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This is a much more robust way to do recompilation checking. The idea
is to create a fingerprint of the ABI of an interface, and track
dependencies by recording the fingerprints of ABIs that a module
depends on. If any of those ABIs have changed, then we need to
recompile.
In bug #1372 we weren't recording dependencies on package modules,
this patch fixes that by recording fingerprints of package modules
that we depend on. Within a package there is still fine-grained
recompilation avoidance as before.
We currently use MD5 for fingerprints, being a good compromise between
efficiency and security. We're not worried about attackers, but we
are worried about accidental collisions.
All the MD5 sums do make interface files a bit bigger, but compile
times on the whole are about the same as before. Recompilation
avoidance should be a bit more accurate than in 6.8.2 due to fixing
#1959, especially when using -O.
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