Add the "null" garbage collector (#9484)

* Add the "null" garbage collector

The null collector does not actually collect any garbage, it simply
bump-allocates until the heap is exhausted, at which point further allocation
attempts will fail. It does not require any GC barriers.

Users can configure which collector to use via the `wasmtime::Config::collector`
method or the `-C collector=drc|null` CLI flag. The `wasmtime::Collector` enumeration,
similar to the `wasmtime::Strategy` enumeration but for choosing a collector
rather than a compiler, additionally has a table summarizing the properties and
characteristics of our current set of collector implementations.

Finally, we also run `.wast` tests that exercise GC types under both the DRC and
null collectors. I tried to avoid running tests that are not related to GC under
both configurations just to avoid a combinatorial blow up.

* cargo fmt

* fix +gc -gc-null -gc-drc build

* Fix some warnings in various cargo feature combo builds

* Fix some more warnings in certain build configs

* Fix unit tests for null GC

* Fill in some placeholder comments that I forgot to write

* Fix issues where we ask for a GC store when we don't actually need one

Which was causing test failures, since we no longer return a GC store with a
dummy heap.

* Add fuzzing config support for different collectors

* address review feedback

* fix cmake tests

* Fix test compilation after rebase

* Fix GC tests under MIRI

.github/workflows/main.yml

@@ -369,6 +369,12 @@ jobs:
               -p wasmtime --no-default-features --features gc
               -p wasmtime --no-default-features --features runtime,gc
               -p wasmtime --no-default-features --features cranelift,gc
+              -p wasmtime --no-default-features --features gc-drc
+              -p wasmtime --no-default-features --features runtime,gc-drc
+              -p wasmtime --no-default-features --features cranelift,gc-drc
+              -p wasmtime --no-default-features --features gc-null
+              -p wasmtime --no-default-features --features runtime,gc-null
+              -p wasmtime --no-default-features --features cranelift,gc-null
               -p wasmtime --no-default-features --features runtime
               -p wasmtime --no-default-features --features threads
               -p wasmtime --no-default-features --features runtime,threads

Cargo.toml

@@ -391,6 +391,8 @@ default = [
   "component-model",
   "threads",
   "gc",
+  "gc-drc",
+  "gc-null",
   "winch",
 
   # Enable some nice features of clap by default, but they come at a binary size
@@ -441,7 +443,9 @@ coredump = ["wasmtime-cli-flags/coredump"]
 addr2line = ["wasmtime/addr2line"]
 debug-builtins = ["wasmtime/debug-builtins"]
 threads = ["wasmtime-cli-flags/threads"]
-gc = ["wasmtime-cli-flags/gc"]
+gc = ["wasmtime-cli-flags/gc", "wasmtime/gc"]
+gc-drc = ["gc", "wasmtime/gc-drc", "wasmtime-cli-flags/gc-drc"]
+gc-null = ["gc", "wasmtime/gc-null", "wasmtime-cli-flags/gc-null"]
 
 # CLI subcommands for the `wasmtime` executable. See `wasmtime $cmd --help`
 # for more information on each subcommand.

crates/c-api/Cargo.toml

@@ -52,6 +52,8 @@ addr2line = ["wasmtime/addr2line"]
 demangle = ["wasmtime/demangle"]
 threads = ["wasmtime/threads"]
 gc = ["wasmtime/gc"]
+gc-drc = ["wasmtime/gc-drc"]
+gc-null = ["wasmtime/gc-null"]
 cranelift = ['wasmtime/cranelift']
 winch = ['wasmtime/winch']
 # ... if you add a line above this be sure to change the other locations

crates/c-api/artifact/Cargo.toml

@@ -34,6 +34,8 @@ default = [
   'demangle',
   'threads',
   'gc',
+  'gc-drc',
+  'gc-null',
   'cranelift',
   'winch',
   # ... if you add a line above this be sure to change the other locations
@@ -52,6 +54,8 @@ demangle = ["wasmtime-c-api/demangle"]
 wat = ["wasmtime-c-api/wat"]
 threads = ["wasmtime-c-api/threads"]
 gc = ["wasmtime-c-api/gc"]
+gc-drc = ["wasmtime-c-api/gc-drc"]
+gc-null = ["wasmtime-c-api/gc-null"]
 cranelift = ["wasmtime-c-api/cranelift"]
 winch = ["wasmtime-c-api/winch"]
 # ... if you add a line above this be sure to read the comment at the end of

crates/c-api/build.rs

@@ -15,6 +15,8 @@ const FEATURES: &[&str] = &[
     "DEMANGLE",
     "THREADS",
     "GC",
+    "GC_DRC",
+    "GC_NULL",
     "CRANELIFT",
     "WINCH",
 ];

crates/c-api/cmake/features.cmake

@@ -38,6 +38,8 @@ feature(addr2line ON)
 feature(demangle ON)
 feature(threads ON)
 feature(gc ON)
+feature(gc-drc ON)
+feature(gc-null ON)
 feature(async ON)
 feature(cranelift ON)
 feature(winch ON)

