Wasm: Allow environment to translate some global.set/get operations

Spidermonkey will need to emit pre/post barriers for global.set/get to a
reference type. #1176 and #1299 plan to add a template concept that could
be used to implement this. Once that has been stabilized, we should be able
to remove this code in favor of templates easily.

cranelift/wasm/src/code_translator.rs

@@ -28,7 +28,7 @@ use crate::state::{ControlStackFrame, ElseData, FuncTranslationState, ModuleTran
 use crate::translation_utils::{
     blocktype_params_results, ebb_with_params, f32_translation, f64_translation,
 };
-use crate::translation_utils::{FuncIndex, MemoryIndex, SignatureIndex, TableIndex};
+use crate::translation_utils::{FuncIndex, GlobalIndex, MemoryIndex, SignatureIndex, TableIndex};
 use crate::wasm_unsupported;
 use core::{i32, u32};
 use cranelift_codegen::ir::condcodes::{FloatCC, IntCC};
@@ -95,6 +95,10 @@ pub fn translate_operator<FE: FuncEnvironment + ?Sized>(
                     let flags = ir::MemFlags::trusted();
                     builder.ins().load(ty, flags, addr, offset)
                 }
+                GlobalVariable::Custom => environ.translate_custom_global_get(
+                    builder.cursor(),
+                    GlobalIndex::from_u32(*global_index),
+                )?,
             };
             state.push1(val);
         }
@@ -108,6 +112,14 @@ pub fn translate_operator<FE: FuncEnvironment + ?Sized>(
                     debug_assert_eq!(ty, builder.func.dfg.value_type(val));
                     builder.ins().store(flags, val, addr, offset);
                 }
+                GlobalVariable::Custom => {
+                    let val = state.pop1();
+                    environ.translate_custom_global_set(
+                        builder.cursor(),
+                        GlobalIndex::from_u32(*global_index),
+                        val,
+                    )?;
+                }
             }
         }
         /********************************* Stack misc ***************************************

cranelift/wasm/src/environ/dummy.rs

@@ -504,6 +504,23 @@ impl<'dummy_environment> FuncEnvironment for DummyFuncEnvironment<'dummy_environ
     ) -> WasmResult<ir::Value> {
         Ok(pos.ins().null(self.reference_type()))
     }
+
+    fn translate_custom_global_get(
+        &mut self,
+        mut pos: FuncCursor,
+        _global_index: GlobalIndex,
+    ) -> WasmResult<ir::Value> {
+        Ok(pos.ins().iconst(I32, -1))
+    }
+
+    fn translate_custom_global_set(
+        &mut self,
+        _pos: FuncCursor,
+        _global_index: GlobalIndex,
+        _val: ir::Value,
+    ) -> WasmResult<()> {
+        Ok(())
+    }
 }
 
 impl TargetEnvironment for DummyEnvironment {

cranelift/wasm/src/environ/spec.rs

@@ -36,6 +36,9 @@ pub enum GlobalVariable {
         /// The global variable's type.
         ty: ir::Type,
     },
+
+    /// This is a global variable that needs to be handled by the environment.
+    Custom,
 }
 
 /// A WebAssembly translation error.
@@ -406,6 +409,23 @@ pub trait FuncEnvironment: TargetEnvironment {
     /// Translate a `ref.func` WebAssembly instruction.
     fn translate_ref_func(&mut self, pos: FuncCursor, func_index: u32) -> WasmResult<ir::Value>;
 
+    /// Translate a `global.get` WebAssembly instruction at `pos` for a global
+    /// that is custom.
+    fn translate_custom_global_get(
+        &mut self,
+        pos: FuncCursor,
+        global_index: GlobalIndex,
+    ) -> WasmResult<ir::Value>;
+
+    /// Translate a `global.set` WebAssembly instruction at `pos` for a global
+    /// that is custom.
+    fn translate_custom_global_set(
+        &mut self,
+        pos: FuncCursor,
+        global_index: GlobalIndex,
+        val: ir::Value,
+    ) -> WasmResult<()>;
+
     /// Emit code at the beginning of every wasm loop.
     ///
     /// This can be used to insert explicit interrupt or safepoint checking at