Coarse-grained recompilation checking

[simonmar]

GHC's recompilation checker is very clever: it tracks changes at the level of an individual function or type, and only recompiles a module when the entities it actually depends on have changed.

However, there are some drawbacks to doing this:

• It is complicated and hard to get right
• It adds extra information to .hi files
• It takes time to check the dependencies (probably not much, though)

Furthermore,

• we have virtually no tests for it
• when it goes wrong, the bug is very hard to reproduce, which means that failures are often not reported.

So we propose:

• Simplify the recompilation checker so that it tracks changes at the level of a module only. This will cause more recompilation, but it will be much easier to get right and to test.
• As an intermediate step, we could implement both schemes and generate some output to indicate when they give different results.

See Commentary/Compiler/RecompilationAvoidance for more discussion.

[simonmar]

I decided not to do this: it was easier to convert the existing recompilation checker to use fingerprints, and at the same time audit it to look for bugs (I did find a couple).