A new JS MIME parser

Here is one of those guessing games whose answers will depress you: how many (partial) MIME parsers do we have in Thunderbird? If you guessed zero, ha-ha, very funny; now go sit in the corner. If you guessed one, you've obviously never worked with libmime before. The actual answer depends on how liberal you want to be. On the conservative end, there are no fewer than 5 parsers which go at least as far as parsing multipart messages (including two in a single file). If you choose to go liberally, counting everybody who attempts to split header values to look for a specific value, you gain an additional six that I am aware of… and I have no doubt that more are lurking behind them. I suppose this means that we now have more implementations of message header parsing than we do base64-decoding (although nowadays, it seems like most base64-decoding got stuffed into one method with several wrappers). The complete list as I know it:

• nsMsgBodyHandler
• nsMsgDBFolder::GetMsgTextFromStream
• libmime
• IMAP fakeserver (one for body parts, one for body structure, and another spot happily hunts down headers)
• NNTP fakeserver (hunts down headers)
• nsParseMailbox
• nsNNTPProtocol (hunts down headers)
• nsMsgLocalStoreUtils (hunts down headers)
• Somewhere in compose code. Probably, although it's not clear how much is being funneled back to libmime and how much is not.
• A class in necko implements in the RFC 2231 and RFC 2047 decoding.

Well, it's time for to increment that count by one. And then decrement it by at least three (two of the IMAP and the NNTP fakeserver decoders are switched over in a queued patch, and I hope to get the third IMAP fakeserver switched over). I've been working on a new JS MIME parser to Thunderbird for a bit at a time over the past several months, and I have local patches that start using it in tests (and as a replacement for nsIMimeHeaders).

So, why replace libmime instead of consolidating everyone onto it? The short answer is because libmime is a festering pile of crap. It uses an internal object system that can be summarized as "reimplementing C++ in C" and has existed largely in the same form since at least Netscape 5 (the Mozilla classic version gives a date of May 15, 1996). All of that would be manageable were it not for the fact that the architecture is clearly broken. Reliably parsing multipart MIME messages is not a trivial task (indeed, not only does one of the above do it incorrectly, but there is actually a test which may rely on it being wrong. Guess which one it is.), and the fact that so many people have variants on it is a clear indication that the API fails to expose what it ought to expose. This means that the code is in need of change, and the implementation is a form which makes changing things extremely difficult.

The choice to do it in JS was motivated mostly by Andrew Sutherland. There has been a long-term ideal in Thunderbird dating back to at least around 2008 to move more implementation of core code into JS, which would help avoid massive churn spurred on by mozilla-central; nowadays, there is the added benefit that it would aid in efforts like B2G or porting to platforms where native code is frowned upon. MIME code, being extremely self-contained (charset conversion and S/MIME encryption make up the biggest dependencies in the core parser). As of my current implementation, the only external dependency that the MIME parser has is atob, although charset conversion (via the proposed string encoding API) will be added when I get there. In other words, this code is usable by anyone who wants to write a mail client in JS, not just the internal mailnews code.

Another advantage to writing my own library is that it gives me a lot of time to lay out specifications in clearer terms. One called out in the spec are on how to handle seeing boundaries for the non-leaf-most part. My own brain went further and started musing on non-leaf parts getting QP or base64 content-transfer-encodings (which RFC 6532 went ahead and allowed anyways), or multiple nested parts having the same boundary (which the specification, in my view, hints at resolving in a particular fashion). Other developments include the fact that most MIME parsers do not permit as much CFWS as the specification indicates could be present ("Content-Type: multipart / mixed" would be unusable in every MIME parser source I read)I also produced a list of all the specifications that the parser will need to refer to that I have found so far (13 RFCs and a handful of non-RFCs, plus 9 obsoleted RFCs that may still warrant viewing). As for size? The JS implementation is about 600-700 lines right now (including over 300 lines of comments), while the equivalent C++ portions take over a thousand lines to do more or less the same thing.

