wasmtime: Implement global.{get,set} for externref globals

We use libcalls to implement these -- unlike `table.{get,set}`, for which we
create inline JIT fast paths -- because no known toolchain actually uses
externref globals.

Part of #929

build.rs

@@ -201,11 +201,7 @@ fn ignore(testsuite: &str, testname: &str, strategy: &str) -> bool {
             ("simd", "simd_splat") => return true, // FIXME Unsupported feature: proposed SIMD operator I32x4TruncSatF32x4S
 
             // Still working on implementing these. See #929.
-            ("reference_types", "global")
-            | ("reference_types", "linking")
-            | ("reference_types", "ref_func")
-            | ("reference_types", "ref_null")
-            | ("reference_types", "table_fill") => {
+            ("reference_types", "table_fill") => {
                 return true;
             }
 

crates/environ/src/func_environ.rs

@@ -175,6 +175,10 @@ declare_builtin_functions! {
     /// Returns an index to do a GC and then insert a `VMExternRef` into the
     /// `VMExternRefActivationsTable`.
     activations_table_insert_with_gc(vmctx, reference) -> ();
+    /// Returns an index for Wasm's `global.get` instruction for `externref`s.
+    externref_global_get(vmctx, i32) -> (reference);
+    /// Returns an index for Wasm's `global.get` instruction for `externref`s.
+    externref_global_set(vmctx, i32, reference) -> ();
 }
 
 impl BuiltinFunctionIndex {
@@ -432,6 +436,28 @@ impl<'module_environment> FuncEnvironment<'module_environment> {
 
         new_ref_count
     }
+
+    fn get_global_location(
+        &mut self,
+        func: &mut ir::Function,
+        index: GlobalIndex,
+    ) -> (ir::GlobalValue, i32) {
+        let pointer_type = self.pointer_type();
+        let vmctx = self.vmctx(func);
+        if let Some(def_index) = self.module.defined_global_index(index) {
+            let offset = i32::try_from(self.offsets.vmctx_vmglobal_definition(def_index)).unwrap();
+            (vmctx, offset)
+        } else {
+            let from_offset = self.offsets.vmctx_vmglobal_import_from(index);
+            let global = func.create_global_value(ir::GlobalValueData::Load {
+                base: vmctx,
+                offset: Offset32::new(i32::try_from(from_offset).unwrap()),
+                global_type: pointer_type,
+                readonly: true,
+            });
+            (global, 0)
+        }
+    }
 }
 
 // TODO: This is necessary as if Lightbeam used `FuncEnvironment` directly it would cause
@@ -1042,19 +1068,56 @@ impl<'module_environment> cranelift_wasm::FuncEnvironment for FuncEnvironment<'m
 
     fn translate_custom_global_get(
         &mut self,
-        _: cranelift_codegen::cursor::FuncCursor<'_>,
-        _: cranelift_wasm::GlobalIndex,
+        mut pos: cranelift_codegen::cursor::FuncCursor<'_>,
+        index: cranelift_wasm::GlobalIndex,
     ) -> WasmResult<ir::Value> {
-        unreachable!("we don't make any custom globals")
+        debug_assert_eq!(
+            self.module.globals[index].wasm_ty,
+            WasmType::ExternRef,
+            "We only use GlobalVariable::Custom for externref"
+        );
+
+        let builtin_index = BuiltinFunctionIndex::externref_global_get();
+        let builtin_sig = self
+            .builtin_function_signatures
+            .externref_global_get(&mut pos.func);
+
+        let (vmctx, builtin_addr) =
+            self.translate_load_builtin_function_address(&mut pos, builtin_index);
+
+        let global_index_arg = pos.ins().iconst(I32, index.as_u32() as i64);
+        let call_inst =
+            pos.ins()
+                .call_indirect(builtin_sig, builtin_addr, &[vmctx, global_index_arg]);
+
+        Ok(pos.func.dfg.first_result(call_inst))
     }
 
