externref: implement stack map-based garbage collection

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 #929

Fixes #1804

crates/jit/src/compiler.rs

@@ -15,7 +15,7 @@ use wasmtime_environ::wasm::{DefinedFuncIndex, DefinedMemoryIndex, MemoryIndex,
 use wasmtime_environ::{
     CacheConfig, CompileError, CompiledFunction, Compiler as _C, Module, ModuleAddressMap,
     ModuleMemoryOffset, ModuleTranslation, ModuleVmctxInfo, Relocation, RelocationTarget,
-    Relocations, Traps, Tunables, VMOffsets, ValueLabelsRanges,
+    Relocations, StackMaps, Traps, Tunables, VMOffsets, ValueLabelsRanges,
 };
 use wasmtime_runtime::{InstantiationError, VMFunctionBody, VMTrampoline};
 
@@ -138,6 +138,7 @@ pub struct Compilation {
     pub jt_offsets: PrimaryMap<DefinedFuncIndex, ir::JumpTableOffsets>,
     pub dwarf_sections: Vec<DwarfSection>,
     pub traps: Traps,
+    pub stack_maps: StackMaps,
     pub address_transform: ModuleAddressMap,
 }
 
@@ -165,27 +166,34 @@ impl Compiler {
     ) -> Result<Compilation, SetupError> {
         let mut code_memory = CodeMemory::new();
 
-        let (compilation, relocations, address_transform, value_ranges, stack_slots, traps) =
-            match self.strategy {
-                // For now, interpret `Auto` as `Cranelift` since that's the most stable
-                // implementation.
-                CompilationStrategy::Auto | CompilationStrategy::Cranelift => {
-                    wasmtime_environ::cranelift::Cranelift::compile_module(
-                        translation,
-                        &*self.isa,
-                        &self.cache_config,
-                    )
-                }
-                #[cfg(feature = "lightbeam")]
-                CompilationStrategy::Lightbeam => {
-                    wasmtime_environ::lightbeam::Lightbeam::compile_module(
-                        translation,
-                        &*self.isa,
-                        &self.cache_config,
-                    )
-                }
+        let (
+            compilation,
+            relocations,
+            address_transform,
+            value_ranges,
+            stack_slots,
+            traps,
+            stack_maps,
+        ) = match self.strategy {
+            // For now, interpret `Auto` as `Cranelift` since that's the most stable
+            // implementation.
+            CompilationStrategy::Auto | CompilationStrategy::Cranelift => {
+                wasmtime_environ::cranelift::Cranelift::compile_module(
+                    translation,
+                    &*self.isa,
+                    &self.cache_config,
+                )
             }
-            .map_err(SetupError::Compile)?;
+            #[cfg(feature = "lightbeam")]
+            CompilationStrategy::Lightbeam => {
+                wasmtime_environ::lightbeam::Lightbeam::compile_module(
+                    translation,
+                    &*self.isa,
+                    &self.cache_config,
+                )
+            }
+        }
+        .map_err(SetupError::Compile)?;
 
         let dwarf_sections = if debug_data.is_some() && !compilation.is_empty() {
             transform_dwarf_data(
@@ -239,6 +247,7 @@ impl Compiler {
             jt_offsets,
             dwarf_sections,
             traps,
+            stack_maps,
             address_transform,
         })
     }