Implement the ref.test Wasm GC instruction
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This commit implements the `ref.test` instruction, which tests whether a reference is of a given type. We implement inline fast paths for abstract types, but currently rely on an out-of-line libcall for concrete types in the general case. This is known to be suboptimal. (FWIW, we also emit a fast path in front of the libcall where we first check the actual type and expected type for equality and skip the libcall if they are equal.) This implementation is expected to be improved in the future by exposing a module's types' supertypes arrays to Wasm, so that the Wasm can do the O(1) subtype checks inline. This will make the vast majority of all `ref.test`s inlinable. After that, the only remaining case that would require out-of-line libcalls would be when a module is given an instance of a type that it did not itself define (but which could be a subtype of a type it defined, for example, and which itself might not even have been defined until after this module's instance was created!)
alexcrichton
approved these changes
Oct 8, 2024
…fusion between similarly named spec test
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This commit implements the
ref.testinstruction, which tests whether a reference is of a given type.We implement inline fast paths for abstract types, but currently rely on an out-of-line libcall for concrete types in the general case. This is known to be suboptimal. (FWIW, we also emit a fast path in front of the libcall where we first check the actual type and expected type for equality and skip the libcall if they are equal.)
This implementation is expected to be improved in the future by exposing a module's types' supertypes arrays to Wasm, so that the Wasm can do the O(1) subtype checks inline. This will make the vast majority of all
ref.tests inlinable. After that, the only remaining case that would require out-of-line libcalls would be when a module is given an instance of a type that it did not itself define (but which could be a subtype of a type it defined, for example, and which itself might not even have been defined until after this module's instance was created!)