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Br and BrOn can consider the code before and after them connected if it might
be reached (which is the case if the Br has a condition, which BrOn always has).
The wasm2js changes may look a little odd as some of them have this:
i64toi32_i32$1 = i64toi32_i32$2;
i64toi32_i32$1 = i64toi32_i32$2;
I looked into that and the reason is that those outputs are not optimized, and
also even in unoptimized wasm2js we do run simplify-locals once (to try to
reduce the downsides of flatten). As a result, this PR makes a difference there,
and that difference can lead to such odd duplicated code after other operations.
However, there are no changes to optimized wasm2js outputs, so there is no
actual problem.
Followup to #5860.
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SimplifyLocals (#5860)
This addresses most of the minor regression from the correctness fix in #5857.
That PR makes us consider calls as branching, but in some cases it is ok to
ignore that branching (see the comment in the code here), which this PR allows as
an option.
This undoes one test change from that PR, showing it undoes the regression for
SimplifyLocals. More tests are added to cover this specifically as well.
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Calls were simply not handled there, so we could think we were still in the same
basic block when we were not, affecting various passes (but somehow this went
unnoticed until the TNHOracle #5850 ran on some particular Java code).
One existing test was affected, and two new tests are added: one for TNHOracle
where I detected this, and one in OptimizeCasts which is perhaps a simpler way
to see the problem.
All the cases but the TNH one, however, do not need this fix for correctness
since they actually don't care if a call would throw. As a TODO, we should find a
way to undo this minor regression. The regression only affects builds with EH
enabled, though, so most users should be unaffected even in the interm.
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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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RTTs were removed from the GC spec and if they are added back in in the future,
they will be heap types rather than value types as in our implementation.
Updating our implementation to have RTTs be heap types would have been more work
than deleting them for questionable benefit since we don't know how long it will
be before they are specced again.
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This caused no noticeable bugs, but it could in theory in new passes - in fact
in a pass I will open later this week it did.
Also fix the order in wasm.h. That part has no effect, but it is nice to be
consistent. After this PR, everything should match the single source of
truth which is wasm-delegations-fields.h (as that is used in printing, binary
reading/writing, etc., so it has to be correct). Also Switch now matches
the ordering in Break.
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That traversal did not mention BrOn, which led to it doing incorrect work in
SimplifyLocals.
Also add assertions at the end, that aim to prevent future issues.
The rest of the fix is to make SimplifyLocals not assume that things
are a Switch if they are not an If/Block/etc., so that we don't crash
on a BrOn.
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