On 16 Jan 2014, at 11:44 am, Szczepan Faber <szczepan.fa...@gradleware.com> wrote:
> > > > On Fri, Dec 20, 2013 at 4:37 AM, Adam Murdoch <adam.murd...@gradleware.com> > wrote: > Hi, > > Just some thoughts on how we might spike a solution for incremental java > compilation, to see if it’s worthwhile and what the effort might be: > > The goal is to improve the Java compile tasks, so that they do less work for > certain kinds of changes. Here, ‘less work’ means compiling fewer source > files, and also touching fewer output files so that consumers of the task > output can also do less work. It doesn’t mean compiling the *fewest* possible > number of source files - just fewer than we do now. > > The basic approach comes down to keeping track of dependencies between source > files and the other compilation inputs - where inputs are source files, the > compile classpath, the compile settings, and so on. Then, when an input > changes, we would recompile the source files that depend on that input. > Currently, we assume that every source file depends on every input, so that > when an input changes we recompile everything. > > Note that we don’t necessarily need to track dependencies at a fine-grained > level. For example, we may track dependencies between packages rather than > classes, or we may continue to assume that every source file depends on every > class in the compile classpath. > > A basic solution would look something like: > > 1. Determine which inputs have changed. > 2. If the compile settings have changed, or if we don’t have any history, > then schedule every source file for compilation, and skip to #5. > 3. If a class in the compile classpath has changed, then schedule for > compilation every source file that depends on this class. > 4. If a source file has changed, then schedule for compilation every source > file that depends on the classes of the source file. > 5. For each source file scheduled for compilation, remove the previous output > for that source file. > 6. Invoke the compiler. > 7. For each successfully compiled source file, extract the dependency > information for the classes in the source file and persist this for next time. > > For the above, “depends on” includes indirect dependencies. > > Steps #1 and #2 are already covered by the incremental task API, at least > enough to spike this. > > Step #3 isn’t quite as simple as it is described above: > - Firstly, we can ignore changes for a class with a given name, if a class > with the same name appears before it in the classpath (this includes the > source files). > - If a class is removed, this counts as a ‘change’, so that we recompile any > source files that used to depend on this class. > - If a class is added before some other class with the same name in the > classpath, then we recompile any source files that used to depend on the old > class. > - Dependencies can travel through other classes in the classpath, or source > files, or a combination of both (e.g. a source class depends on a classpath > class depends on a source class depends on a classpath class). > > Re whole #3 step > > Do you have some thoughts on how to figure out what classes have changed in > the compile classpath given the classpath consists mostly of jars and the > incremental api only tells us currently that jar X have changed? I can > imagine that we have this information for project dependencies, because > project dependencies are 'internal' jars, also compiled incrementally, so we > should have all information. This may be tricky for 'external' jars, though. > Are you after a generic solution that works for internal and external jars? > If so, do you have some thoughts on how to approach it. Not at this stage. To start with, it’ll be the compile task’s problem to scan the jars that have changed to decide which particular classes have changed or not, and to take care of persisting whatever it needs to make this decision. > > In what ways you see improving the incremental task api here? After we’ve made some progress on this and the incremental native compilation, we can roll some stuff back into the incremental task API. > > Step #4 is similar to step #3. > > For a spike, it might be worth simply invalidating everything when the > compile classpath changes, and just deal with changes in the source files. > > For step #7 we have three basic approaches for extracting the dependencies: > > The first approach is to use asm to extract the dependencies from the byte > code after compilation. The upside is that this is very simple to implement > and very fast. We have an implementation already that we use in the tooling > API (ClasspathInferer - but it’s mixed in with some other stuff). It also > works for things that we only have the byte code for. > > The downside is that it’s lossy: the compiler inlines constants into the byte > code and discards source-only annotations. We also don’t easily know what > type of dependency it is (is it an implementation detail or is is visible in > the API of the class?) > > Both these downsides can be addressed: For example we might treat a class > with a constant field or a class for a source-only annotation as a dependency > of every source file, so that when one of these things change, we would > recompile everything. And to determine the type of dependency, we just need > to dig deeper into the byte code. > The second approach is to use the compiler API that we are already using to > invoke the compiler to query the dependencies during compilation. The upside > is that we get the full source dependency information. The downsides are that > we have to use a sun-specific extension of the compiler API to do this and > it’s a very complicated API, which means fiddly to get right. > > What compiler API do you mean? What is the downside of using sun-specific > extension here? The compiler tree API (the com.sun.source packages). The downside is that it's not guaranteed to be stable and it's not guaranteed to be present. > > The third approach is to parse and analyse the source separately from > compilation. > > Do you mean Compiler Tree Api here? No, I meant something like the source parser we use to extract the documentation metadata for the DSL generation. In fact, this might be a better option than using the compiler tree API if we go for source analysis - it already does much of the work, and it happens to understand groovy too. > > Perhaps we can mix the approaches. E.g. for certain classes we could go for > source analysis. > > Source analysis might be interesting from the standpoint of the daemon's > background activity. Imagine the daemon works out the class dependencies as > the developer edits the java code. A nice idea. There is certainly no reason we couldn’t mix the approaches (apart from the cost of implementing two approaches instead of one). > > I’d probably try out the first option, as it’s the simplest to implement and > probably the fastest at execution time. > > There are some issues around making this efficient. > First, we need to make the persistence mechanism fast. For the spike, let’s > assume we can do this. I would just keep the state in some static field > somewhere and not bother with persistence. > > We could use daemon's state as we do for task history cache. > > Second, we need to make the calculation of affected source files fast. One > option is to calculate this when something changes rather than each time we > run the compilation task, so that we keep, basically, a map from input file > to the closure of all source files affected by that input file. > > Third, we need to keep the dependency graph as small as we can. So, we might > play around with tracking dependencies between packages rather than classes. > We should also ignore dependencies that are not visible to the consumer, so > that we don’t traverse the dependencies of method bodies, or private elements. > > Finally, we should ignore changes that are not visible to the consumer, so > that we ignore changes to method bodies, private elements of a class, the > annotations of classes, debug info and so on. This is relatively easy for > changes to the compile classpath. For changes to source files, it’s a bit > trickier, as we don’t know what’s changed until we compile the source file. > We could, potentially, compile in two passes - first source files that have > changed and then second source files that have not change but depend on those > that have. Something, potentially, to play with as part of a spike. > > Can you elaborate why it's easy to obtain this information from the compile > classpath? Because it’s in byte code format. All the structural stuff has already been figured out for us and described in the byte code and we just need to diff the structure to decide whether the changes to the class are visible to the consumers or not. When the change is not visible, we don’t need to recompile the consumer source files. For source files, all we have is some text so we don’t know whether the change can safely be ignored or not. We either need to recompile all consumer source files regardless of whether they need it, or we need to preprocess the source file to decide if the change is visible to consumers or not. -- Adam Murdoch Gradle Co-founder http://www.gradle.org VP of Engineering, Gradleware Inc. - Gradle Training, Support, Consulting http://www.gradleware.com
