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The LocalGraph there was used for two purposes:
1. Get the list of gets and sets.
2. Get only the reachable gets and sets.
It is trivial to get all the gets and sets in a much faster way, by just walking the
code as this PR does. The downside is that we also consider unreachable gets
and sets, so unreachable code can prevent us from optimizing, but that seems
worthwhile as many passes make that assumption (and they all become
maximally effective after --dce). That is the only non-NFC part here.
Removing LocalGraph + the fixup code for unreachability makes this
significantly shorter, and also 2-3x faster.
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Now that the WasmGC spec has settled on a way of validating non-nullable locals,
we no longer need this experimental feature that allowed nonstandard uses of
non-nullable locals.
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Before this PR, if a call had no paths to a catch in the same function then we skipped
creating a new basic block right after it. As a result, we could have a call in the middle
of a basic block. If EH is enabled that means we might transfer control flow out of
the function from the middle of a block. But it is better to have the property that
any transfer of control flow - to another basic block, or outside of the function - can
only happen at the end of a basic block.
This causes some overhead, but a subsequent PR (#5838) will remove that as a
followup, and this PR adds a little code to pass the module and check if EH is enabled,
and avoid the overhead if not, which at least avoids regressing the non-EH case
until that followup lands.
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Before we implemented bottom heap types, `ref.null` had to be annotated with
specific types. The `LUBFinder` utility ignored these types so that it could
find the best LUB from all considered non-null expressions, then go back and
update the type annotations on the nulls to match that LUB. Now that we have
bottom types, however, none of that is necessary, and in fact ignoring nulls can
miss possible refinements to bottom types.
Update and simplify `LUBFinder` so that it is a simple wrapper around the
underlying `Type::getLeastUpperBound` utility with no additional logic. Update
tests to account for the more powerful optimizations.
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Previously only WalkerPasses had access to the `getPassRunner` and
`getPassOptions` methods. Move those methods to `Pass` so all passes can use
them. As a result, the `PassRunner` passed to `Pass::run` and
`Pass::runOnFunction` is no longer necessary, so remove it.
Also update `Pass::create` to return a unique_ptr, which is more efficient than
having it return a raw pointer only to have the `PassRunner` wrap that raw
pointer in a `unique_ptr`.
Delete the unused template `PassRunner::getLast()`, which looks like it was
intended to enable retrieving previous analyses and has been in the code base
since 2015 but is not implemented anywhere.
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An overview of this is in the README in the diff here (conveniently, it is near the
top of the diff). Basically, we fix up nn locals after each pass, by default. This keeps
things easy to reason about - what validates is what is valid wasm - but there are
some minor nuances as mentioned there, in particular, we ignore nameless blocks
(which are commonly added by various passes; ignoring them means we can keep
more locals non-nullable).
The key addition here is LocalStructuralDominance which checks which local
indexes have the "structural dominance" property of 1a, that is, that each get has
a set in its block or an outer block that precedes it. I optimized that function quite
a lot to reduce the overhead of running that logic after each pass. The overhead
is something like 2% on J2Wasm and 0% on Dart (0%, because in this mode we
shrink code size, so there is less work actually, and it balances out).
Since we run fixups after each pass, this PR removes logic to manually call the
fixup code from various places we used to call it (like eh-utils and various passes).
Various passes are now marked as requiresNonNullableLocalFixups => false.
That lets us skip running the fixups after them, which we normally do automatically.
This helps avoid overhead. Most passes still need the fixups, though - any pass
that adds a local, or a named block, or moves code around, likely does.
This removes a hack in SimplifyLocals that is no longer needed. Before we
worked to avoid moving a set into a try, as it might not validate. Now, we just do it
and let fixups happen automatically if they need to: in the common code they
probably don't, so the extra complexity seems not worth it.
Also removes a hack from StackIR. That hack tried to avoid roundtrip adding a
nondefaultable local. But we have the logic to fix that up now, and opts will
likely keep it non-nullable as well.
Various tests end up updated here because now a local can be non-nullable -
previous fixups are no longer needed.
Note that this doesn't remove the gc-nn-locals feature. That has been useful for
testing, and may still be useful in the future - it basically just allows nn locals in
all positions (that can't read the null default value at the entry). We can consider
removing it separately.
Fixes #4824
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It is common in GC code to have stuff like this:
x = null;
..
x = Data();
Nulls in wasm have a type, and if that initial null has say anyref then
before this PR we would keep the type of x as anyref. However,
while nulls have types, all null values are identical, and so we can
in fact change x's type to a nullable reference of Data, by also
changing the null's type to something more specific.
LUBFinder now has an API that can return the best possible LUB
so far, and that can be told to update nulls if we decide that the
new LUB is worth using. This updates the passes using LUBFinder
to use the new API. Note how TypeRefining becomes simpler
because the special logic it had in a subclass of LUBFinder is now
part of the main class (it used to remember if there was a null
default; LUBFinder now handles both a null default as well as
other nulls).
This requires some changes to existing tests to avoid them from
optimizing using nulls in ways that ends up not testing the
original intent. Specifically the dae-gc-refine-params.wast now
has calls to get a null of a type, instead of just having a ref.null
of that type (which could be optimized now). And
dae-gc-refine-return uses locals instead of ref.nulls.
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This is a minor refactoring in DAE to have a helper class that does the
incremental LUB calculation. The class is also used in LocalSubtyping,
where it has the effect of making the work incremental which it was not
before (that would have no observable consequence, but it should make
us faster in the common case where we fail to find a new LUB).
This will allow further optimization in a central place later.
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(#4051)
We ignore sets in unreachable code, but their values may not be
compatible with a new type we specialize a local for. That is, the
validator cares about unreachable sets, while logically we don't need
to, and this pass doesn't. Fix up such unreachable sets at the end.
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It is ok to use the default value of a reference even if we refine the type,
as it would be a more specifically-typed null, and all nulls compare the
same. However, if the default is used then we *cannot* alter the type to
be non-nullable, as then we'd use a null where that is not allowed.
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(#4031)
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The pass handled non-nullability, but another case is a tuple with nullable
values in it that is assigned non-nullable values, and in general, other stuff
that is nondefaultable (but not non-nullable). Ignore those.
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If a local is say anyref, but all values assigned to it are something
more specific like funcref, then we can make the type of the local
more specific. In principle that might allow further optimizations, as
the local.gets of that local will have a more specific type that their
users can see, like this:
(import .. (func $get-funcref (result funcref)))
(func $foo (result i32)
(local $x anyref)
(local.set $x (call $get-funcref))
(ref.is_func (local.get $x))
)
=>
(func $foo (result i32)
(local $x funcref) ;; updated to a subtype of the original
(local.set $x (call $get-funcref))
(ref.is_func (local.get $x)) ;; this can now be optimized to "1"
)
A possible downside is that using more specific types may not end
up allowing optimizations but may end up increasing the size of
the binary (say, replacing lots of anyref with various specific
types that compress more poorly; also, for recursive types the LUB
may be a unique type appearing nowhere else in the wasm). We
should investigate the code size factors more later.
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