For host VM code, we use plain reference counting, where cloning increments
the reference count, and dropping decrements it. We can avoid many of the
on-stack increment/decrement operations that typically plague the
performance of reference counting via Rust's ownership and borrowing system.
Moving a `VMExternRef` avoids mutating its reference count, and borrowing it
either avoids the reference count increment or delays it until if/when the
`VMExternRef` is cloned.
When passing a `VMExternRef` into compiled Wasm code, we don't want to do
reference count mutations for every compiled `local.{get,set}`, nor for
every function call. Therefore, we use a variation of **deferred reference
counting**, where we only mutate reference counts when storing
`VMExternRef`s somewhere that outlives the activation: into a global or
table. Simultaneously, we over-approximate the set of `VMExternRef`s that
are inside Wasm function activations. Periodically, we walk the stack at GC
safe points, and use stack map information to precisely identify the set of
`VMExternRef`s inside Wasm activations. Then we take the difference between
this precise set and our over-approximation, and decrement the reference
count for each of the `VMExternRef`s that are in our over-approximation but
not in the precise set. Finally, the over-approximation is replaced with the
precise set.
The `VMExternRefActivationsTable` implements the over-approximized set of
`VMExternRef`s referenced by Wasm activations. Calling a Wasm function and
passing it a `VMExternRef` moves the `VMExternRef` into the table, and the
compiled Wasm function logically "borrows" the `VMExternRef` from the
table. Similarly, `global.get` and `table.get` operations clone the gotten
`VMExternRef` into the `VMExternRefActivationsTable` and then "borrow" the
reference out of the table.
When a `VMExternRef` is returned to host code from a Wasm function, the host
increments the reference count (because the reference is logically
"borrowed" from the `VMExternRefActivationsTable` and the reference count
from the table will be dropped at the next GC).
For more general information on deferred reference counting, see *An
Examination of Deferred Reference Counting and Cycle Detection* by Quinane:
https://openresearch-repository.anu.edu.au/bitstream/1885/42030/2/hon-thesis.pdf
cc #929Fixes#1804
* Implement trap info in Lightbeam
* Start using wasm-reader instead of wasmparser for parsing operators
* Update to use wasm-reader, some reductions in allocation, support source location tracking for traps, start to support multi-value
The only thing that still needs to be supported for multi-value is stack returns, but we need to make it compatible with Cranelift.
* Error when running out of registers (although we'd hope it should be impossible) instead of panicking
* WIP: Update Lightbeam to work with latest Wasmtime
* WIP: Update Lightbeam to use current wasmtime
* WIP: Migrate to new system for builtin functions
* WIP: Update Lightbeam to work with latest Wasmtime
* Remove multi_mut
* Format
* Fix some bugs around arguments, add debuginfo offset tracking
* Complete integration with new Wasmtime
* Remove commented code
* Fix formatting
* Fix warnings, remove unused dependencies
* Fix `iter` if there are too many elements, fix compilation for latest wasmtime
* Fix float arguments on stack
* Remove wasm-reader and trap info work
* Allocate stack space _before_ passing arguments, fail if we can't zero a xmm reg
* Fix stack argument offset calculation
* Fix stack arguments in Lightbeam
* Re-add WASI because it somehow got removed during rebase
* Workaround for apparent `type_alias_impl_trait`-related bug in rustdoc
* Fix breakages caused by rebase, remove module offset info as it is unrelated to wasmtime integration PR and was broken by rebase
* Add TODO comment explaining `lightbeam::ModuleContext` trait
* Implement interrupting wasm code, reimplement stack overflow
This commit is a relatively large change for wasmtime with two main
goals:
* Primarily this enables interrupting executing wasm code with a trap,
preventing infinite loops in wasm code. Note that resumption of the
wasm code is not a goal of this commit.
* Additionally this commit reimplements how we handle stack overflow to
ensure that host functions always have a reasonable amount of stack to
run on. This fixes an issue where we might longjmp out of a host
function, skipping destructors.
Lots of various odds and ends end up falling out in this commit once the
two goals above were implemented. The strategy for implementing this was
also lifted from Spidermonkey and existing functionality inside of
Cranelift. I've tried to write up thorough documentation of how this all
works in `crates/environ/src/cranelift.rs` where gnarly-ish bits are.
