This change avoids needing to keep around an unused compiled
module. Instead, the result of compiling the wasm bytes is
given to the first instance. The module object and that instance object
point to the same compiled module. Instances are, then, cloned from
the compiled module the module object points to. When an instance is
collected, we make sure that the module object still has a clone
available, and, if the last instance is GC-ed, we also reset the compiled
module so that it does not reference its heap, so that it (==heap) may
be collected.
This is achieved by linking the clones in a double-linked list and
registering a finalizer for each. When we create an instance, we tie it
in the front of the list, making the module object point to it (O(1)). When
the finalizer is called, we relink the list over the dying object (O(1)). The
costliest operation is finalizing the last instance, since we need to visit
all wasm functions and reset heap references.
BUG=v8:5316
Review-Url: https://codereview.chromium.org/2305903002
Cr-Commit-Position: refs/heads/master@{#39153}
01f5af5157