Merge pull request #65 from WebAssembly/add-producers-section

Add initial ProducersSection.md

ProducersSection.md

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+# Producers Section
+
+The purpose of the producers section is to provide an optional,
+highly-structured record of all the distinct tools that were used to produce
+a given WebAssembly module. A primary purpose of this record is to allow
+broad analysis of toolchain usage in the wild, which can help inform both wasm
+producers and consumers.
+
+The producers section is a
+[custom section](https://webassembly.github.io/spec/core/binary/modules.html#custom-section)
+and thus has no semantic effects and can be stripped at any time.
+Since the producers section is relatively small, tools are encouraged to emit
+the section or include themselves in an existing section by default, keeping
+the producers section even in release builds.
+
+WebAssembly consumers should avoid using the producers section to derive
+optimization hints. To ensure portable performance, hints should be
+standardized in a separate custom section, probably in the core spec's
+[Custom Sections appendix](https://webassembly.github.io/spec/core/appendix/custom.html).
+
+An additional goal of the producers section is to provide a discrete, but
+easily-growable [list of known tools/languages](#known-list) for each
+record field. This avoids the skew that otherwise happens with unstructured
+strings. Unknown names do not invalidate an otherwise-valid producers section.
+However, wasm consumers may provide less accurate telemetry results for unknown
+names or even emit diagnostics encouraging the name to be put on the known list.
+
+Since version information is useful, but highly-variable, each field value
+is accompanied with a version string so that the name can remain stable
+over time without requiring frequent updates to the known list.
+
+## Custom Section
+
+Custom section `name` field: `producers`
+
+The producers section may appear only once, and only after the
+[Name section](https://webassembly.github.io/spec/core/appendix/custom.html#name-section).
+
+The producers section contains a sequence of fields with unique names, where the
+end of the last field must coincide with the last byte of the producers section:
+
+| Field       | Type        | Description |
+| ----------- | ----------- | ----------- |
+| field_count | `varuint32` | number of fields that follow |
+| fields      | `field*`    | sequence of field_count `field` records |
+
+where a `field` is encoded as:
+
+| Field             | Type | Description |
+| ----------------- | ---- | ----------- |
+| field_name        | [name][name-ref] | name of this field |
+| field_value_count | `varuint32` | number of value strings that follow |
+| field_values      | `versioned-name*` | sequence of field_value_count name-value pairs |
+
+where a `versioned-name` is encoded as:
+
+| Field   | Type | Description |
+| ------- | ---- | ----------- |
+| name    | [name][name-ref] | name of the language/tool |
+| version | [name][name-ref] | version of the language/tool |
+
+with the additional constraint that each field_name in the list must be unique
+and found in the first column of the following table, and each of a given field_name's
+field_values's name strings must be unique and found in the second column of
+the field_name's row.
+
+| field_name     | field_value name strings |
+| -------------- | -------------------- |
+| `language`     | [source language list](#source-languages) |
+| `processed-by` | [individual tool list](#individual-tools) |
+| `sdk`          | [SDK list](#sdks)    |
+
+[name-ref]: https://webassembly.github.io/spec/core/binary/values.html#names
+
+## Known list
+
+The following lists contain all the known names for the fields listed above.
+**If your tool is not on this list and you'd like it to be, please submit a PR.**
+
+### Source Languages
+
+It is possible for multiple source languages to be present in a single module
+when the output of multiple compiled languages are statically linked together.
+
+* `wat`
+* `C`
+* `C++`
+
+### Individual Tools
+
+It is possible (and common) for multiple tools to be used in the overall
+pipeline that produces and optimizes a given wasm module.
+
+* `wabt`
+* `LLVM`
+* `lld`
+* `Binaryen`
+
+### SDKs
+
+While an SDK is technically just another tool, the `sdk` field designates the
+top-level "thing" that the developer installs and interacts with directly to
+produce the wasm module.
+
+* `Emscripten`
+
+## Text format
+
+TODO