As I said earlier, one of the problems with libmime is its oppressive architecture. It is primarily designed to drive the message pane while living as a streaming converter. However, it encompasses roughly three steps: the production of the raw MIME tree (or a tree that combine the raw MIME data with pseudo-MIME alternatives), conversion of that tree into a more traditional body-and-attachments view, and then the final driving of the UI. Getting it to stop any earlier is between difficult and impossible; what I have now, once it gets some more testing, can satisfy people who just need the first step. Doing the second step would require me to sit down and document how libmime makes its choices, which is not a task I look forward to doing.

Hate speech? Really?

One of my goals in life is to participate in a true political debate. Not a shouting match, but a true debate where both sides argue their points reasonably and, crucially, are open to the fact that they may be holding the wrong ideas and willing to change their beliefs. The Internet, allowing for an easy way to facilitate discussions across a very large, very diverse group of participants ought to have made this easier; alas, it seems that the past few years has only increased the vitriol and spitting bile in these shouting matches.

When I read news articles, sometimes I think "this sounds thought-provoking", and I turn to the online comments to see if it started one. Almost invariably, most of the comments turn out to be rants that are only distantly germane to the article (e.g., any article on the Middle East sets off a firestorm with ... well, you can guess; any article on Ron Paul sets off commenters screaming about how he's being "ignored" by the media, etc.). Sometimes, the ones I think are most thought-provoking have no comments (typically any article that mentions Africa). Go figure.

It is also surprising how many people you think ought to know better end up doing nothing but shouting. I went to a well-regarded selective-admissions public high school (the mean score for the standard college-admissions test in the US is not far from perfect) (and no, I did not get a perfect score on said test), but when we had a sponsored debate between the Democratic and Republican groups in our school, it still devolved into a (literal) shouting match.

Which brings me, in a roundabout manner, to the true topic of this post. Apparently, there is a firestorm currently brewing over a post on planet.mozilla.org that dealt with a political topic of current interest to some people. I discovered, years ago, that you catch a lot more flak if you write a post that goes to an aggregator that people disagree with than if you write one they agree with (I explained why I did not like the results of the 2008 presidential election, and caught more flak than the few posts earlier celebrating those results). I don't mind political discussion—indeed, as I described earlier, I'd love one. But I can understand why people might not like sensitive political topics being collected on a site which can be interpreted as official Mozilla policy.

However, there is one thing I don't like about it. Some people have taken it upon themselves to call the viewpoint presented as hate speech. I don't know what hate speech is exactly, but I don't think it should ever exist, no matter what one defines. That is because terming something as hate speech is an attempt to avoid discussing it: it is a way to say "that statement is so wrong I am not going to bother to refute it", or, equivalently, "I am so right that any opposition to my idea cannot be rationally argued." It is at best an insult to one's opponent and at worst an admission of outright arrogance.

Even worse, I think, are people who want to outlaw hate speech. Speech is, fundamentally, a reflection of one's belief. Attempting to outlaw the utterance of specific beliefs is nothing more than trying to outlaw those beliefs in the first place. Sure, some of them are injurious insults (such as those who deny that the Holocaust happened), but there is, to me, a steep slippery slope you start out on. Why outlaw denials of the Holocaust but not also outlaw denials of other historical religiously-motivated genocides? Hell, why stop there? We could outlaw any false statement: think how much more rational our discussions would be if people didn't state such lies as "Sharia law is invading the US legal system" or "the New Deal did nothing to bring us out of the Great Depression." Wait, maybe that's why.

I remember, when doing a project for a US Government class that involved redebating several key cases on freedom speech, that I found a Supreme Court decision which mentioned something to the effect of "Free speech is at its best when it causes discomfort" (I cannot find the exact quote right now, sadly enough). At that moment, I think, I solidified my stance on free speech to the following: all speech ought to be free, no matter how insulting, radical, or slanderous it may be. It may be that your speech causes injurious actions, but it's only the intent and the action that matters (e.g., falsely yelling "Fire!" in a crowded theater is grounds for manslaughter, but because the action was intended to create a stampede, not because you actually said "Fire"). Attempting to censor others' opinions—whether through legal means, or through attempts to shame them by calling their words "hate speech"—is not an action I can support.

And if you disagree with me, feel free to try to discuss it with me. Who knows, I may just change my mind.

