Add support for snapshotting tables
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调研方向
首先跟踪 issue 中描述的 Wizer instrumentation 和 snapshotting 路径；其中没有指定文件或测试。确定如何表示跨链接实例的 funcref 标识，以及如何处理导入函数和 WASI 函数，同时保持 externref 的初始化行为不变。完成标准是两种 table 类型都可以进行 snapshot，并覆盖 module-linking 的情况。
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描述
Right now there are two potential kinds of tables:
funcreftablesexternreftables
The latter are easy to support, since Wasm can't construct an externref, only receive it from the outside world. As long as we don't allow any externrefs into the Wasm during initialization, then all we have to do is record the initialized size of these tables.
funcref tables are a little harder. Wasm can create funcrefs via the ref.func instruction. So we need a way to figure out which function each wasmtime::FuncRef in a table we are snapshotting is (i.e. what is the local function index of the ith table element?). wasmtime doesn't expose anything like that. I have an idea, however:
- during Wizer's instrumentation phase:
- add two new globals to the module:
$wizer_getting_func_indexand$wizer_func_index - add the following snippet to the start of every function in the module:
block global.get $wizer_getting_func_index i32.eqz br_if 0 i32.const <this function's local index> global.set $wizer_func_index unreachable end
- add two new globals to the module:
- then, during snapshotting, to get a given
funcref's local function index, we set the$wizer_getting_func_indexglobal non-zero, call the function, and then it will set its local function index to the$wizer_func_indexglobal and trap. - the dummy functions we create for imports will need a similar dance, but implemented in native code rather than Wasm. Something similar for wasi functions too.
The one problem is that local function index isn't quite enough in the face of module linking. Instead we will need both the instance "id" (whatever that this; DFS index?) as well as the local function index in that instance. More to think about w.r.t. module linking here...
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bytecodealliance/wizer 的其他 Issue
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Broken link (2021 WebAssembly Summit)可能重新可做 关联的 PR 已关闭且未合并。 未关闭
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