     fn translate_custom_global_set(
         &mut self,
-        _: cranelift_codegen::cursor::FuncCursor<'_>,
-        _: cranelift_wasm::GlobalIndex,
-        _: ir::Value,
+        mut pos: cranelift_codegen::cursor::FuncCursor<'_>,
+        index: cranelift_wasm::GlobalIndex,
+        value: ir::Value,
     ) -> WasmResult<()> {
-        unreachable!("we don't make any custom globals")
+        debug_assert_eq!(
+            self.module.globals[index].wasm_ty,
+            WasmType::ExternRef,
+            "We only use GlobalVariable::Custom for externref"
+        );
+
+        let builtin_index = BuiltinFunctionIndex::externref_global_set();
+        let builtin_sig = self
+            .builtin_function_signatures
+            .externref_global_set(&mut pos.func);
+
+        let (vmctx, builtin_addr) =
+            self.translate_load_builtin_function_address(&mut pos, builtin_index);
+
+        let global_index_arg = pos.ins().iconst(I32, index.as_u32() as i64);
+        pos.ins()
+            .call_indirect(builtin_sig, builtin_addr, &[vmctx, global_index_arg, value]);
+
+        Ok(())
     }
 
     fn make_heap(&mut self, func: &mut ir::Function, index: MemoryIndex) -> WasmResult<ir::Heap> {
@@ -1141,28 +1204,19 @@ impl<'module_environment> cranelift_wasm::FuncEnvironment for FuncEnvironment<'m
         func: &mut ir::Function,
         index: GlobalIndex,
     ) -> WasmResult<GlobalVariable> {
-        let pointer_type = self.pointer_type();
-
-        let (ptr, offset) = {
-            let vmctx = self.vmctx(func);
-            if let Some(def_index) = self.module.defined_global_index(index) {
-                let offset =
-                    i32::try_from(self.offsets.vmctx_vmglobal_definition(def_index)).unwrap();
-                (vmctx, offset)
-            } else {
-                let from_offset = self.offsets.vmctx_vmglobal_import_from(index);
-                let global = func.create_global_value(ir::GlobalValueData::Load {
-                    base: vmctx,
-                    offset: Offset32::new(i32::try_from(from_offset).unwrap()),
-                    global_type: pointer_type,
-                    readonly: true,
-                });
-                (global, 0)
-            }
-        };
+        // Although `ExternRef`s live at the same memory location as any other
+        // type of global at the same index would, getting or setting them
+        // requires ref counting barriers. Therefore, we need to use
+        // `GlobalVariable::Custom`, as that is the only kind of
+        // `GlobalVariable` for which `cranelift-wasm` supports custom access
+        // translation.
+        if self.module.globals[index].wasm_ty == WasmType::ExternRef {
+            return Ok(GlobalVariable::Custom);
+        }
 
+        let (gv, offset) = self.get_global_location(func, index);
         Ok(GlobalVariable::Memory {
-            gv: ptr,
+            gv,
             offset: offset.into(),
             ty: self.module.globals[index].ty,
         })

crates/runtime/src/instance.rs

@@ -29,7 +29,7 @@ use wasmtime_environ::entity::{packed_option::ReservedValue, BoxedSlice, EntityR
 use wasmtime_environ::wasm::{
     DataIndex, DefinedFuncIndex, DefinedGlobalIndex, DefinedMemoryIndex, DefinedTableIndex,
     ElemIndex, FuncIndex, GlobalIndex, GlobalInit, MemoryIndex, SignatureIndex, TableElementType,
-    TableIndex,
+    TableIndex, WasmType,
 };
 use wasmtime_environ::{ir, DataInitializer, EntityIndex, Module, TableElements, VMOffsets};
 
@@ -226,6 +226,21 @@ impl Instance {
         unsafe { self.globals_ptr().add(index) }
     }
 
