The optimization to skip cloning the destination object on partial
stores when the buffer is already owned by the current memory view
mutated obj.buffer.buf in place. The previous v.buf clone only handled
the case where the store value itself aliased that buffer; it did not
cover the more common case where an earlier partial load had returned
a slice into the same buffer. The in-place store would then corrupt
the previously loaded value, breaking code such as cloudflare/bn256
where the gfP arithmetic loads, computes, and stores within the same
global during package initialization.
Fix this in load instead: when the source object is owned by the
current memory view, copy the loaded slice so the returned value is
independent of the live buffer. The v.buf clone in store is no longer
needed and is removed.
Updates the regression test added in the previous commit to reflect
the corrected behavior.
Adds a third init function to the store testdata: an initial partial
store puts the buffer into the current memory view, then a partial load
is followed by another partial store at the same offset, and finally the
loaded value is written to a separate global.
The expected output captures the current (incorrect) behavior of the
in-place partial store optimization: the loaded value gets corrupted by
the subsequent in-place store. The next commit fixes this.
memoryView.store cloned the entire destination object on every partial
store. Compiling a program that pulls in golang.org/x/text/collate hit
this hard: its generated tables are initialized one element at a time,
and each element store copied the whole global, making interp quadratic
in the table size.
Switch to copy-on-first-write per memory view: clone the object the
first time this view writes to it, then mutate that private copy in
place on later partial stores. Rollback is unaffected because revert
still discards the view's objects wholesale.
Two extra safety tweaks fall out of this:
- Full overwrites now clone the incoming value, so the stored buffer
can never alias state held by the caller.
- Partial in-place stores snapshot the source buffer, so a store
whose source is a load from the same object (overlapping or not)
still sees the pre-store bytes.
Add an interp golden test exercising both the overlapping load/store
case and a cross-object aliased load/store case.
Jake Bailey