is Decl, where the inheritance picture is actually helpful. It, however, manages to have failed to document DeclCXX.h, which is a rather glaring omission if what you are working with is C++ source.

The last set of changes I did was rearchitecting the code to make it hackable by other people. I have started on a basic pluggable architecture for actually implementing new language modules, although most of the information is still hardcoded to just use cxx-clang. In addition, I've begun work on ripping out SQL as the exchange medium of choice: the sidebar list is now directly generated using the post-processing ball of information, and linkification is now set up so that it can be generated independent of tokenization. In the process, I've greatly sped up HTML generation times only to regress a lot of it by tokenizing the file twice (the latter part will unfortunately remain until I get around to changing tokenization API). It shouldn't take that much longer for me to rip SQL fully out of the HTML builder and shove SQL generation into a parallel process for improved end-to-end time.

Some things I've discovered about python along the way. First, python closures don't let you mutate closed-over-variables. That breaks things. Second, if you have a very large python object, don't pass it around as an argument to multiprocessing: just make it a global and let the kernel's copy-on-write code make cloning cheap. Third, apparently the fastest way to concatenate strings in python is to use join with a list comprehension. Finally, python dynamic module importing sucks big time.

Where am I going from here? I'll have HTMLification (i.e., remove SQL queries and replace with ball lookups) fixed up tomorrow; after that, I'll make the cgi scripts work again. The next major change after that is getting inheritance and references in good shape, and then making sure that I can do namespaces correctly. At that point in time, I'll think I'll be able to make a prerelease of DXR.

DXR thinking aloud

A primary goal of DXR is to be easy to use. Like any simple statement, translating that into design decisions is inordinately difficult, and I best approach such issues by thinking out loud. Normally, I do this in IRC channels, but today I think it would be best to think in a louder medium, since the problem is harder.

There are three distinct things that need to be made "easy to use". The first is the generation of the database and the subsequent creation of the web interface. The second is extension of DXR to new languages, while the last is the customization of DXR to provide more information. All of them have significant issues.

Building with DXR

Starting with the first one, build systems are complicated. For a simple GNU-ish utility, ./configure && make is a functioning build system. But where DXR is most useful is on the large, hydra-like projects where figuring out how to build the program is itself a nightmare: Mozilla, OpenJDK, Eclipse, etc. There is also a substantial number of non-autoconf based systems which throw great wrenches in everything. At the very least, I know this much about DXR: I need to set environment variables before configuring and building (i.e., replace the compiler), I need to "watch" the build process (i.e., follow warning spew), and I need to do things after the build finishes (post-processing). Potential options:

1. Tell the user to run this command before the build and after the build. On the plus side, this means that DXR needs to know absolutely nothing about how the build system works. On the down side, this requires confusing instructions: in particular, since I'm setting environment variables, the user has to specifically type ". " in the shell to get them set up properly. There are a lot of people who do not have significant shell exposure to actually understand why that is necessary, and general usage is different enough from the commands that people are liable to make mistakes doing so.
2. Guess what the build system looks like and try to do it all by ourselves. This is pretty much the opposite extreme, in that it foists all the work on DXR. If your program is "normal", this won't be a problem. If your program isn't... it will be a world of pain. Take also into consideration that any automated approach is likely to fail hard on Mozilla code to begin with, which effectively makes this a non-starter.
3. Have the user input their build system to a configuration file and go from there. A step down from the previous item, but it increases the need for configuration files.
4. Have DXR spawn a shell for the build system. Intriguing, solves some problems but causes others.

Conclusion: well, I don't like any of those options. While the goal of essentially being able to "click" DXR and have it Just Work™ is nice, I have reservations about such an approach being able to work in practice. I think I'll go for a "#1 and punt on this issue to someone with more experience."

Multiple language

I could devote an entire week's worth of blog posts to this topic, I think, and I would wager that this is more complicated and nebulous than even build systems are. In the end, all we really need to worry about with build systems is replacing compilers with our versions and getting to the end; with languages, we actually need to be very introspective and invasive to do our job.

Probably the best place to start is actually laying out what needs to be done. If the end goal is to produce the source viewer, then we need to at least be able to do syntax highlighting. That by itself is difficult, but people have done it before: I think my gut preference at this point is to basically ask authors of DXR plugins to expose something akin to vim's syntax highlighting instead of asking them to write a full lexer for their language.

On the other end of the spectrum is generating the database. The idea is to use an instrumenting compiler, but while that works for C++ or Java, someone whose primary code is a website in Perl or a large Python utility has a hard time writing a compiler. Perhaps the best option here is just parsing the source code when we walk the tree. There is also the question about what to do with the build system: surely people might want help understanding what it is their Makefile.in is really doing.

So what does the database look like? For standard programming languages, we appear to have a wide-ranging and clear notion of types/classes, functions, and variables, with slightly vaguer notions of inheritance, macros (in both the lexical preprocessing sense and the type-based sense of C++'s templates), and visibility. Dynamic languages like JavaScript or Python might lack some reliable information (e.g., variables don't have types, although people often still act as if they have implicit type information), but they still uphold this general contract. If you consider instead things like CSS and HTML or Makefiles in the build system, this general scheme completely fails to hold, but you can still desire information in the database: for example, it would help to be able to pinpoint which CSS rules apply to a given HTML element.