crates/c-api/include/wasmtime/conf.h.in

@@ -20,6 +20,8 @@
 #cmakedefine WASMTIME_FEATURE_DEMANGLE
 #cmakedefine WASMTIME_FEATURE_THREADS
 #cmakedefine WASMTIME_FEATURE_GC
+#cmakedefine WASMTIME_FEATURE_GC_DRC
+#cmakedefine WASMTIME_FEATURE_GC_NULL
 #cmakedefine WASMTIME_FEATURE_ASYNC
 #cmakedefine WASMTIME_FEATURE_CRANELIFT
 #cmakedefine WASMTIME_FEATURE_WINCH

crates/cli-flags/Cargo.toml

@@ -31,5 +31,7 @@ logging = ["dep:file-per-thread-logger", "dep:tracing-subscriber"]
 cranelift = ["wasmtime/cranelift"]
 coredump = ["wasmtime/coredump"]
 gc = ["wasmtime/gc"]
+gc-drc = ["gc", "wasmtime/gc-drc"]
+gc-null = ["gc", "wasmtime/gc-null"]
 threads = ["wasmtime/threads"]
 memory-protection-keys = ["wasmtime/memory-protection-keys"]

crates/cli-flags/src/lib.rs

@@ -182,6 +182,16 @@ wasmtime_option_group! {
         /// Currently only `cranelift` and `winch` are supported, but not all
         /// builds of Wasmtime have both built in.
         pub compiler: Option<wasmtime::Strategy>,
+        /// Which garbage collector to use: `drc` or `null`.
+        ///
+        /// `drc` is the deferred reference-counting collector.
+        ///
+        /// `null` is the null garbage collector, which does not collect any
+        /// garbage.
+        ///
+        /// Note that not all builds of Wasmtime will have support for garbage
+        /// collection included.
+        pub collector: Option<wasmtime::Collector>,
         /// Enable Cranelift's internal debug verifier (expensive)
         pub cranelift_debug_verifier: Option<bool>,
         /// Whether or not to enable caching of compiled modules.
@@ -535,6 +545,11 @@ impl CommonOptions {
             strategy => config.strategy(strategy),
             _ => err,
         }
+        match_feature! {
+            ["gc" : self.codegen.collector]
+            collector => config.collector(collector),
+            _ => err,
+        }
         match_feature! {
             ["cranelift" : target]
             target => config.target(target)?,

crates/cli-flags/src/opt.rs

@@ -385,6 +385,17 @@ impl WasmtimeOptionValue for wasmtime::Strategy {
     }
 }
 
+impl WasmtimeOptionValue for wasmtime::Collector {
+    const VAL_HELP: &'static str = "=drc|null";
+    fn parse(val: Option<&str>) -> Result<Self> {
+        match String::parse(val)?.as_str() {
+            "drc" => Ok(wasmtime::Collector::DeferredReferenceCounting),
+            "null" => Ok(wasmtime::Collector::Null),
+            other => bail!("unknown collector `{other}` only `drc` and `null` accepted",),
+        }
+    }
+}
+
 impl WasmtimeOptionValue for WasiNnGraph {
     const VAL_HELP: &'static str = "=<format>::<dir>";
     fn parse(val: Option<&str>) -> Result<Self> {

crates/cranelift/Cargo.toml

@@ -41,4 +41,6 @@ component-model = ["wasmtime-environ/component-model"]
 incremental-cache = ["cranelift-codegen/incremental-cache"]
 wmemcheck = ["wasmtime-environ/wmemcheck"]
 gc = ["wasmtime-environ/gc"]
+gc-drc = ["gc", "wasmtime-environ/gc-drc"]
+gc-null = ["gc", "wasmtime-environ/gc-null"]
 threads = ["wasmtime-environ/threads"]

crates/cranelift/src/func_environ.rs

@@ -118,7 +118,7 @@ pub struct FuncEnvironment<'module_environment> {
     /// Offsets to struct fields accessed by JIT code.
     pub(crate) offsets: VMOffsets<u8>,
 
-    tunables: &'module_environment Tunables,
+    pub(crate) tunables: &'module_environment Tunables,
 
     /// A function-local variable which stores the cached value of the amount of
     /// fuel remaining to execute. If used this is modified frequently so it's
@@ -1130,7 +1130,8 @@ impl<'module_environment> FuncEnvironment<'module_environment> {
         *builder.func.dfg.inst_results(call).first().unwrap()
     }
 
-    fn vmshared_type_index_ty(&self) -> Type {
+    /// Get the `ir::Type` for a `VMSharedTypeIndex`.
+    pub(crate) fn vmshared_type_index_ty(&self) -> Type {
         Type::int_with_byte_size(self.offsets.size_of_vmshared_type_index().into()).unwrap()
     }
 

crates/cranelift/src/gc.rs

@@ -26,7 +26,7 @@ pub use imp::*;
 
 /// How to initialize a newly-allocated array's elements.
 #[derive(Clone, Copy)]
-#[cfg_attr(not(feature = "gc"), allow(dead_code))]
+#[cfg_attr(not(any(feature = "gc-null", feature = "gc-drc")), allow(dead_code))]
 pub enum ArrayInit<'a> {
     /// Initialize the array's elements with the given values.
     Elems(&'a [ir::Value]),