A brief summary of how this works is that each function and each loop
header now checks to see if they're interrupted. Interrupts and the
stack overflow check are actually folded into one now, where function
headers check to see if they've run out of stack and the sentinel value
used to indicate an interrupt, checked in loop headers, tricks functions
into thinking they're out of stack. An interrupt is basically just
writing a value to a location which is read by JIT code.
When interrupts are delivered and what triggers them has been left up to
embedders of the `wasmtime` crate. The `wasmtime::Store` type has a
method to acquire an `InterruptHandle`, where `InterruptHandle` is a
`Send` and `Sync` type which can travel to other threads (or perhaps
even a signal handler) to get notified from. It's intended that this
provides a good degree of flexibility when interrupting wasm code. Note
though that this does have a large caveat where interrupts don't work
when you're interrupting host code, so if you've got a host import
blocking for a long time an interrupt won't actually be received until
the wasm starts running again.
Some fallout included from this change is:
* Unix signal handlers are no longer registered with `SA_ONSTACK`.
Instead they run on the native stack the thread was already using.
This is possible since stack overflow isn't handled by hitting the
guard page, but rather it's explicitly checked for in wasm now. Native
stack overflow will continue to abort the process as usual.
* Unix sigaltstack management is now no longer necessary since we don't
use it any more.
* Windows no longer has any need to reset guard pages since we no longer
try to recover from faults on guard pages.
* On all targets probestack intrinsics are disabled since we use a
different mechanism for catching stack overflow.
* The C API has been updated with interrupts handles. An example has
also been added which shows off how to interrupt a module.
Closes#139Closes#860Closes#900
* Update comment about magical interrupt value
* Store stack limit as a global value, not a closure
* Run rustfmt
* Handle review comments
* Add a comment about SA_ONSTACK
* Use `usize` for type of `INTERRUPTED`
* Parse human-readable durations
* Bring back sigaltstack handling
Allows libstd to print out stack overflow on failure still.
* Add parsing and emission of stack limit-via-preamble
* Fix new example for new apis
* Fix host segfault test in release mode
* Fix new doc example
This commit makes the following changes to unwind information generation in
Cranelift:
* Remove frame layout change implementation in favor of processing the prologue
and epilogue instructions when unwind information is requested. This also
means this work is no longer performed for Windows, which didn't utilize it.
It also helps simplify the prologue and epilogue generation code.
* Remove the unwind sink implementation that required each unwind information
to be represented in final form. For FDEs, this meant writing a
complete frame table per function, which wastes 20 bytes or so for each
function with duplicate CIEs. This also enables Cranelift users to collect the
unwind information and write it as a single frame table.
* For System V calling convention, the unwind information is no longer stored
in code memory (it's only a requirement for Windows ABI to do so). This allows
for more compact code memory for modules with a lot of functions.
* Deletes some duplicate code relating to frame table generation. Users can
now simply use gimli to create a frame table from each function's unwind
information.
Fixes#1181.
* wasmtime: Pass around more contexts instead of fields
This commit refactors some wasmtime internals to pass around more
context-style structures rather than individual fields of each
structure. The intention here is to make the addition of fields to a
structure easier to plumb throughout the internals of wasmtime.
Currently you need to edit lots of functions to pass lots of parameters,
but ideally after this you'll only need to edit one or two struct fields
and then relevant locations have access to the information already.
Updates in this commit are:
* `debug_info` configuration is now folded into `Tunables`. Additionally
a `wasmtime::Config` now holds a `Tunables` directly and is passed
into an internal `Compiler`. Eventually this should allow for direct
configuration of the `Tunables` attributes from the `wasmtime` API,
but no new configuration is exposed at this time.
* `ModuleTranslation` is now passed around as a whole rather than
passing individual components to allow access to all the fields,
including `Tunables`.
This was motivated by investigating what it would take to optionally
allow loops and such to get interrupted, but that sort of codegen
setting was currently relatively difficult to plumb all the way through
and now it's hoped to be largely just an addition to `Tunables`.
* Fix lightbeam compile
* Improve robustness of cache loading/storing
Today wasmtime incorrectly loads compiled compiled modules from the
global cache when toggling settings such as optimizations. For example
if you execute `wasmtime foo.wasm` that will cache globally an
unoptimized version of the wasm module. If you then execute `wasmtime -O
foo.wasm` it would then reload the unoptimized version from cache, not
realizing the compilation settings were different, and use that instead.
This can lead to very surprising behavior naturally!
