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Clarify the ID and location of the exception section #27

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Clarify the ID and location of the exception section #27

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0dcdc92
Clarify the ID and location of the exception section
eholk Aug 15, 2017
710acf8
Refine section position and give rationale
eholk Aug 15, 2017
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21 changes: 19 additions & 2 deletions proposals/Exceptions.md
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Expand Up @@ -306,14 +306,31 @@ or defined:

### Exception section

The `exception` section is the named section 'exception'. The exception section
declares exception types using exception type signatures.
The `exception` section is the named section 'exception' with id 13. The
exception section may occur anywhere after the import section and before the
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The order of all known sections is fixed. We shouldn't lose that property. For maximum future-proofness I suggest inserting it between import section (2) and function section (3).

export section. There may be at most one exception section. The exception
section declares exception types using exception type signatures.

| Field | Type | Description |
|-------|------|-------------|
| count | `varuint32` | count of the number of exceptions to follow |
| sig | `except_type*` | The type signature of the data fields for each exception |

#### Rationale

The exception is assigned the number 13 because this is the next available
identifier.
The [spec](https://webassembly.github.io/spec/binary/modules.html#sections)
currently shows numbers 0 through 11 are assigned. Section 12 appears to be
reserved for the name section.
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@binji binji Aug 15, 2017 •
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Where is that mentioned?

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Where is what? The name section? I was going based on this line of code: https://cs.chromium.org/chromium/src/v8/src/wasm/module-decoder.h?type=cs&q=NameSection+f:v8&l=36

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Yeah, I meant where is the name sections specified as section 12. I'd say that v8 is wrong here -- the spec says nothing about the name section being numbered.

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Name section is a custom section, numbered zero, and using an explicit string as name.


The position of the exception section is driven by the requirement that items be
defined before they are used. Having the exception section after the import
section allows imported exceptions to be assigned the lower number
tags. Similarly, defining the exceptions before the export section allows
exceptions to be exported. Beyond these constraints, no additional requirements
are imposed.

### Import section

The import section is extended to include exception types by extending an
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