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| -rw-r--r-- | src/pass.h | 22 |
1 files changed, 22 insertions, 0 deletions
diff --git a/src/pass.h b/src/pass.h index 60d6692a1..f4b15a320 100644 --- a/src/pass.h +++ b/src/pass.h @@ -191,6 +191,28 @@ struct PassOptions { // to the outside may be inspected in detail, which prevents us from e.g. // changing a type that escapes (so we can't remove or refine fields on an // escaping struct type, for example). + // + // Note that the module can still have imports and exports - otherwise it + // could do nothing at all! - so the meaning of "closed world" is a little + // subtle here. We do still want to keep imports and exports unchanged, as + // they form a contract with the outside world. For example, if an import has + // two parameters, we can't remove one of them. A nuance regarding that is how + // type equality works between wasm modules using the isorecursive type + // system: not only do we need to not remove a parameter as just mentioned, + // but we also want to keep types of things on the boundary unchanged. For + // example, we should not change an exported function's signature, as the + // outside may need that type to properly call the export. + // * For now we disallow nontrivial types on the boundary, that is, you + // cannot use a custom struct type as a function parameter type. Such a + // type would not be optimizable due to the constraint to not modify the + // type of the import/export, but it is very simple to use a type in a + // single import/export and then all of its supertypes become + // unoptimizable; likewise, in some optimizations all subtypes may be + // affected (say in not being able to remove a field from them). Overall, + // there is a risk of missing out on significant optimization + // opportunities here, and for that reason we error on using such types on + // the boundary for now. Instead, use basic types like anyref, externref, + // etc. bool closedWorld = false; // Whether to try to preserve debug info through, which are special calls. bool debugInfo = false; |