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[FuzzExec] Give host imported functions exact types - #9314
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In WebAssembly with Custom Descriptors, an inexact function import has a static inexact type `(ref $t)` because at instantiation time an open type `$t` could be satisfied by a subtype. However, at runtime, every concrete function instance (including host functions) has an exact dynamic type, and when `$t` is a final type (`!isOpen()`), `GCTypeUtils::evaluateCastCheck` optimizes casts of `(ref $t)` to `(ref (exact $t))` to succeed. When `LoggingExternalInterface::getImportedFunction` was updated in #7993 to pass `import->type` instead of `import->type.getHeapType()`, host imported functions were given an inexact runtime `Literal` type, causing runtime casts of host imported functions to `(ref (exact $t))` to fail in `--fuzz-exec` after passes like `--remove-unused-brs` folded `br_on_cast` to `ref.cast`. Fix this by passing `import->type.with(Exact)` when constructing host function literals in `LoggingExternalInterface::getImportedFunction`.
tlively
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stevenfontanella
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October 27, 2026 04:13
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This pull request updates the LoggingExternalInterface in src/tools/execution-results.h to ensure that host functions are returned with their exact import type by using import->type.with(Exact). It also adds corresponding test cases in imported-func.wast and imported-func.wast.second to verify the exact type casting behavior of host imports. There are no review comments, so I have no feedback to provide.
stevenfontanella
approved these changes
Oct 27, 2026
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In WebAssembly with Custom Descriptors, an inexact function import has a static inexact type
(ref $t)because at instantiation time an open type$tcould be satisfied by a subtype. However, at runtime, every concrete function instance (including host functions) has an exact dynamic type, and when$tis a final type (!isOpen()),GCTypeUtils::evaluateCastCheckoptimizes casts of(ref $t)to(ref (exact $t))to succeed.When
LoggingExternalInterface::getImportedFunctionwas updated in #7993 to passimport->typeinstead ofimport->type.getHeapType(), host imported functions were given an inexact runtimeLiteraltype, causing runtime casts of host imported functions to(ref (exact $t))to fail in--fuzz-execafter passes like--remove-unused-brsfoldedbr_on_casttoref.cast. Fix this by passingimport->type.with(Exact)when constructing host function literals inLoggingExternalInterface::getImportedFunction.