+    /// Get a raw pointer to the global at the given index regardless whether it
+    /// is defined locally or imported from another module.
+    ///
+    /// Panics if the index is out of bound or is the reserved value.
+    pub(crate) fn defined_or_imported_global_ptr(
+        &self,
+        index: GlobalIndex,
+    ) -> *mut VMGlobalDefinition {
+        if let Some(index) = self.module.local.defined_global_index(index) {
+            self.global_ptr(index)
+        } else {
+            self.imported_global(index).from
+        }
+    }
+
     /// Return a pointer to the `VMGlobalDefinition`s.
     fn globals_ptr(&self) -> *mut VMGlobalDefinition {
         unsafe { self.vmctx_plus_offset(self.offsets.vmctx_globals_begin()) }
@@ -1390,8 +1405,16 @@ fn initialize_globals(instance: &Instance) {
                     };
                     *to = from;
                 }
+                GlobalInit::RefFunc(f) => {
+                    *(*to).as_anyfunc_mut() = instance.get_caller_checked_anyfunc(f).unwrap()
+                        as *const VMCallerCheckedAnyfunc;
+                }
+                GlobalInit::RefNullConst => match global.wasm_ty {
+                    WasmType::FuncRef => *(*to).as_anyfunc_mut() = ptr::null(),
+                    WasmType::ExternRef => *(*to).as_externref_mut() = None,
+                    ty => panic!("unsupported reference type for global: {:?}", ty),
+                },
                 GlobalInit::Import => panic!("locally-defined global initialized as import"),
-                GlobalInit::RefNullConst | GlobalInit::RefFunc(_) => unimplemented!(),
             }
         }
     }

crates/runtime/src/libcalls.rs

@@ -60,9 +60,11 @@ use crate::externref::VMExternRef;
 use crate::table::Table;
 use crate::traphandlers::raise_lib_trap;
 use crate::vmcontext::{VMCallerCheckedAnyfunc, VMContext};
-use std::ptr::NonNull;
+use std::mem;
+use std::ptr::{self, NonNull};
 use wasmtime_environ::wasm::{
-    DataIndex, DefinedMemoryIndex, ElemIndex, MemoryIndex, TableElementType, TableIndex,
+    DataIndex, DefinedMemoryIndex, ElemIndex, GlobalIndex, MemoryIndex, TableElementType,
+    TableIndex,
 };
 
 /// Implementation of f32.ceil
@@ -430,3 +432,47 @@ pub unsafe extern "C" fn wasmtime_activations_table_insert_with_gc(
     let registry = &**instance.stack_map_registry();
     activations_table.insert_with_gc(externref, registry);
 }
+
+/// Perform a Wasm `global.get` for `externref` globals.
+pub unsafe extern "C" fn wasmtime_externref_global_get(
+    vmctx: *mut VMContext,
+    index: u32,
+) -> *mut u8 {
+    let index = GlobalIndex::from_u32(index);
+    let instance = (&mut *vmctx).instance();
+    let global = instance.defined_or_imported_global_ptr(index);
+    match (*global).as_externref().clone() {
+        None => ptr::null_mut(),
+        Some(externref) => {
+            let raw = externref.as_raw();
+            let activations_table = &**instance.externref_activations_table();
+            let registry = &**instance.stack_map_registry();
+            activations_table.insert_with_gc(externref, registry);
+            raw
+        }
+    }
+}
+
+/// Perform a Wasm `global.set` for `externref` globals.
+pub unsafe extern "C" fn wasmtime_externref_global_set(
+    vmctx: *mut VMContext,
+    index: u32,
+    externref: *mut u8,
+) {
+    let externref = if externref.is_null() {
+        None
+    } else {
+        Some(VMExternRef::clone_from_raw(externref))
+    };
+
+    let index = GlobalIndex::from_u32(index);
+    let instance = (&mut *vmctx).instance();
+    let global = instance.defined_or_imported_global_ptr(index);
+
+    // Swap the new `externref` value into the global before we drop the old
+    // value. This protects against an `externref` with a `Drop` implementation
+    // that calls back into Wasm and touches this global again (we want to avoid
+    // it observing a halfway-deinitialized value).
+    let old = mem::replace((*global).as_externref_mut(), externref);
+    drop(old);
+}

crates/runtime/src/vmcontext.rs

@@ -1,6 +1,7 @@
 //! This file declares `VMContext` and several related structs which contain
 //! fields that compiled wasm code accesses directly.
 
