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# Binary Encoding
This document describes the [portable](Portability.md) binary encoding of the
[Abstract Syntax Tree](AstSemantics.md) nodes.
The binary encoding is designed to allow fast startup, which includes reducing
download size and allow for quick decoding.
Reducing download size, is achieved through three layers:
* The **raw** binary encoding itself, natively decoded by the browser, and to
be standardized in the [MVP](MVP.md).
* **Specific** compression to the binary encoding, that is unreasonable to
expect a generic compression algorithm like gzip to achieve.
* This is not meant to be standardized, at least not initially, as it can be
done with a downloaded decompressor that runs as web content on the client,
and in particular can be implemented in a [polyfill](Polyfill.md).
* We can do better than generic compression because we are aware of the AST
structure and other details:
* For example, macro compression that
[deduplicates AST trees](https://github.com/WebAssembly/design/issues/58#issuecomment-101863032)
can focus on AST nodes + their children, thus having `O(nodes)` entities
to worry about, compared to generic compression which in principle would
need to look at `O(bytes*bytes)` entities. Such macros would allow the
logical equivalent of `#define ADD1(x) (x+1)`, i.e., to be
parametrized. Simpler macros (`#define ADDX1 (x+1)`) can implement useful
features like constant pools.
* Another example is reordering of functions and some internal nodes, which
we know does not change semantics, but
[can improve general compression](http://www.rfk.id.au/blog/entry/cromulate-improve-compressibility/).
* **Generic** compression, such as gzip, already supported in browsers, or LZMA
and other compression algorithms, which might be standardized as well.
Each of the three layers work to find compression opportunities to the best of
its abilities, without encroaching upon the subsequent layer's compression
opportunities.
## Why a binary encoding instead of a text-only representation
Given that text is so compressible and it is well known that it is hard to beat
gzipped source, is there any win from having a binary format over a text format?
Yes:
* Large reductions in payload size can still significantly decrease the
compressed file size.
* Experimental results from a
[polyfill prototype](https://github.com/WebAssembly/polyfill-prototype-1) show the
gzipped binary format to be about 20-30% smaller than the corresponding
gzipped asm.js.
* A binary format that represents the names of variables and functions with raw
indices instead of strings is much faster to decode: array indexing
vs. dictionary lookup.
* Experimental results from a
[polyfill prototype](https://github.com/WebAssembly/polyfill-prototype-1) show that
decoding the binary format is about 23× faster than parsing the
corresponding asm.js source (using
[this demo](https://github.com/lukewagner/AngryBotsPacked), comparing
*just* parsing in SpiderMonkey (no validation, IR generation) to *just*
decoding in the polyfill (no asm.js code generation).
## Building blocks
### Variable-length integers
* 31% size reduction before compression, 7% size reduction after compression.
* [LEB128](http://en.wikipedia.org/wiki/LEB128) except limited to uint32_t payloads.
## Global structure
* A module contains:
* a header followed by
* a table containing, for each section, its type, offset (within the module), and byte length, followed by
* a sequence of sections.
* A section contains:
* a header followed by
* the section contents (specific to the section type)
* A code section contains:
* the generic section header
* a table containing, for each function, it's signature, offset (within the section), sorted by offset, followed by
* a sequence of functions
* A function contains:
* a table containing, for each type, how many locals are indexed by the function body of that type
* the serialized AST
## Serialized AST
* Use a preorder encoding of the AST
* Efficient single-pass validation+compilation and polyfill
* Allows context-dependent index spaces (described above)
* The data of a node (if there is any), is written immediately after the opcode and before child nodes
* The opcode statically determines what follows, so no generic metadata is necessary.
* Examples
* Given a simple AST node: `struct I32Add { AstNode *left, *right; }`
* First write the opcode of `I32Add` (1 byte)
* Then recursively write the left and right nodes.
* Given a call AST node: `struct Call { uint32_t callee; vector<AstNode*> args; }`
* First write the opcode of `Call` (1 byte)
* Then write the (variable-length) integer `Call::callee` (1-5 bytes)
* Then recursively write each arg node (arity is determined by looking up `callee` in table of signatures)
## Backwards Compatibility
As explained above, for size- and decode-efficiency, the binary format will serialize AST nodes,
their contents and children using dense integer indices and without any kind of embedded metadata
or tagging. This raises the question of how to reconcile the efficient encoding with the
backwards-compatibility goals.
Specifically, we'd like to avoid the situation where a future version of WebAssembly has features
F1 and F2 and vendor V1 implements F1, assigning the next logical opcode indices to F1's new
opcodes, and V2 implements F2, assigning the same next logical opcode indices to F2's new opcodes
and now a single binary has ambiguous semantics if it tries to use either F1 or F2. This type of
non-linear feature addition is commonplace in JS and Web APIs and is guarded against by
having unique names for unique features (and associated [conventions](https://hsivonen.fi/vendor-prefixes)).
The current proposal is to maintain both the efficiency of indices in the [serialized AST](BinaryEncoding.md#serialized-ast) and the established
conflict-avoidance practices surrounding string names:
* The WebAssembly spec doesn't define any global index spaces
* So, as a general rule, no magic numbers in the spec (other than the literal [magic number](http://en.wikipedia.org/wiki/Magic_number_%28programming%29)).
* Instead, a module defines its *own* local index spaces of opcodes by providing tables *of names*.
* So what the spec *would* define is a set of names and their associated semantics.
* If the implementation encounters a name it doesn't implement, by default an error is thrown while loading.
* However, a name *may* include a corresponding polyfill function (identified by index
into the function array) to be called if the name isn't natively implemented. (There are a lot
more details to figure out here.)
* To avoid (over time) large index-space declaration sections that are largely the same
between modules, finalized versions of standards would define named baseline index spaces
that modules could optionally use as a starting point to further refine.
* For example, to use all of [the MVP](MVP.md) plus [SIMD](EssentialPostMVPFeatures.md#fixed-width-simd)
the declaration could be "base" followed by the list of SIMD opcodes used.
* This feature would also be most useful for people handwriting the [text format](TextFormat.md).
* However, such a version declaration does not establish a global "version" for the module
or affect anything outside of the initialization of the index spaces; decoders would
remain versionless and simply add cases for new *names* (as with current JS parsers).
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