How to use bugpoint

One of the problems with working with compilers is that you tend not to find some bugs until they get used on real code. And unlike most test suites, real code can involve some very large files, which is where you tend to find bugs. As luck would have it, my debugging fun today led to two bugs being found in the same function... which is only a 200 line function that hides infinite loops in macros and is written using continuations to boot. The LLVM IR for this function, after being compiled with -O3, was 4500 lines (and one block had 87 predecessors and several ϕ instructions as well). At such a size, finding out why my code crashes on such a function is impossible, let alone figuring out which optimizations I need to blame for it.

Fortunately, LLVM has a tool called bugpoint which can reduce this IR into a more manageable size of work. Doing manual reduction via an iterative process of "this doesn't look necessary; cut it out" the first time took me about an hour to produce a pile of code small enough to actually analyze. Doing it via bugpoint on the second bug took closer to 30 minutes. Unfortunately, the hard part is figuring out how to actually use the tool in the first place: none of the manuals give an example command line invocation, and they start playing games of "look at that documentation over there". So, I am going to remedy this situation by actually giving functional documentation.

In this case, I have an assert in an LLVM pass that is being triggered. This pass isn't being run as part of opt, but rather its own tool that takes as input an LLVM IR file. So the first step is to get the IR file (clang -cc1 -emit-llvm -O3 is sufficient for my needs). After that, it's time to prepare a shell script that actually compiles the code; you can skip this step if you don't actually need to provide arguments to the program. For example, my bugpoint.sh script would look like:

#!/bin/bash
/path/to/tool "$@" -arg1 -arg2 -arg3=bleargh -o /dev/null

After that, the next step is to actually invoke bugpoint. Here's the command line that's running as I write this post: bugpoint --compile-custom -compile-command ./bugpoint.sh io_lat4.ll. Since my program causes an assertion failure, bugpoint figures out that I'm trying to crash on code generation (it can also detect miscompilation errors). Hopefully, the first bit of output you get should look like the following:

Error running tool:
  ./bugpoint.sh bugpoint-test-program.bc-FB1NoU
Text indicating your program crashed
  *** Debugging code generator crash!

If you've seen this much (you may need to wait for it to crash; it can take a long time if you doing Debug+Asserts builds), you know that it's trying to find code that makes your tool crash. After that, bugpoint tries to first reduce global initializers and then tries to eliminate as many functions as possible. After that, it tries eliminating basic blocks and then goes to work eliminating instructions. You will see lots of streaming output telling you what stuff it's removing; the documentation says it's helpful to capture this output, but I've found it useless.

When everything is done, you should get several files of the form bugpoint-*.bc; the most useful one is bugpoint-reduced-simplified.bc, which is the most reduced testcase. What you get now is a nice, reduced testcase? Not quite. First, it gives you just a bitcode file (I'd prefer .ll files, simply because my first thought is to read them to figure out what's going on). Another problem I have with bugpoint is that it doesn't do things like attempt to eliminate unused struct entries, nor does it bother to clean up some of its names. Take a look at this struct monstrosity: %struct.su3_matrix.4.134.147.173.199.212.238.290.303.329.381.394.407.459.472.485.498.524.550.563.576.589.602.615.628.654.667.680.719.758.823.836.849.862.1044.1057.1096.1808 = type { [3 x [3 x %struct.complex.3.133.146.172.198.211.237.289.302.328.380.393.406.458.471.484.497.523.549.562.575.588.601.614.627.653.666.679.718.757.822.835.848.861.1043.1056.1095.1807]] }.

Anyways, I hope this helps anyone else who has looked at bugpoint before and wondered "How do I actually use this tool to do something useful?".

Fun and games with assembly

This is a problem that came up during my research. Suppose I have a function foo whose assembly looks like the following:

foo:
  mov global($rip), %rax
  mov (%rax), %rax
  retq

Are the following two lines of code equivalent?

  /* defines somewhere */
  int val;
  extern int foo(void);
  /* actual code */
  asm("callq foo" : "=a"(val));
  val = foo();

The answer, as you might guess by the fact that I'm writing this post, is "no." The reason why is a lot more complex. In the case that caused me endless toil and grief, the function foo is not located in the same executable as the caller; it is in an external library file. Now, the platform ABI allows functions to clobber a fair number of registers which callers must assume are unusable. If the loader needs to perform work when calling a global relocated function, then it is a good idea to use those registers where possible, since you don't need to bother saving those values. When the asm construct is invoked, we've declared that only the return register is clobbered, so the compiler is free to store some values over the call. When the function call is direct and doesn't need to go through the GOT or anything similar, this is perfectly fine and works as one would expect. But if you need to invoke the loader when calling through the GOT, then you now have registers whose values have suddenly mysteriously changed on you when calling a function which doesn't (appear to) use them. Hope you have more fun debugging those cases than I did!