This commit updates how the cache is managed in an attempt to make it
much more robust against these sorts of issues. This takes a leaf out of
rustc's playbook and models the cache with a function that looks like:
fn load<T: Hash>(
&self,
data: T,
compute: fn(T) -> CacheEntry,
) -> CacheEntry;
The goal here is that it guarantees that all the `data` necessary to
`compute` the result of the cache entry is hashable and stored into the
hash key entry. This was previously open-coded and manually managed
where items were hashed explicitly, but this construction guarantees
that everything reasonable `compute` could use to compile the module is
stored in `data`, which is itself hashable.
This refactoring then resulted in a few workarounds and a few fixes,
including the original issue:
* The `Module` type was split into `Module` and `ModuleLocal` where only
the latter is hashed. The previous hash function for a `Module` left
out items like the `start_func` and didn't hash items like the imports
of the module. Omitting the `start_func` was fine since compilation
didn't actually use it, but omitting imports seemed uncomfortable
because while compilation didn't use the import values it did use the
*number* of imports, which seems like it should then be put into the
cache key. The `ModuleLocal` type now derives `Hash` to guarantee that
all of its contents affect the hash key.
* The `ModuleTranslationState` from `cranelift-wasm` doesn't implement
`Hash` which means that we have a manual wrapper to work around that.
This will be fixed with an upstream implementation, since this state
affects the generated wasm code. Currently this is just a map of
signatures, which is present in `Module` anyway, so we should be good
for the time being.
* Hashing `dyn TargetIsa` was also added, where previously it was not
fully hashed. Previously only the target name was used as part of the
cache key, but crucially the flags of compilation were omitted (for
example the optimization flags). Unfortunately the trait object itself
is not hashable so we still have to manually write a wrapper to hash
it, but we likely want to add upstream some utilities to hash isa
objects into cranelift itself. For now though we can continue to add
hashed fields as necessary.
Overall the goal here was to use the compiler to expose what we're not
hashing, and then make sure we organize data and write the right code to
ensure everything is hashed, and nothing more.
* Update crates/environ/src/module.rs
Co-Authored-By: Peter Huene <peterhuene@protonmail.com>
* Fix lightbeam
* Fix compilation of tests
* Update the expected structure of the cache
* Revert "Update the expected structure of the cache"
This reverts commit 2b53fee426a4e411c313d8c1e424841ba304a9cd.
* Separate the cache dir a bit
* Add a test the cache is busted with opt levels
* rustfmt
Co-authored-by: Peter Huene <peterhuene@protonmail.com>
* Remove all global state from the caching system
This commit is a continuation of an effort to remove usages of
`lazy_static!` and similar global state macros which can otherwise be
accomodated with passing objects around. Previously there was a global
cache system initialized per-process, but it was initialized in a bit of
a roundabout way and wasn't actually reachable from the `wasmtime` crate
itself. The changes here remove all global state, refactor many of the
internals in the cache system, and makes configuration possible through
the `wasmtime` crate.
Specifically some changes here are:
* Usage of `lazy_static!` and many `static` items in the cache module
have all been removed.
* Global `cache_config()`, `worker()`, and `init()` functions have all
been removed. Instead a `CacheConfig` is a "root object" which
internally owns its worker and passing around the `CacheConfig` is
required for cache usage.
* The `wasmtime::Config` structure has grown options to load and parse
cache files at runtime. Currently only loading files is supported,
although we can likely eventually support programmatically configuring
APIs as well.
* Usage of the `spin` crate has been removed and the dependency is removed.
* The internal `errors` field of `CacheConfig` is removed, instead
changing all relevant methods to return a `Result<()>` instead of
storing errors internally.
* Tests have all been updated with the new interfaces and APIs.
Functionally no real change is intended here. Usage of the `wasmtime`
CLI, for example, should still enable the cache by default.
* Fix lightbeam compilation
* Fix fuzz target compilation.
* Bump version to 0.7.0
* Temporarily disable fuzz tests
Temporarily disable fuzz tests until https://github.com/bytecodealliance/cranelift/issues/1216 is resolved.
* Fix publish-all.sh to not modify the witx crate.
* Remove the "publish = false" attribute from Lightbeam.
* Add a README.md for wasmtime-interface-types.
* Remove the "rust" category.
This fixes the following warning:
warning: the following are not valid category slugs and were ignored: rust. Please see https://crates.io/category_slugs for the list of all category slugs.
* Mark wasmtime-cli as "publish = false".
* Sort the publishing rules in topological order.
Also, publish nightly-only crates with cargo +nightly.
