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This pass may do multiple iterations, and before this PR it scanned the entire
module each time. That is simpler than tracking stale data, but it can be quite
slow. This PR adds staleness tracking, which makes it over 3x faster (and this
can be one of our slowest passes in some cases, so this is significant).
To achieve this:
* Add a staleness marker on function info.
* Rewrite how we track unseen calls. Previously we used atomics in a clever way,
* now we just accumulate the data in a simple way (easier for staleness tracking).
* Add staleness invalidation in the proper places.
* Add a param to localizeCallsTo to allow us to learn when a function is changed.
This kind of staleness analysis is usually not worthwhile, but given the 3x plus
speedup it seems justified. I fuzzed it directly, and also any staleness bug
can lead to validation errors, so normal fuzzing also gives us good coverage here.
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This constructed a LocalGraph, which computes the sets that reach each get. But
all we need to know is which params are live, so instead we can do a liveness
computation (which is just a boolean, not the list of sets). Also, it is simple to get
the liveness computation to only work on the parameters and not all the locals,
as a further optimization.
Existing tests cover this, though I did find that the case of unreachability needed
a new test.
On a large testcase I am looking at, this makes --dae 17% faster.
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effects (#6395)
Before this PR, when we saw a param was unused we sometimes could not remove it.
For example, if there was one call like this:
(call $target
(call $other)
)
That nested call has effects, so we can't just remove it from the outer call - we'd need to
move it first. That motion was hard to integrate which was why it was left out, but it
turns out that is sometimes very important. E.g. in Java it is common to have such calls
that send the this parameter as the result of another call; not being able to remove such
params meant we kept those nested calls alive, creating empty structs just to have
something to send there.
To fix this, this builds on top of #6394 which makes it easier to move all children out of
a parent, leaving only nested things that can be easily moved around and removed. In
more detail, DeadArgumentElimination/SignaturePruning track whether we run into effects that
prevent removing a field. If we do, then we queue an operation to move the children
out, which we do using a new utility ParamUtils::localizeCallsTo. The pass then does
another iteration after that operation.
Alternatively we could try to move things around immediately, but that is quite hard:
those passes already track a lot of state. It is simpler to do the fixup in an entirely
separate utility. That does come at the cost of the utility doing another pass on the
module and the pass itself running another iteration, but this situation is not the most
common.
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DeadArgumentElimination (#4547)
Similar to #4544, this moves the code to a utility function, and also
slightly generalizes it to support a list of functions (and not just 1)
and also a list of call_refs (and not just calls).
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DeadArgumentElimination (#4544)
In preparation for removing dead arguments from all functions sharing a heap
type (which seems useful for j2wasm output), first this PR refactors that code
so it is reusable. This moves the code out of the pass into FunctionUtils, and
also generalizes it slightly by
supporting a set of functions and not just a single one, and
receiving a list of call_refs and not just calls
(no other changes to anything).
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