Achievement Unlocked: Use 100GB of memory in a single process

I discovered that the compiler I was using last night managed to run out of memory while compiling some code. This was on our research group's server which has a whopping 128GB of memory. And by "ran out of memory", I literally watched in top as clang went from dozens of megabytes of memory to 120GB of memory, evict everything from the cache and thrash swap like crazy before the out-of-memory killer decided it ought to go.

What was I doing? No, I wasn't linking the source code to the universe; I was merely compiling a file in the Spec 2006 benchmarks. And this wasn't anything large--only 3700 lines of code or so in a single C++ file. And the pass that wasn't dying wasn't some horribly written pass that leaked memory like a leaky bucket. Indeed, LLVM tends to get you to free memory too quickly, judging by how many times I've chased down memory corruption. If you don't believe me, the offending code was this:

    Code = CloneBasicBlock(CodeOrig, ValMap, "", F, NULL);
    for (II = Code->begin(), EE = Code->end(); II != EE; ++II) {
      if (!isa(II))
        RemapInstruction(II, ValMap, RF_IgnoreMissingEntries);
    }

For those of you who don't know what the code is doing, it roughly amounts to this: make a copy of a basic block of a function, and then tell all of the instructions to use the new instructions instead. On some programs, though, this simple instruction seems to have compelled the code to eat up gigabytes of memory.

Fortunately, I am now proud to announce that I have reduced my memory consumption by 99%… with a one-line fix. So I guess I should get the achievement for "Fix the Memory Leak" as well?

How bugs get fixed

Recently, I have had the misfortuneopportunity to fix bug 695309. This bug is perhaps a good exemplar of why "obvious" bugs take weeks or months to fix; it is also a good example of why just reporting that a bug occurs for the filer is insufficient to fix. In the hope that others might find this useful, I will explain in a fake liveblogging format how I fixed the bug (fake because I'm writing most of these entries well after they happened).

October 20

There's a bug that "Thunderbird sometimes marks entire newsgroups as unread" in my email. Time to open up the bug report… reported in version 8… I'm pretty sure I've seen this once or twice before, so I don't think it's just a "the news server borked itself" issue. Time to file it away until when I have more time.

November 12

Another comment reminded me that I have this tab open. I've definitely seen it a few times, but I need to remember to keep Thunderbird open with logging enabled to figure out what's going on. It's reported as being a regression from version 8, when I checked in a slew of the NNTP URI parsing patches, so that seems like a probable target to look at. The question is why URI parsing would be causing an intermittent problem instead of a constant issue?

December 3

Some NNTP logs. Nothing is obviously amiss (not terribly unexpected, since logging either tends to drown you in useless information or omit the things you really want to know). Note after the fact: the NNTP logs in fact contain the smoking gun; it's just that the poster trimmed it out.

December 6

Bienvenu comments that no developer has seen the problem; this isn't true, as I have seen it (but just taken to avoiding it as much as possible). At some point in time, I figured out that the issue could be avoided by shutting down Thunderbird before putting my laptop to sleep. After being prodded over IRC, I finally sat down and attempted to debug it. The working thesis is that the problem is that newsrc files are getting corrupted, so I faithfully save several copies of the newsrc for confirmation. The msf files are also generally good candidates, but since other flags are untouched, it's highly unlikely in this bug.

I successfully trigger the bug once. The reports are mixed: the newsrc didn't get trashed as expected at first, but later it did. However, there is a brief window of time after the bug happens which allows you to fix it, if you shut down Thunderbird. Knowing what I now know, this is because the newsrc file is written out on a timer, so the newly-all-unread messages wouldn't have been saved to disk to show the bug. Since I always think of what to log after the fact, I try to trigger it a few more times.

