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WebAssembly specification, reference interpreter, and test suite.
https://webassembly.github.io/spec/
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A question about the order of exports #1762

Open yamt opened 3 months ago

yamt commented 3 months ago

does the order of exports in a module have any significance on the semantics of the module?

in the spec, i couldn't find anything based on the order.

otoh, some of wasm-c-api examples seem to rely on the order of exports: https://github.com/WebAssembly/wasm-c-api/blob/2ce1367c9d1271c83fb63bef26d896a2f290cd23/example/global.c#L136-L151

toywasm has an option to sort exports in-place to speed up export uniqueness check and later lookup. https://github.com/yamt/toywasm/blob/fce8455ae4adf73d99f7e9f8893122e536c4a63c/lib/module.c#L2294 (toywasm doesn't provide wasm-c-api.)

rossberg commented 3 months ago

Semantically, it doesn't matter for the module itself. This is in fact the case for the internal order of any section in the binary format — except for globals, where validation has recently been relaxed to allow initialisation dependencies in definition order.

But yes, the order of imports and exports is reflected in the module type, and some interfaces, such as the C API, use that order for simplicity.

yamt commented 3 months ago

i thought users of wasm-c-api were supposed to iterate over the list of exports to find relevant entries by their names/types/etc. so, i got a bit surprised to see there are wasm-c-api samples using the order as if there is externally visible "export index". is it even a valid use? if so, i suspect eg. wast should have a way to use the "export index" to test it.

rossberg commented 3 months ago

Users of the C API can still look up by name using the export types of the module. Just iterate both lists together, they don't have to assume a particular order. But if they happen to know the positional order then they can use that directly.

In other words, imports/exports can be viewed as both named and positional parameters/results.

Not sure how we could add a test for positional order to wast, though, since the JS API does not currently expose the order.

yamt commented 3 months ago

Not sure how we could add a test for positional order to wast, though, since the JS API does not currently expose the order.

ok.

i guess it's probably just one of cases where some api allow lower-level investigations than others. in an extreme case, an api can provide a way for users to perform byte-to-byte processing. well, it's even reasonable for things like custom sections.