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Add architecture document for wasm export (#2049)

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core/iwasm/README.md
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## Wasm function
- [Wasm function architecture](./doc/wasm_function.MD)
## Exports
- [Wasm export architecture](./doc/wasm_exports.MD)

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core/iwasm/doc/wasm_exports.MD Normal file
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# Wasm exports
The internal data structure for Wasm exports:
![](./images/wasm_exports.svg)
## Setup exports for Module
The array data structure pointed by `WASMModule::exports` is setup during loading a module. Basically the runtime will load the exports sections from the module file content, and construct an array of C struct `WASMExport`.
A `WASMExport` item contains three elements that map the Wasm file exports section structure:
- name: the name shown to external
- kind: total 4 export types: function, globals, memory, table
- index: As all the 4 export types are organized in array, this refers to index in target export type
## Function exports
### use function exports
function exports are often used in two situations:
1. **call by host**: runtime API `wasm_runtime_lookup_function` will walk through the array of `WASMModuleInstance::export_functions` and compare the exported name with given target symbol name in the function parameter. If any array item matches the name, it then returns the value of field `function` which points to associated function instance (WASMFunctionInstance)
2. **import by another module**: During linking multiple modules, the runtime saves the pointer of exported WASMFunctionInstance in the local WASMFunctionInstance of importing module.
### setup for instance
The data structure pointed by `WASMModuleInstance::export_functions` is set up during instantiating module instance.
The runtime will walk through the `WASMModule::exports` array and find all the item with kind equal to "function". Create a node of `WASMExportFuncInstance` for each matching, find the associated `WASMFunctionInstance` object and save its address in the field `function`.