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This PR is part of a series that adds basic support for the typed continuations proposal.
This PR relaxes the restriction that tags must not have results , only params. Tags with
results must not be used for exception handling and are only allowed if the typed
continuations feature is enabled.
As a minor point, this PR also changes the printing of tags without params: To make the
presentation consistent, (param) is omitted when printing a tag.
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When printing Binaryen IR, we previously generated names for unnamed heap types
based on their structure. This was useful for seeing the structure of simple
types at a glance without having to separately go look up their definitions, but
it also had two problems:
1. The same name could be generated for multiple types. The generated names did
not take into account rec group structure or finality, so types that differed
only in these properties would have the same name. Also, generated type names
were limited in length, so very large types that shared only some structure
could also end up with the same names. Using the same name for multiple types
produces incorrect and unparsable output.
2. The generated names were not useful beyond the most trivial examples. Even
with length limits, names for nontrivial types were extremely long and visually
noisy, which made reading disassembled real-world code more challenging.
Fix these problems by emitting simple indexed names for unnamed heap types
instead. This regresses readability for very simple examples, but the trade off
is worth it.
This change also reduces the number of type printing systems we have by one.
Previously we had the system in Print.cpp, but we had another, more general and
extensible system in wasm-type-printing.h and wasm-type.cpp as well. Remove the
old type printing system from Print.cpp and replace it with a much smaller use
of the new system. This requires significant refactoring of Print.cpp so that
PrintExpressionContents object now holds a reference to a parent
PrintSExpression object that holds the type name state.
This diff is very large because almost every test output changed slightly. To
minimize the diff and ease review, change the type printer in wasm-type.cpp to
behave the same as the old type printer in Print.cpp except for the differences
in name generation. These changes will be reverted in much smaller PRs in the
future to generally improve how types are printed.
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This attribute is always 0 and reserved for future use. In Binayren's
unofficial text format we were writing this field as `(attr 0)`, but we
have recently come to the conclusion that this is not necessary.
Relevant discussion:
https://github.com/WebAssembly/exception-handling/pull/160#discussion_r653254680
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We recently decided to change 'event' to 'tag', and to 'event section'
to 'tag section', out of the rationale that the section contains a
generalized tag that references a type, which may be used for something
other than exceptions, and the name 'event' can be confusing in the web
context.
See
- https://github.com/WebAssembly/exception-handling/issues/159#issuecomment-857910130
- https://github.com/WebAssembly/exception-handling/pull/161
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singleton (#3581)
The fix here is to remove the code with
// maybe we don't need a block here?
That would remove a try's block if we thought it wasn't needed. However,
it just checked for exception branches, but not normal branches, which are
also possible.
At that location, we don't have a good way to find out if the block has other
branches to it aside from scanning its contents. So this PR just gives up on
doing so, which means we add an unnecessary block if the optimizer is not
run. If this matters we could make the binary parser more complicated by
remembering whether a block had branches in the past, but I'm not sure if
it's worth it.
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This adds support for reading/writing of the new `delegate` instruction
in the folded wast format, the stack IR format, the poppy IR format, and
the binary format in Binaryen. We don't have a formal spec written down
yet, but please refer to WebAssembly/exception-handling#137 and
WebAssembly/exception-handling#146 for the informal semantics. In the
current version of spec `delegate` is basically a rethrow, but with
branch-like immediate argument so that it can bypass other
catches/delegates in between.
`delegate` is not represented as a new `Expression`, but it is rather
an option within a `Try` class, like `catch`/`catch_all`.
One special thing about `delegate` is, even though it is written
_within_ a `try` in the folded wat format, like
```wasm
(try
(do
...
)
(delegate $l)
)
```
In the unfolded wat format or in the binary format, `delegate` serves as
a scope end instruction so there is no separate `end`:
```wasm
try
...
delegate $l
```
`delegate` semantically targets an outer `catch` or `delegate`, but we
write `delegate` target as a `try` label because we only give labels to
block-like scoping expressions. So far we have not given `Try` a label
and used inner blocks or a wrapping block in case a branch targets the
`try`. But in case of `delegate`, it can syntactically only target `try`
and if it targets blocks or loops it is a validation failure.
So after discussions in #3497, we give `Try` a label but this label can
only be targeted by `delegate`s. Unfortunately this makes parsing and
writing of `Try` expression somewhat complicated. Also there is one
special case; if the immediate argument of `try` is the same as the
depth of control flow stack, this means the 'delegate' delegates to the
caller. To handle this case this adds a fake label
`DELEGATE_CALLER_TARGET`, and when writing it back to the wast format
writes it as an immediate value, unlike other cases in which we write
labels.
This uses `DELEGATE_FIELD_SCOPE_NAME_DEF/USE` to represent `try`'s label
and `delegate`'s target. There are many cases that `try` and
`delegate`'s labels need to be treated in the same way as block and
branch labels, such as for hashing or comparing. But there are routines
in which we automatically assume all label uses are branches. I thought
about adding a new kind of defines such as
`DELEGATE_FIELD_TRY_NAME_DEF/USE`, but I think it will also involve some
duplication of existing routines or classes. So at the moment this PR
chooses to use the existing `DELEGATE_FIELD_SCOPE_NAME_DEF/USE` for
`try` and `delegate` labels and makes only necessary amount of changes
in branch-utils. We can revisit this decision later if necessary.
Many of changes to the existing test cases are because now all `try`s
are automatically assigned a label. They will be removed in
`RemoveUnusedNames` pass in the same way as block labels if not targeted
by any delegates.
This only supports reading and writing and has not been tested against
any optimization passes yet.
---
Original unfolded wat file to generate test/try-delegate.wasm:
```wasm
(module
(event $e)
(func
try
try
delegate 0
catch $e
end)
(func
try
try
catch $e
i32.const 0
drop
try
delegate 1
end
catch $e
end
)
)
```
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