Re: Incremental Java Compile

"Mike Schilling" <>
Sun, 9 May 2010 17:41:51 -0700
Joshua Maurice wrote:

On May 9, 9:30 am, "Mike Schilling" <>

Joshua Maurice wrote:

It is technically impossible to do a true minimal recompile
algorithmically. Let's define it as "Let a build be a set of file
compilations. Let the minimum recompile be the minimum such set for
which the output class file are equivalent to the class files of a
full clean build." First, we'd have to prove such a minimum exists.
That's relatively straightforward. With that out of the way, I
think I could then prove that the problem is equivalent to the
Halting problem. If you define "equivalent" generously, I'm pretty
sure this is the case. If you define it as "same binary file
content", then perhaps not, though still possibly yes.

Why would you think that? The ways in which a change to A can affect
B is finite and well-defined.

Let's suppose the source changed from "2" to "(1+1)". Using the strict
interpretation, the output class files would probably be equivalent
(ignoring debug information like original source code). Thus a rebuild
is technically not required.

If the change is from

    public static final int I = 2;


    public static final int I = 1 + 1;

I'd accept a system that recompiles all users of I as still "minimal". (Not
that it's a difficult optimization to make, since the rules for what's a
compile-time constant are straightforward.) If this change isn't to a
compile-time constant, it would have no effect on anything defined in a
different source file.

Or, a slightly less trivial case: Suppose we have class A which has 2
public static functions A1 and A2 which have independent
implementations. Class C uses A1. Suppose someone comes along and
changes A2 in some meaningful way.

That is, chaged its signature. Merely chaning its implementation would not
rquire C to be recompiled.

Class C does not need to be
recompiled, but any sort of file level dependency analysis which is
correct would recompile Class C.

Right, strictly minimal recompilation wouild need to know not only that C
uses A but what parts of A it uses. Quite strightforward, though perhaps not
a good idea. (It's possbile that gathering too much dependency information,
and evaluationing it at too granular a level, slows the whole process down,
compared to allowing some recompilations that are strictly speaking

If you define it as binary file contents as equivalent output files,
then it's not equivalent to the Halting problem, I think. However, if
you define it as "The output class files display the same visible
behavior across all valid input", then I think the general case \is\
the Halting problem.

If I understand you, you're distinguishing between:

    1. A recompilation would result in the same class file that already
exists, and
    2. A recompilation would result in a change to the class file, but the
result of running the two will always be the same

I agree that a system that tries to determine 2 is infeasible. And my point
about adding the comment isn't that the file itself would be recompiled.
That's acceptable, and might even be necessary to get the line numbers seen
by a debugger to be correct. My point is that all files which refer to it
are recompiled, even though nothing they use (the class definitions, the
field and method definitions, and the values of the compile-time constants)
has changed. The same is true when e.g. method implementations change or an
init block is added; I was using "add a comment" as the limiting case of a
change no other file would care about.

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