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Unlike other module elements, types are not stored on the `Module`.
Instead, they are collected by traversing the IR before printing and
binary writing. The code that collects the types tries to optimize the
order of rec groups based on the number of times each type is used. As a
result, the output order of types generally has no relation to the input
order of types. In addition, most type optimizations rewrite the types
into a single large rec group, and the order of types in that group is
essentially arbitrary. Changes to the code for counting type uses,
sorting types, or sorting rec groups can yield very large changes in the
output order of types, producing test diffs that are hard to review and
potentially harming the readability of tests by moving output types away
from the corresponding input types.
To help make test output more stable and readable, introduce a tool
option that causes the order of output types to match the order of input
types as closely as possible. It is implemented by having the parsers
record the indices of the input types on the `Module` just like they
already record the type names. The `GlobalTypeRewriter` infrastructure
used by type optimizations associates the new types with the old indices
just like it already does for names and also respects the input order
when rewriting types into a large recursion group.
By default, wasm-opt and other tools clear the recorded type indices
after parsing the module, so their default behavior is not modified by
this change.
Follow-on PRs will use the new flag in more tests, which will generate
large diffs but leave the tests in stable, more readable states that
will no longer change due to other changes to the optimizing type
sorting logic.
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Now that the header includes topological sort utilities in addition to
the utility that iterates over all topological orders, it makes more
sense for it to be named topological_sort.h
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As with all type optimizations, MinimizeRecGroups only changes private
types, which are the only types that are safe to modify. However, it is
important for correctness that MinimimizeRecGroups maintain separate
type identities for all types, whether public or private, to ensure that
casts that should differentiate two types cannot change behavior.
Previously the pass worked exclusively on private types, so there was
nothing preventing it from constructing a minimial rec group that
happened to have the same shape, and therefore type identity, as a
public rec group. #6886 exhibits a fuzzer test case where this happens
and changes the behavior of the program.
Fix the bug by recording all public rec group shapes and resolve
conflicts with these shapes by updating the shape of the conflicting
non-public type.
Fixes #6886.
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This saves memory and could in principle improve performance, although a
quick experiment with 30 samples on ReorderGlobals did not yield a
statistically significant improvement. At any rate, using Index is more
consistent with other parts of the code base.
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Most of our type optimization passes emit all non-public types as a
single large rec group, which trivially ensures that different types
remain different, even if they are optimized to have the same structure.
Usually emitting a single large rec group is fine, but it also means
that if the module is split, all of the types will need to be repeated
in all of the split modules. To better support this use case, add a pass
that can split the large rec group back into minimal rec groups, taking
care to preserve separate type identities by emitting different
permutations of the same group where possible or by inserting unused
brand types to differentiate them.
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