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This avoids cluttering the main wasm namespace, and clarifies what the
scanner does.
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If we write an immutable global to a field, and that is the only thing
we ever write, then we can replace reads of the field with a get of
the global. To do that, this tracks immutable globals written to
fields and not just constant values.
Normally this is not needed, as if the global is immutable then we
propagate its constant value to everywhere anyhow. However, for
references this is useful: If we have a global immutable vtable,
for example, then we cannot replace a get of it with a constant.
So this PR helps with immutable reference types in globals, allowing
us to propagate global.gets to them to more places, which then
can allow optimizations there.
This + later opts removes 25% of array.gets from j2wasm. I believe
almost all of those are itable calls, so this means those are getting
devirtualized now. I see something like a 5% speedup due to that.
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Saves a little code size and might prevent some bugs.
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Add struct.get tracking, and if a field is never read from, simply remove
it.
This will error if a field is written using struct.new with a value with side
effects. It is not clear we can handle that, as if the struct.new is in a
global then we can't save the other values to locals etc. to reorder
things. We could perhaps use other globals for it (ugh) but at least for
now, that corner case does not happen on any code I can see.
This allows a quite large code size reduction on j2wasm output (20%). The
reason is that many vtable fields are not actually read, and so removing
them and the ref.func they hold allows us to get rid of those functions,
and code that they reach.
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This method is in parallel to runOnFunction above it. It sets the runner
and then does the walk, like that method.
Also set runner to nullptr by default. I noticed ubsan was warning on
things here, which this should avoid, but otherwise I'm not aware of an
actual bug, so this should be NFC. But it does provide a safer API
that should avoid future bugs.
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This just moves code outside and makes it more generic. One set of
functionality are "struct utils", which are tools to scan wasm for info
about the usage of struct fields, and to analyze that data. The other
tool is a general analysis of nominal subtypes.
The code will be useful in a few upcoming passes, so this will avoid a
significant amount of code duplication.
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This finishes the refactoring started in #4115 by doing the
same change to pass a Module into EffectAnalyzer instead of
features. To do so this refactors the fallthrough API and a few
other small things. After those changes, this PR removes the
old feature constructor of EffectAnalyzer entirely.
This requires a small breaking change in the C API, changing
BinaryenExpressionGetSideEffects's feature param to a
module. That makes this change not NFC, but otherwise it is.
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When looking for all values written to a field, we can ignore
values that are loaded from that same field, i.e., are copied from
something already present there. Such operations never introduce
new values.
This helps by a small but non-zero amount on j2cl.
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ConstantFieldPropagation (#4064)
Previously we tracked them in the same way. That means that we did the same
when seeing if either a struct.new or a struct.set can write to the memory that
is read by a struct.get, where the rule is that if either type is a subtype of the
other then they might. But with struct.new we know the precise type, which
means we can do better.
Specifically, if we see a new of type B, then only a get of a supertype of B can
possibly read that data: it is not possible for our struct of type B to appear in
a location that requires a subtype of B. Conceptually:
A = type struct
B = type extends A
C = type extends B
x = struct.new<B>
struct.get<A>(y) // x might appear here, as it can be assigned to a
// variable y of a supertype
struct.get<C>(y) // x cannot appear here
This allows more devirtualization. It is a followup for #4052 that implements
a TODO from there.
The diff without whitespace is simpler.
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A field in a struct is constant if we can see that in the entire program we
only ever write the same constant value to it. For example, imagine a
vtable type that we construct with the same funcrefs each time, then (if
we have no struct.sets, or if we did, and they had the same value), we
could replace a get with that constant value, since it cannot be anything
else:
(struct.new $T (i32.const 10) (rtt))
..no other conflicting values..
(struct.get $T 0) => (i32.const 10)
If the value is a function reference, then this may allow other passes
to turn what was a call_ref into a direct call and perhaps also get
inlined, effectively a form of devirtualization.
This only works in nominal typing, as we need to know the supertype
of each type. (It could work in theory in structural, but we'd need to do
hard work to find all the possible supertypes, and it would also
become far less effective.)
This deletes a trivial test for running -O on GC content. We have
many more tests for GC at this point, so that test is not needed, and
this PR also optimizes the code into something trivial and
uninteresting anyhow.
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