+use crate::externref::VMExternRef;
 use crate::instance::Instance;
 use std::any::Any;
 use std::ptr::NonNull;
@@ -296,6 +297,11 @@ mod test_vmglobal_definition {
         let offsets = VMOffsets::new(size_of::<*mut u8>() as u8, &module.local);
         assert_eq!(offsets.vmctx_globals_begin() % 16, 0);
     }
+
+    #[test]
+    fn check_vmglobal_can_contain_externref() {
+        assert!(size_of::<VMExternRef>() <= size_of::<VMGlobalDefinition>());
+    }
 }
 
 impl VMGlobalDefinition {
@@ -423,6 +429,30 @@ impl VMGlobalDefinition {
     pub unsafe fn as_u128_bits_mut(&mut self) -> &mut [u8; 16] {
         &mut *(self.storage.as_mut().as_mut_ptr() as *mut [u8; 16])
     }
+
+    /// Return a reference to the value as an externref.
+    #[allow(clippy::cast_ptr_alignment)]
+    pub unsafe fn as_externref(&self) -> &Option<VMExternRef> {
+        &*(self.storage.as_ref().as_ptr() as *const Option<VMExternRef>)
+    }
+
+    /// Return a mutable reference to the value as an externref.
+    #[allow(clippy::cast_ptr_alignment)]
+    pub unsafe fn as_externref_mut(&mut self) -> &mut Option<VMExternRef> {
+        &mut *(self.storage.as_mut().as_mut_ptr() as *mut Option<VMExternRef>)
+    }
+
+    /// Return a reference to the value as an anyfunc.
+    #[allow(clippy::cast_ptr_alignment)]
+    pub unsafe fn as_anyfunc(&self) -> *const VMCallerCheckedAnyfunc {
+        *(self.storage.as_ref().as_ptr() as *const *const VMCallerCheckedAnyfunc)
+    }
+
+    /// Return a mutable reference to the value as an anyfunc.
+    #[allow(clippy::cast_ptr_alignment)]
+    pub unsafe fn as_anyfunc_mut(&mut self) -> &mut *const VMCallerCheckedAnyfunc {
+        &mut *(self.storage.as_mut().as_mut_ptr() as *mut *const VMCallerCheckedAnyfunc)
+    }
 }
 
 /// An index into the shared signature registry, usable for checking signatures
@@ -559,6 +589,10 @@ impl VMBuiltinFunctionsArray {
             wasmtime_drop_externref as usize;
         ptrs[BuiltinFunctionIndex::activations_table_insert_with_gc().index() as usize] =
             wasmtime_activations_table_insert_with_gc as usize;
+        ptrs[BuiltinFunctionIndex::externref_global_get().index() as usize] =
+            wasmtime_externref_global_get as usize;
+        ptrs[BuiltinFunctionIndex::externref_global_set().index() as usize] =
+            wasmtime_externref_global_set as usize;
 
         if cfg!(debug_assertions) {
             for i in 0..ptrs.len() {

crates/wasmtime/src/externals.rs

@@ -7,6 +7,8 @@ use crate::{
     ValType,
 };
 use anyhow::{anyhow, bail, Result};
+use std::mem;
+use std::ptr;
 use std::slice;
 use wasmtime_environ::wasm;
 use wasmtime_runtime::{self as runtime, InstanceHandle};
@@ -227,6 +229,15 @@ impl Global {
                 ValType::I64 => Val::from(*definition.as_i64()),
                 ValType::F32 => Val::F32(*definition.as_u32()),
                 ValType::F64 => Val::F64(*definition.as_u64()),
+                ValType::ExternRef => Val::ExternRef(
+                    definition
+                        .as_externref()
+                        .clone()
+                        .map(|inner| ExternRef { inner }),
+                ),
+                ValType::FuncRef => {
+                    from_checked_anyfunc(definition.as_anyfunc() as *mut _, &self.instance.store)
+                }
                 ty => unimplemented!("Global::get for {:?}", ty),
             }
         }
@@ -256,6 +267,19 @@ impl Global {
                 Val::I64(i) => *definition.as_i64_mut() = i,
                 Val::F32(f) => *definition.as_u32_mut() = f,
                 Val::F64(f) => *definition.as_u64_mut() = f,
+                Val::FuncRef(f) => {
+                    *definition.as_anyfunc_mut() = f.map_or(ptr::null(), |f| {
+                        f.caller_checked_anyfunc().as_ptr() as *const _
+                    });
+                }
+                Val::ExternRef(x) => {
+                    // In case the old value's `Drop` implementation is
+                    // re-entrant and tries to touch this global again, do a
+                    // replace, and then drop. This way no one can observe a
+                    // halfway-deinitialized value.
+                    let old = mem::replace(definition.as_externref_mut(), x.map(|x| x.inner));
+                    drop(old);
+                }
                 _ => unimplemented!("Global::set for {:?}", val.ty()),
             }
         }