This begs the question, how does one handle multiple languages in the database? As I ponder this, I realize that there are multiple domains of knowledge: what is available in one language is not necessarily available in another. Of the languages Mozilla uses, C, assembly, C++, and Objective-C[++] all share the same ability to access any information written in the other languages; contrast this to JS code, which can only interact with native code via the use of a subset of IDL or interactions with native functions. IDL is a third space, which is a middle ground between native and JS code, but is insufficiently compatible with either to be lumped in with one. Options:

1. Dump each language into the same tables with no distinction. This has problems in so far as some languages can't be shoehorned into the same models, but I think that in such cases, one is probably looking for different enough information anyways that it doesn't matter. The advantage of this is that searching for an identifier will bring it up everywhere. The disadvantage... is that it gets brought up everywhere.
2. Similar to #1, but make an extra column for language, and let people filter by language.
3. Going a step further, take the extra language information and build up the notion of different bindings: this foo.bar method on a python object may be implemented by this Python_foo_bar C binding. In other words, add another table which lists this cross-pollination and takes it into account when searching or providing detailed information<.
4. Instead of the language column in tables, make different tables for every language.
5. Instead of tables, use databases?

Hmm. I think the binding cross-reference is important. On closer thought, it's not really languages themselves that are the issue here, it's essentially the target bindings: if we have a system that is doing non-trivial build system work that involves cross-compiling, it matters if what we are doing is being done for the host or being done for the target. Apart from that, I think right now that the best approach is to have different tables.

Extraneous information

The previous discussion bleeds into this final one, since they both ultimately concern themselves with one thing: the database. This time, the question is how to handle generation of information beyond the "standard" set of information. Information, as I see it, comes in a few forms. There is additional information at the granularity of identifiers (this function consumes 234 bytes of space or this is the documentation for the function), lines (this line was not executed in the test suite), files (this file gets compiled to this binary library), and arguably directories or other concepts not totally mappable to constructs in the source tree (e.g., output libraries).

The main question here is not on the design of the database: it's only a question of extra tables or extra columns (or both!). Instead, the real question is in the design of the programmatic mechanisms. In dxr+dehydra, the simple answer is to load multiple scripts. For dxr+clang, however, the question becomes a lot more difficult since the code is written in C++ and isn't dynamically loading modules like dehydra does. It also begins to beg the question of the exposed API. On the other hand, I'm not sure I know enough of the problem space to be able to actually come up with solutions. I think I'll leave this one for later

Building with dxr-clang

As I'm building yet another program from scratch (I've now run dxr on four separate trees in the past week, and I'm still looking for a good one to use for tests), let me take the time to explain how to set up DXR on your own computer. Instead of stuffing it in this blogpost, most of the information is instead on wiki.mozilla.org. One thing not on that page is that I currently have the sql files in the clang dumper dump to the source directory instead of the object directory, so the DXR config needs to have the source and object directory be the same setting, even if you don't actually build the source in-place!

clang-DXR

As mentioned previously, the first thing I started doing was getting DXR to use libclang to build the indexing. As of right now, I have a tool which kind of works. In that it can successfully build a program but utterly fails to produce reliable SQL output due to constantly swallowed segfaults (I think). My test program uses cmake to build, so it "helpfully" hides the noise of what is actually going on, so debugging will have to wait until next week.

The main unexpected hurdle is that for all of the wonderful praise clang has had lavished on it, I have found it surprisingly difficult to use. I had expected to be able to do a more or less straightforward port of dxr.js, with some cosmetic changes to handle differences in naming, etc., between dehydra and libclang. That expectation is now hopelessly gone; libclang provides nothing near the clean API that dehydra does. For example, to get the fully-qualified name of a class, instead of being able to say type.name, I have to walk up the entire AST hierarchy of the class to find all of its containing classes and namespaces to do it myself. If I want to get one for a function (which, in C++, requires differentiating between overloads, so I need type parameters), I have to build my own type-to-string dumper. Indeed, no less than half of my code so far is merely trying to build the type names I need to dump out. In addition, it seems that the clang people are dismissive of an idea to include a higher level API than is currently present.

If you didn't notice from the links, by the way, dxr is now on github.

Summer DXR work

So, far this summer, I will be mostly putting away Thunderbird work and will instead be focusing my attention on reviving DXR. Sometime this week, I expect to be updating the installation on dxr.mozilla.org (simply so I can match up better on DXR to what I'm doing locally).

What will I be doing on DXR? First and foremost, I will be making it easier to run DXR on anything that is not Mozilla. Or anything that is Mozilla, for that matter. This will be done by cleanly separating the build/instrumentation steps from the rest of DXR. In other words, setting CC/CXX/LD/etc. should be all you need to do build your program.

After that, I will be working on using Clang to get information instead of using gcc+dehydra. If I understand the current Apple development process, supporting clang is essential to supporting Mac OS X builds in the future. I will also work on getting IDL and JS support added.

One other thing I want to do is (time permitting) to get DXR to represent the results of different builds, so that you can get results other than just the current Linux build. I have a list of other ideas to play around with, but I'll leave that for later, since I don't know if I'll time to start them or not.

Don't expect dxr.mozilla.org to see all of the new features immediately. I'll update when I feel like it (i.e., I see enough stability to do it), and it's easier for me to work more locally on my laptop. I also find it easier to do rapid updates on a smaller project than Mozilla which takes quite some time to do clobber rebuilds; I don't know which project(s) I'll be using for all of my work yet, so don't bother asking.