None of the later tests are quite so successful as the first one. It does start to dawn on me that the bug probably has two parts. There is a first step that puts the group into a pre-bug situation; a second step actually produces the symptoms of the bug. Omniscient addendum: the first step is where the bug happens; the second step is just the delay in getting the bug to occur. I report my findings, and subsequent comments all but confirm my hypothesis for a necessary component.

December 12

This bug seems to be happening for me only when I don't want it to happen. I thought of more things to test earlier, and had them ready to copy-paste into my error console to try to find a smoking gun. This test confirms that the fault originates with the newsrcLine (so something reset that); more investigation leads to only one truly likely scenario to cause this to happen. At this point, I am all but convinced that the bug happens in two parts. All of the debugging needs to focus on when the first part happens; the symptoms (everything being marked read) are a result of Thunderbird attempting to repair the damage that had already been done. Since most people are going to try to report based on when they see the problems, I'm probably not going to get anything useful.

December 13

Hey, I found it again. Tests confirm that the high water number was first set to 0. This means either we have memory corruption or we are setting the value incorrectly initially. Looking at the code, this is set from a call to sscanf. A simple test indicates that I can set the input value to 0 if I don't have it scan a number. Now all I need to do is figure out network traffic that can cause this. Time to try to trigger it with xpcshell tests. And then get frustrated because I still can't do it reliably.

December 18

With Wireshark (logging on Windows is rather annoying), I finally can track down the network traffic of interest. It turns out that we are off-by-one in terms of responses. This really should be enough to get xpcshell to report the error. However, it also means I probably should finally give up and go for the NNTP logs again to catch the error: it is painfully obvious that the problem is that the internal state is futzed up. This also means that other various newly-reported issues (more frequent auth failures, various alerts like "Not in a newsgroup", etc.) are pretty much confirmed to be the same bug.

January 9

I finally buckled down and turned my very old hackish code for NSPR log management into an extension. This means that I don't have to futz with environment variables on Windows, and it also gives me an easy way to trim down my log size (since unlike environment variables, I can stop logging). I test and finally get the NNTP log of interest.

Now that I have a log, I can try to work backwards and figure out how this bug happens. The failure is easily ascribable to somehow thinking we've already opened the socket when we open a socket; this much I have known for almost a month (the Wireshark told me). What the NNTP log gives me is a more fine-grained ability to try to trace the code after-the-fact to figure out where the bug is.

Working backwards from the log

NNTP logs helpfully record the address of the the NNTP protocol object when emitting output, so the best place to start is from the first line of that object. Since the numbers are hard to read, so I replace the addresses with a more obvious string like "evil." The next state was set to SEND_FIRST_NNTP_COMMAND—that clearly should be NNTP_LOGIN_RESPONSE, so let's see why it might be skipped. The latter is set if m_socketIsOpen is false, so perhaps someone could open a socket if it isn't set.

This variable is set only by nsMsgProtocol, specifically the LoadUrl method. So who might call that? Still nothing out of the ordinary is popping out at me, so it's time to return to the logs (A particularly astute developer might be able to find out the bug without returning to the log here, but I doubt most people would come up with the final flash of insight yet).

To figure out the problem, it's important (to me, at least) to set out the time frames. The natural sequence of events for a connection is to be created, connected to the server, and then go through several iterations of running URLs. Is this bad connection a new connection or an old one (since I started the log after TB started up, I can't guarantee that this is truly a new connection)? Thunderbird stupidly (but helpfully, in this case) tells me when the connection is created, since we get a log message of "creating" in the constructor. Since this message doesn't appear, it's being reused.

Wait—that message is there, tucked just underneath the lines that tell me LoadUrl is run. As I am often wont to do, I announce my revelation to IRC: "< jcranmer> bienvenu: please slap me". The code for the constructor is pretty simple, but there is one little function call that ends up causing the problem. If I tell you the exact revision that caused the regression (specifically, the first hunk of that the patch), can you find the regression? Click me if you can't

Reproducing the bug

Now that I know the exact sequence of events to cause the bug, I can write a test to reliably reproduce the bug. I can also explain why the symptoms occur. First, a connection's socket dies, but it doesn't fail the timeout check in mailnews code, so it remains in the cache. Next, the server schedules a URL on the connection, which promptly dies. If there are many more URLs to be run (when we're doing, say, a timed update), then we have several elements in the queue waiting to be processed. Now, we attempt to run a new URL; with no connections available in the cache, we now create a new connection and run the new URL on that. Since the queue is not empty, the SetIsBusy(false) call ends up pulling a queued URL and setting up the state (including opening up the connection) and running that. Then the constructor finishes, and the new URL for which the connection was constructed is used to initialize it. Since the connection is by now open, the code moves straight to the run-my-URL part of the state machine, which misinterprets the login response as the answer to the command it just sent. This ends up leading to all of the various badness subsequently observed.

The description of these events is pretty complicated. A condensed version is added to the test I constructed to get this to happen, which is called, fittingly enough, test_bug695309.js. I am not going to try to come up with a better name, as I think most people might agree that this is where naming tests after bug numbers is most desirable. Naturally, the code to actually trigger this is complicated: I started with code to trigger just the unread effect, and then switched to monitoring the highwater mark instead (one less download of messages). I need to fill up the connection cache and then figure out how to kill the connections (restarting the server seems to do an adequate job). After that, I need to run enough URLs to cause the connections to queue, and then trigger a load of the folder in question at precisely the right moment. All of these requires paying careful attention to what happens synchronously and what does not, and it all relies on knowing the precise steps of what will happen as a result of each action. Any of a small number of changes I could make in the future is probably going to "break" the test, in that it will cease to test anything meaningful.

Postmortem on fixing it

I'm sure some people might look at this bug and ask several questions. Let me try to anticipate some of them and answer them.

Why did the reviewers fail to notice this change?

This bug is a result of the confluence of several changes. In the beginning, SetIsBusy was mostly a no-op, so calling it from the constructor was safe. Then the URL queue was added, and SetIsBusy was used to indicate that the connection was ready to receive a new URL from the queue. This turned it into a time bomb, since the code was correct only because the server was null in the constructor and the queue should have been empty during connection construction anyways. The final change was moving initialization of the server, which triggered the time bomb. But even then, it triggered the bomb only in a rather abnormal circumstance. At no time would the code have failed any automated tests or any of the "does your patch work" tests that a reviewer normally goes through: robustness during connection interruption is both sufficiently difficult to test and rare enough that it generally doesn't get tested.

Why did this bug take so long to track down?

The time since December 1 is truly a result of lack of time: I have had maybe 1 good week to work on anything due to outside factors. There are two, maybe three, people in active participation with Mozilla who know this code well, and out of those, only one was able to somewhat reliably reproduce it (specifically, the one with the least amount of time on his hands). I could have relied on others to gather the information I needed, but my previous efforts in QA have taught me that getting people to give you necessary information is often very difficult, especially when the information collectable at the obvious point in time is completely useless. I realize in retrospect that there were some more technically inclined people in the bug (at which point in time I was already devoting as much attention as I felt I could spare on the bug anyways). The bug is also particularly pernicious in that the most valuable information is that which is gathered before any visible symptoms occur; it was after I figured out how to discover that the bug was going to occur that I could track it down further.

Will the fix be backported to more stable branches?

At this stage, I am looking at two fixes: the small one-liner that fixes the specific bug, and the larger patch which fixes the larger issue of "the NNTP URL connection queue is a ticking time bomb". The former I would definitely like to see backported to aurora and beta (and possibly release if there are plans for another one before the next uplift), and I can't see any reason a small patch on the most-highly-voted TB bug filed since 01-01-2011 would get rejected (it is the newest bug to have more than 5 votes, and the next largest has 1/3 less and is almost twice as old).

Why is this bug so hard to reproduce?

If you think to the cause of the bug, it is that it is passing a check in a function that is always called. It therefore seems like this bug should be triggered a majority of the time, and not an undependable minority. As I alluded to earlier, the problem is that this also has to interact with a buggy pending URL queue management system. In short, most of the time, this queue will be empty during the initial, buggy call; with a poor timing of events (less likely to happen on most developers' computers, in short), the queue will cease to be empty and cause the problem. Indeed, when crafting my test for reproduction, I discovered that letting the event loop spin in one particular location (after killing connections) would fail to trigger the bug. In the real world, that is one of the places where the event loop is most liable to spinning for the longest.

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.